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1 December 2008 Impacts of piscivorous birds on salmonid populations and game fisheries in Scotland: a review
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The Scottish populations of salmonids are important both ecologically and economically. Game fisheries for Atlantic salmon Salmo salar, sea trout Salmo trutta trutta and brown trout Salmo trutta fari are all highly acclaimed and generate substantial levels of income for Scotland, but many populations are in decline and efforts are being made to ensure the future sustainability of these species. These declines have led to a focus on the impact of piscivorous bird predation on fish populations. The purpose of our review was to assess the evidence for population-level impacts on salmonid populations, and/or economic impacts on Scottish game fisheries of predation by the four primary UK freshwater piscivorous bird species: cormorant Phalacrocorax carbo, goosander Mergus merganser, red-breasted merganser Mergus serrator and grey heron Ardea cinerea. There is evidence that these birds can, in some situations, remove large numbers of fish from stocked and natural fisheries. However, a lack of information on fish population levels, the numbers and species composition of feeding birds, and robust calculations of fish consumption has hampered the conversion of the results of the existing studies into useful quantitative measures of impact. As a consequence, few studies have demonstrated any reductions in numbers of breeding fish or fish productivity due to predation by piscivorous birds, or direct economic loss to fisheries in Scotland. We support a previous recommendation for a reiterative procedure of modelling, experimentation and remodelling to assess the impacts of piscivorous birds on fisheries. Wide-scale studies of the movements of piscivorous birds, their feeding locations in relation to river characteristics, and the factors that make fish particularly vulnerable to predation are seen as important areas for future research.

The future sustainability of Scottish freshwater salmonid fisheries, game fisheries in particular, is being threatened by a number of contributing factors. The factors that receive most attention, however, are often those that provoke emotive responses rather than those that necessarily have the greatest impact (Duffy 1995, Butler et al. 2006). Predation of salmonids by birds and mammals is one such factor: the conflict that has arisen as a consequence of competition between humans and predators has become increasingly controversial as fish populations have declined and predator populations have increased (Kirby et al. 1996, Carss 2003). Such conflict often results in calls for predator reduction but in the absence of evidence of predator impact, the efficacy of such management remains unknown (Marquiss & Carss 1994b, Butler et al. 2006). In this review, we aim to summarise the evidence for population-level and economic impacts of the four piscivorous bird species great cormorant Phalacrocorax carbo, grey heron Ardea cinerea, goosander Mergus merganser and red-breasted merganser Mergus serrator on salmonid populations and game fisheries in Scotland. Other factors that potentially impact upon salmonid stocks are also reviewed briefly.

Reviewing process

We conducted literature searches that encompassed published (peer-reviewed), ‘grey’ and web-based literature. Published material was identified initially using the ISI Web of Knowledge database (up to the year end of 2006). Other published material and ‘grey’ literature was identified by carrying out web searches using the Google search engine and from the reference sections of papers and reports already obtained. Literature covered in this review focuses on piscivorous birds and salmonid populations in Scotland, but studies investigating bird-salmonid/fishery interactions elsewhere were included in the discussion section where the issue of ‘impacts’ on fish populations from piscivorous birds was addressed specifically. We also conducted a small number of one-to-one consultations and workshops with key stakeholder groups (see the Acknowledgements), in order to assess research needs and identify further sources of unpublished data that might not have been found as a result of the literature search.

The level of critical evaluation to which a piece of ‘evidence’ can be subjected necessarily depends on the amount of supporting information that is provided. In the context of the reviewing for our study, full critical scientific appraisal was only possible where (at the very least) data were presented together with full details of the methodology used to collect and analyse them. Reports are generally not formally peer-reviewed, although some will have undergone internal review from the source organisation, or in some cases, there may have been a more formal external review process. For this review, we occasionally used unpublished data, and this has been made clear in the text. The full details of the data collection and analytical methods were not always available for these data.

What does impact of predation mean?

Any factor such as food availability, space for breeding or predation can be considered limiting (i.e. having a population-level impact) if it prevents a population from increasing or causes it to decline (Newton 1998). The impact of predation on prey species depends largely on whether and how predators respond to changes in prey density (Begon et al. 1990). They can respond by changing individual predation rates (the functional response) or their density (the numerical response; Solomon 1949). Predation mortality may be wholly or partially offset by improved survival of the remaining individuals, i.e. the predation may be compensated for via reduced mortality from other factors. This is thought to be the case for young salmonids in their first year of life (Armstrong 1997, Milner et al. 2003). For predation to have an impact at the population level, it must represent additive mortality (Begon et al. 1990). For example, it is thought that mortality of salmon at sea is largely density-independent (e.g. Milner et al. 2003), and if this is the case, predation on smolts (juvenile salmon physiologically adapted for the migration from fresh water to salt water) leaving to enter the marine environment may inflict additive mortality and reduce the size of the returning population.

Regardless of whether piscivorous birds exert population-level impacts on prey populations, they may inflict an economic impact on fisheries through competition with humans. There is only a direct economic cost if predators remove fish that would otherwise have been available for anglers, and that would have been caught. The economic impact of predation, therefore, does not necessarily equate to the number of individual prey taken by a predator. Predators may also exert economic impacts indirectly, without necessarily reducing population abundance. For example, when anglers perceive that there are fewer fish to catch, this can lead to a reduction in the number of anglers purchasing permits. For systems involving piscivorous birds and fisheries, the nature of the impacts may differ slightly between natural and stocked systems, and consist of:

  • reductions in the number of fish reaching maturity and/or returning to the river;

  • changes in fish behaviour reducing catchability;

  • reductions in the number of fish available to anglers;

  • reductions in the number of permits sold to anglers;

  • reductions in the value of fish due to damage;

  • costs of fishery protection measures;

  • reductions in the breeding success of the fish population.

Status of Scotland's freshwater fisheries

Most freshwater sport fishing in Scotland has traditionally revolved around the game species that form the focus of this review: Atlantic salmon Salmo salar, sea trout Salmo trutta trutta and brown trout Salmo trutta fari. More recently, important sport fisheries have developed for other species such as rainbow trout Oncorhynchus mykiss, grayling Thymallus thymallus and coarse fish such as pike Esox lucius.

Information on numbers of salmon and sea trout caught by rod and line throughout Scotland each year is collated by the Fisheries Research Services, alongside the numbers caught by the netting industry (e.g. Fisheries Research Services 2006c). The catch statistics for the rod and line industry are believed to represent a suitable index of fish abundance, although there is no record of changes in angling effort over time (Youngson et al. 2002). For salmon, figures on abundance are broken down into seasonal components: spring salmon, summer salmon and grilse (salmon that have returned to fresh water after one winter at sea). In an effort to conserve stocks, anglers have increased the numbers of salmon and sea trout that are released again immediately after having been caught. These releases have been monitored since 1994. The term ‘total rod catch’ refers to the annual numbers of fish retained by the rod and line industry plus those caught and released.

Salmon fishing is the most highly acclaimed and economically important of Scotland's fisheries. However, whilst total rod catches of all salmon have remained relatively stable since 1950, there has been a long-term decline in the total rod catch of spring salmon (Fisheries Research Services 2006c), thought to be due, predominantly, to a decrease in survival at sea (Fisheries Research Services 2006a,c). It has been suggested that the decline over the last 50 years in fishing effort by the netting industry has partially compensated for the effect of declines in the survival of salmon at sea (e.g. Fisheries Research Services 2006c). Any such compensatory effect is now believed to be almost completely utilised, however, and if survival in the marine environment continues to decline, an increasing number of spawning populations are likely to be at risk (Fisheries Research Services 2006c). There has been relatively little research to investigate the threats facing salmonids in the marine environment, in part due to the difficulty of conducting such studies. Existing data indicate a positive linear correlation between numbers going to sea and numbers returning (see Milner et al. 2003 for review). Current management efforts are therefore focussed on populations within the freshwater environment with the objective of maximising the number that survive to go to sea.

As with salmon, net fisheries of sea trout have also declined, with the numbers of fish caught reflecting the decrease in effort (Fisheries Research Services 2006c). Although not so marked as the net fisheries, there has also been a decrease in rod catches of sea trout, with historically low levels of sea trout being caught by the rod and line fisheries in 2003, 2004 and 2005 (Fisheries Research Services 2006c). These trends also vary across Scotland, with the east coast fisheries showing little change, but west coast fisheries showing a marked decrease in the numbers caught (Fisheries Research Services 2006c).

The fishery for brown trout in Scotland is popular with anglers in both rivers and lochs (stillwater fisheries) and generates millions of pounds in revenue in some regions (e.g. Central Scotland and the Highlands; Radford et al. 2004). In areas where demand is high, for example in Loch Leven, both brown and rainbow trout are stocked to maintain angling levels. The rainbow trout is a non-native species in Scotland, but is being stocked increasingly in lochs and ponds for angling purposes, and rainbow trout fisheries have become an important economic and recreational resource in Scotland (Fisheries Research Services 2003, 2004). Of the total farmed rainbow trout production in 2005, 11.7% was supplied to fisheries for the purpose of restocking angling waters (Fisheries Research Services 2006b).

Together, all the freshwater fisheries within Scotland comprise an important component of the Scottish economy but are threatened by uncertainty surrounding the future productivity and economic viability of fish stocks. The economic importance of freshwater sport fisheries in Scotland is substantial, particularly in many rural areas. For example, in 2003, the total expenditure by anglers within the River Spey catchment was £11.8 million, 92% of which was due to salmon and sea trout anglers (Riddington et al. 2004). In 2004, the entire Scottish angling industry was estimated to generate £113 million in output, with salmon and sea trout angling accounting for £73 million of this total (Radford et al. 2004). The industry was shown to support around 2,800 jobs and generate nearly £50 million in wages and self-employment income to Scottish households.

Population estimates and trends of piscivorous birds

There has not been a recent review of the population sizes or population trends in Scotland of the five piscivorous bird species considered here (population estimates of shag Phalacrocorax aristotelis are also presented). We have assembled estimates using the highest quality data available to us to assess the status of these species within Scotland relative to European populations, and, in particular, recent trends (Table 1). Several of the population estimates have a number of caveats attached, in many cases because they have been produced using more recent trend information to extrapolate from older population estimates, some of which were derived from surveys not designed specifically for the species in question. In some cases, there has never been a survey that is comprehensive enough to allow a population estimate to be produced with any confidence (e.g. there has never been a specific survey of all breeding red-breasted mergansers in Scotland or Britain). A fuller consideration of the caveats associated with these population figures is provided in Park et al. (2005). The figures demonstrate that while the existing data suggest that several of the species have favourable conservation status at the UK and European level (e.g. the sawbills and grey heron), both great cormorant and shag are of medium conservation concern in the UK, and the shag has a population that is concentrated within Europe (see Table 1). Regardless of their present conservation listings, and often increasing populations in the early part of the period under consideration (post-1960), the populations of some species have shown evidence of decline in more recent years; i.e. breeding cormorant populations in northern Scotland and possibly Scottish wintering numbers, breeding shags, wintering and possibly breeding red-breasted mergansers, wintering goosanders and some local reports of declines in breeding birds.

Table 1.

Most recent estimates of the population size in Scotland, trends since 1960, and population status within Europe for the five piscivorous bird species covered by the current review. Superscribed numbers refer to the main sources of reference which are listed below the table itself. The populations include breeding (Bre) and wintering (Win) birds. For full details of derivation of estimates see Park et al. (2005).


Impact of piscivorous birds on game fisheries in Scotland


The literature on the great cormorant (subsequently referred to as ‘cormorant’) does not generally distinguish between the two races that are known to occur in England and Wales, but not in Scotland (Carss 2003). Most of the literature refers to cormorant, but it is possible that misidentification of shags may have occurred in some studies encompassing coastal regions.

The perceived level of impact of cormorant predation in Britain has increased in recent years and calls for population regulation have become more frequent (e.g. Kirby et al. 1996), so much that in England and Wales in September 2004, the decision was taken to licence the killing of an increased number of birds (see The main conflict between cormorants and freshwater fisheries in Europe, as a whole, occurs during the winter months, when there are large numbers of cormorants roosting inland (Carss 2003). During the winter period, the main concerns within Scotland relate to cormorants removing large numbers of wild and stocked trout from important stillwater fisheries, such as Loch Leven, and large numbers of young salmonids (particularly large parr and smolts) from rivers (McIntosh 1978, Carss et al. 1997b). There is also concern about cormorants wounding fish, which may reduce their survival and alter their behaviour, so that they become harder for anglers to catch (e.g. Russell et al. 1996).

Most Scottish studies reviewed here consist of observational data either relating to bird numbers and foraging behaviour or dietary data obtained from pellets, regurgitates or stomach samples from shot birds (Table 2). Whilst these studies can be combined with daily food intake (DFI) estimates to calculate consumption (e.g. Carss & Ekins 2002, Wilson et al. 2003), researchers have used a variety of different methods to derive DFI, so that it is hard to make meaningful comparisons between studies (Carss et al. 1997a, Gremillet et al. 2003). In addition, few studies have sufficient fish population data with which to estimate the proportion of fish being removed. In order to demonstrate the scale of any population-level impacts, however, the degree to which the predation is additive to other sources of mortality is required (Armstrong et al. 1998). Some studies have suggested that cormorant predation on salmon in rivers is low and that, of the salmon that are removed, small smolts or pre-smolt parr are preferentially selected (Carss & Marquiss 1997, 1998; but see Armstrong & Stewart 1996, Carss & Marquiss 1999b, Middlemas & Armstrong 2002 for a discussion of the methodology used to distinguish parr and smolt). The stage of the fish taken by predators is important in relation to the likely impact of predation. Thus, whilst mortality in young fish (pre-smolt) is considered to be density dependent, when they become smolts and leave the river system for the sea, it may be largely density independent (e.g. Milner et al. 2003).

Table 2.

Summary of methods used to estimate consumption by piscivorous birds (see also Carss et al. 1997a): Studies reviewed for this paper have been given as examples of where particular techniques have been used. Superscribed numbers refer to the main sources of reference which are listed below the table itself.


The highest quality data relating to a Scottish fishery are for the trout fishery at Loch Leven (e.g. Carss & Marquiss 1992, 1994, Carss et al. 1997b, Wright 2003, Stewart et al. 2005). Since 1983, Loch Leven has been stocked with brown trout (and since 1993 also with rainbow trout), and recent work has indicated that the numbers of brown (both stocked and wild) and rainbow trout removed by cormorants are substantial (Stewart et al. 2005). The diet of cormorants shot under licence at Loch Leven was assessed and used to estimate the likely loss of trout to cormorants roosting on the loch. The model estimated that 80,803 (95% Cl: 41,617–128,248) brown and 5,213 (95% Cl: 830–12,454) rainbow trout were taken by cormorants over a seven-month period (Stewart et al. 2005). The population of brown trout with >260 mm fork length (i.e. sufficiently large to contribute directly to the fishery) in Loch Leven in 1998 was estimated to be 157,000 using a mark-recapture method and 48,000 using sonar and netting (Wright 2003); the reason for the two estimates being so markedly different is not discussed in any depth. A comparison between the estimated number of brown trout (>260 mm fork length) taken by cormorants, and the size of the brown trout population suggested that cormorants were removing 30% (95% Cl: 16–49%) of the stock over the study period using mark-recapture estimates, and 98% (95% CI: 53–159%) of the stock using sonar and netting population estimates (Stewart et al. 2005). The latter, very high estimate in particular reflects the high degree of uncertainty in the estimation of the fish population size. For rainbow trout, the proportion of fish removed by cormorants was 31% (CI: 5–73%) over the study period (Stewart et al. 2005). The study also found a significant positive relationship between stocking levels of yearling brown trout and numbers of cormorants counted the following winter over a 17-year period. There appear to be different patterns of prey selection in juvenile and adult birds, and between the sexes, such that adult males fed almost exclusively on trout whereas juvenile females fed mainly on small shoaling fish (sticklebacks Gasterosteus aculeatus and perch fry Perca fluviatilis; Stewart et al. 2005). In economic terms, cormorant consumption was estimated to be removing 40% of the rainbow trout fishery catch, whereas for brown trout, cormorants removed almost 16 times the fishery catch (Stewart et al. 2005). The degree to which this predation may be compensated for by decreases in other sources of mortality is unknown although the authors conclude that there is currently no evidence for compensation, and that the potential for competition between cormorants and this fishery at this site is therefore high (Stewart et al. 2005).

The predation by piscivorous birds of fish stocked for the purposes of angling is often viewed as a direct economic loss to the fishery concerned. Whilst it is not known to what extent cormorant predation actually reduces the number of fish available to anglers, the perception that there are fewer fish to catch can lead to a reduction in the number of anglers purchasing permits (DEFRA Technical Advice Note 2004, Carss 2003). There are no reported examples of this in Scotland, however, during a conflict resolution workshop held in the Lea Valley in southeastern England, many anglers stated that they had stopped fishing the Lea because of low catch rates and the increase in cormorant numbers. These claims were reflected in the declines in fishing permits (Carss 2003). For example, in 1992/93 (over a period of nine months) approximately 23,000 anglers purchased a day ticket, 600 season tickets were sold and club membership at this time numbered around 6,500. In contrast, the forecast for the 12 months to December 2002 expected to see (at the time of reporting) season and club membership dropping by approximately 54% and day membership decreasing by >70%. This has had knock-on effects for fishing tackle shops across northeastern London, many of which have reportedly closed (Carss 2003).

In summary, there have been few quantitative studies undertaken that have actually demonstrated reductions in population size or productivity as a result of cormorant predation in Scotland. The recent study at Loch Leven indicated that large numbers of fish were removed by cormorants. However, it also highlighted some of the problems that exist in quantifying the scale of the impact on fish stocks and fishery economics, in part due to the uncertainties over fish population sizes.

Sawbill ducks

In Scotland, sawbill ducks (i.e. goosanders and red-breasted merganser) are predominantly perceived as a problem on rivers, where they are thought to consume substantial proportions of salmonid stocks, in particular salmonid smolts (e.g. Marquiss et al. 1991,1998). The majority of studies of sawbill predation carried out in Scottish rivers have been dietary and/or observational, but they are not, however, all independent studies as many relate to dietary information collected from the same individual shot birds (e.g. Marquiss & Carss 1998, Carss & Marquiss 1999a). Doubly-labelled water and respirometry experiments have been carried out on captive birds (Feltham 1995), which have improved estimates of consumption, but some of the assumptions associated with the doubly-labelled water method (see Table 2) and with the comparison of consumption with fish populations are either untested or have been shown to be violated to some degree (see Carss & Marquiss 1998 for details).

For both sawbill ducks, diet is varied, but there are consistent patterns in all the Scottish dietary data sets that are important to consider when gauging potential impact on any given river: diet is less diverse and contains a greater proportion of salmon in northern rivers as compared to southern rivers; and larger numbers of salmon are consumed in early winter, March and April compared to all other times of the year (Marquiss & Carss 1998). Most early studies assumed that birds were feeding on ‘average-sized’ smolts, leading to calculations of sawbills removing up to 35% of salmon production (e.g. Shearer et al. 1987). Since then, however, some studies have indicated that the birds preferentially take parr and small smolts and this has implications for the extent to which any losses can be considered additive and the overall impact of predation on these fish populations, depending on the point at which population regulation switches from density-dependent to density-independent (e.g. Feltham 1990, Marquiss et al. 1998).

By measuring the metabolic rates of nine captive goosanders released onto two Scottish rivers, and using data from previous dietary studies, daily consumption of salmon was estimated and numbers of fish removed was calculated (Feltham 1995). Annual predation of smolts by goosanders was estimated to have been 3–16% of the annual smolt production for one river in eastern Scotland (Feltham 1995). This assumes, however, that captive birds behave as wild birds upon release, the validity of which has been questioned (Carss & Marquiss 1998). A recent quantitative study on the Spey catchment in Scotland attempted to estimate the impact that salmon smolt removal by sawbills has on the number of returning adults and therefore, the number of fish available to anglers (J. Butler, unpubl. data). The minimum loss of salmon to sawbill ducks was calculated as 18,582 smolts between February-April 2003, equating to 3–5% of the smolt run, a loss of 335 salmon to the rod fishery and up to £569,500 of lost revenue to the local economy. This value assumes each rod-caught salmon is worth £1700 to the local economy, which was calculated using the figure derived for total expenditure on the rod fishery for salmon and sea trout on the Spey in 2003 (Riddington et al. 2004), divided by the rod catch for the same year. It is believed that these values represent underestimates, because the study focussed on the impact of predation on the spring salmon stock, so it did not include the losses of large parr and sea trout smolts from the calculations, or losses outside of the period February-April (J. Butler, unpubl. data). Caution is required, however, in assessing the economic impact of changes in fish populations. Specifically, Riddington et al. (2004) highlight the non-linear nature of economic returns from fish such that “a doubling of the returning salmon stock will not result in a doubling of the economic impact of salmon angling” and that “the causal chain between salmon stocks and output, income and employment is complicated and is not linear”.

There may be differences in the proportions of salmon in the diet of sawbills on different rivers, and so any economic impact is also likely to vary between rivers. Salmon was predominant in the stomachs of sawbills from the three northern-most rivers in one study in Scotland, but in the other nine rivers, trout, eel Anguilla anguilla or minnow Phoxinus phoxinus were the main dietary constituents (Marquiss & Carss 1998). These results could reflect differences in river characteristics or the relative ratios of salmon to sawbill populations on the different rivers, and further work is required in order to clarify the reasons for the observed dietary variation.

In summary, there is some evidence that sawbill ducks may have population-level impacts on salmon fisheries in some Scottish rivers, but the extent of any impact has not been quantified. Estimates of impact are likely to vary between studies, however, and all of the studies undertaken to date have necessarily incorporated assumptions, some of which have been proven to be violated to a greater or lesser degree.

Grey herons

Herons have been reported less often as a threat to Scottish fisheries than cormorants and sawbills, and their perceived impact has generally been reported for the fish farming industry (Carss 1993, Quick et al. 2004, Park et al. 2005). However, a questionnaire survey indicated that anglers do perceive herons to be a problem throughout Britain, largely in rivers but also in still waters in northwestern and southeastern England (Carss & Marquiss 1996). To our knowledge, there have been no assessments of grey heron impact on river or stillwater fisheries in Scotland, or in Britain as a whole (see also Hughes et al. 1999).

Other factors affecting Scottish game fisheries

Few of the Scottish studies reviewed here have considered how predation by piscivorous birds compares to other sources of mortality, and the lack of quantitative studies on piscivorous bird predation makes any quantitative comparisons with other mortality factors impossible at present. Below, we discuss briefly other factors thought to be affecting the Scottish salmonid fishery, despite evidence not always existing for the Scottish situation.

Other predators

Aside from piscivorous birds, the main predators of salmonids in freshwater in Scotland are seals, mustelids, piscivorous fish and humans. In Scotland, there are two species of seal, the grey seal Halichoerus grypus and the common seal Phoca vitulina. Most individuals of both species spend all their time at sea, feeding exclusively on marine fish species, although some also feed on estuaries and up rivers, where they are perceived to be removing commercially important salmonids and impacting on salmonid populations (e.g. Last 2000, Butler et al. 2006). The main studies of predation by seals have been largely observational and dietary (e.g. Boyle et al. 1990, Pierce et al. 1991, Greenstreet et al. 1993, Carter et al. 2001). One of the problems encountered when carrying out dietary studies of seals is the high digestibility of salmonid otoliths, which may lead to under-representation of salmon in diet analysis (Pierce et al. 1991 and references therein). It is thought that seals in the UK are unlikely to be responsible for the long-term declines in salmon abundance (SCOS 2005), although modelling work has indicated that the removal of seals from small rivers at the start of a year is likely to have a positive impact on salmon populations, particularly on the spring salmon stocks, which are in decline and at low population levels in some rivers (SCOS 2005, Butler et al. 2006). Otters Lutra lutra and mink Mustela vison are both found in riparian habitats in Scotland and have been reported to feed on salmonids, but due to the population status of both these species in Scotland (in particular, otters), it is not thought that either are currently having a large impact on populations (Carss et al. 1990, Cunningham et al. 2002). Predation on juvenile salmonids by fish such as pike, eels or larger salmonids may be substantial (McIntosh 1978, Henderson & Letcher 2003, Hyvarinen & Vehanen 2004), and there have been some attempts by managers of salmonid fish to control populations of piscivorous fish, such as pike (Morrison 1988). McIntosh (1978) suggested that such predation on juvenile salmonids is compensatory mortality, however, and in fact may reduce density-dependent competition within a fish population.

Habitat degradation

Water quality, quantity and the physical structure of water courses are all important components of salmonid habitat, many of which have been altered over time through anthropogenic activities (e.g. Gilvear et al. 2002). In Scotland, one of the main pollutants of concern is contamination of rivers from the fish farming industry (e.g. Hennessey et al. 1996, Jacobs et al. 2002). Climate change may affect conditions such as temperature within both freshwater and marine ecosystems, (e.g. Friedland 1998, Swansburg et al. 2002, Stefansson et al. 2003), and both river and sea surface temperature have been shown to be related to the growth and survival of salmonids (Friedland et al. 2000, Swansburg et al. 2002). There are many hydro-electric dam constructions on rivers in Scotland, and although there are no studies that report the impact of these on salmonid populations in Scotland, there are a number of other studies from Europe and North America that show that they block the upstream movement of migratory fish such as salmonids, thereby obstructing their return to their spawning grounds (e.g. Levin & Tolimieri 2001, Koed et al. 2002). They also invariably alter flow regimes and change the nature of the habitat available downstream of the dam (Dauble et al. 2003). In addition to this, dams create bottlenecks, where fish are delayed in their migrations and become more vulnerable to predation (eg. Collis et al. 2002 and references therein, Koed et al. 2002, Mathers et al. 2002).

Survival at sea

Poor survival in the marine environment is considered to be one of the main factors contributing to the declining numbers of returning salmonids in Scotland (e.g. Fisheries Research Services 2006a). However, survival at sea is very difficult to quantify directly because of the huge complexity of the marine environment. Food availability in the sea is of great importance to survival, as the marine stage in the life cycle of a salmonid is the main growth stage. Evidence to date suggests that long-term changes in prey availability are linked to changes in the climate and sea-surface temperatures, and this has resulted in a decline in the abundance of salmon, a trend which is likely to continue with predicted temperature changes in the future (Beaugrand & Reid 2003).


In 2005, approximately 900,000 fish were reported as having escaped from fish farms in Scotland (Fisheries Research Service 2006b), and as a result of continuing escapes, there is concern that farmed fish may compromise the genetic integrity of wild stocks. Evidence from a river system in Ireland suggests that both farmed and hybrid salmon progeny can survive to the smolt stage, survive at sea and even return to their river of origin (McGinnity et al. 1997). Although farmed salmon progeny have a lower chance of survival to the smolt stage, they grow faster than hybrid and wild salmon and competitively displace wild fish from preferred habitat (McGinnity et al. 1997). Therefore, the survival of farmed and hybrid progeny is likely to be threatening wild salmon genetic integrity, and levels of adaptation and fitness to some extent (e.g. Fleming et al. 2000). Fish farming has also been linked with an increase in reports of sea-lice on wild salmonids (SEERAD 2001a,b, Butler 2002).


The controversy surrounding the impact of the four piscivorous birds on fisheries covered by this review is not an issue confined to Scotland, or to the game fishing industry. This particularly applies to cormorants because of their widespread distribution. For example, in a recent report on cormorant-fisheries conflicts across Europe, Carss (2003) reported that the highest proportion of fish species recorded in conflicts involving cormorants were cyprinids followed by salmonids, perch/pike and a number of fish species associated with coastal aquaculture. Looking specifically at game species, we found case studies from Europe and North America that report that the estimated proportion of salmonids removed by cormorants from rivers can be high in some cases (e.g. Kennedy & Greer 1988, Lekuona & Campos 1997, Collis et al. 2001, 2002, Koed et al. 2006). It should be noted that much of the literature from North America, including some of the afore-mentioned studies, relates to the double-crested cormorant Phalacrocorax auritus, and Pacific salmonid species; therefore, we have limited our discussion of these studies. Although relevant studies exist from other geographical locations and for closely-related species, focussing the review on the situation in Scotland has highlighted the difficulties associated with assessing the impact of piscivorous birds on fish populations and fisheries in general and the following ‘take-home’ messages can be applied to more than just the Scottish situation.

Most of the relevant studies on the impacts of piscivorous birds on salmonid fisheries have been based on observational and/or dietary data, and the conversion of these data into useful quantitative measures of impact has often been hampered by a lack of information on fish populations and reliable consumption calculations. This does not necessarily mean that there are no impacts, but rather that the difficulties of measuring the key parameters of fish populations and bird predation have not allowed the scale of any impact to be assessed rigorously. Marquiss et al. (1991) reviewed a number of experimental sawbill duck removal studies that were carried out in Canada but concluded that in all cases, experimental design was sufficiently flawed to prevent impact assessment. Experiments in which birds are excluded from sites through netting or removal, and where the outcome (e.g. fish population size) can be measured and compared to similar sites with no manipulation, are the best way to demonstrate impact (Marquiss & Carss 1994a). Steinmetz et al. (2003) carried out such an experiment in the USA and showed that the presence of belted kingfisher Ceryle alcyon and great blue herons Ardea herodias altered both fish abundance and size distribution. Unfortunately, these types of experiments are most often only practical on fish cages or small areas of stocked stillwaters, so may be of limited relevance to larger, more complex systems.

Other than those reviewed in Marquiss et al. (1991), we found only one other study that attempted to quantify the effect of removal on populations of piscivorous birds (Marquiss 1998). Long-term bird survey data from the Rivers Dee and North Esk in Scotland were used to assess the effectiveness of bird removal (Marquiss 1998). In three out of four cases, shooting reduced the subsequent numbers of sawbill ducks on a stretch of river, but the reduction in numbers was always much less than the number of birds shot (e.g. 16 fewer birds resulting from 49 birds being shot), indicating that areas with artificially low bird densities become more attractive (Marquiss 1998). Whilst shooting in this context is designed to scare birds away for periods when fish are most vulnerable to predation, rather than to reduce population densities, it does demonstrate that the scale of reduction in numbers of birds at a site was influenced by the time of year that shooting took place and the overall intensity of shooting (Marquiss 1998). Besides this example, there are few studies that have shown that the removal of predators (either by scaring or killing) actually reduces bird abundance or increases fish yields (Draulans 1987, Russell et al. 1996). Studies, such as those described above, generally assume the ‘surplus-yield calculation’, whereby it is believed that removing the top predator from a system results in more fish becoming available to the fishery (Yodzis 2001). The reality is unlikely to be this straightforward, due to the complexity of the relevant ecosystems and their underlying food webs (Yodzis 2001).

In a previous review of the impacts of piscivorous birds on fisheries, Wires et al. (2003) stress the need for reliable diet assessment for predatory birds, combined with daily food intake estimates, improved information on the biology and demography of each predatory bird species, spatial and temporal abundance and distribution information for the relevant fish populations and an understanding of fish population dynamics. Marquiss & Carss (1994a,b) also emphasised the importance of experimental manipulations in demonstrating ‘cause and effect’ but acknowledged the difficulty of carrying out such experiments on rivers or large stillwater fisheries. Expanding on this recommendation, Marquiss et al. (1998) suggested a combination of modelling and experimentation, i.e. ‘model-field experiment-remodel’, due to the limitations associated with each in isolation. For the development of testable models, however, a number of parameters relating to both bird and fish biology are required and these are listed in Marquiss et al. (1998). We fully support these previous recommendations.

Following our review of the literature, and discussions with stakeholders, we have identified the need for a Scotland-wide survey of piscivorous birds to be carried out along important salmonid rivers, designed carefully to encompass the important seasons of the year when impacts are believed to be greatest and to cover all important salmonid rivers (Park et al. 2005). This work should be combined with research to establish the relationships between bird numbers and distribution and the characteristics of individual river sections. To get closer to assessing the likelihood of impact in this complex system, a review of existing demographic data for fish populations in Scotland is needed, given the quality of existing empirical data and sensitivities of such models to the many parameters involved. In addition, a review of the data on numbers of fish-eating birds (available from applications for licences to control them) would be valuable, to assess their utility for demonstrating spatial and seasonal variation in bird numbers, and hence for informing the design of Scotland-wide river surveys.

In conclusion, piscivorous birds are perceived to impact on fisheries in a number of ways that are all interrelated: economically, ecologically and behaviourally (Kirby et al. 1996, 1997, DEFRA Technical Advice Note 2004). They have the potential to compete directly with anglers for the same species and sizes of fish, reduce recruitment by taking smaller fish, damage fish that are then not marketable and are more prone to disease, and make fish less catchable through stress and behavioural changes. Piscivorous birds also have the potential to impact on fisheries indirectly by influencing the perceptions of anglers as to the ‘quality’ of a fishery, leading to loss of permit sales. However, research is required before many of these putative impacts can be quantified or demonstrated at the population level.


our review arises from a literature survey commissioned by Scotland's Moorland Forum following Petition 449 from the Scottish Gamekeepers’ Association (SGA) to the Petitions Committee of the Scottish Parliament regarding predatory birds in Scotland. We would like to thank the following people for their contribution to this review: John Armstrong, Graham Austin, Bert Burnett, James Butler, Ron Campbell, Humphrey Crick, David Dunkley, Ross Gardiner, Murray Grant, Ronnie Kippen, Nigel Harding, John Harradine, Mark Holling, Andrew Joys, Paul Knight, John Marchant, Mick Marquiss, Martin Milarky, Andy Musgrove, Duncan Orr-Ewing, Mark Rehfisch, Colin Shedden, Roger Sidaway, Dave Stewart, Patrick Stirling-Aird, Des Thompson, Mike Thornton, Jerry Wilson and Jenny Worden. We are grateful to the following organisations for funding our research: British Association for Shooting and Conservation, The Game Conservancy Scottish Research Trust, The Heather Trust, Royal Society for the Protection of Birds, Scottish Association for Country Sports, The Scottish Executive, Scottish Gamekeepers Association, Scottish Natural Heritage, Scottish Rural Property and Business Association and Scottish Raptor Study Groups.



M. J. S. Armitage, M. M. Rehfisch, and C. V. Wernham . 1997. The 1997 breeding sawbill survey. DRAFT BTO Research Report No. 193, BTO. Thetford. pp. Google Scholar


J. D. Armstrong 1997. Self-thinning in juvenile sea trout and other salmonid fishes revisited. Journal of Animal Ecology 66:519–526. Google Scholar


J. D. Armstrong, R. Gardiner, and R. Laughton . 1998. Potential responses by juvenile salmonid populations to predation In M. Marquiss, D. N. Carrs, J. D. Armstrong, and R. Gardiner , editors. (Eds.). Fish Eating Birds and Salmonids in Scotland Report on fish-eating birds research (1990–97), to. The Scottish Office Agriculture, Environment and Fisheries Department. Available at: Scholar


J. D. Armstrong and D. C. Stewart . 1996. The relationship between first vertebra width and body length of Atlantic salmon differs between parr and smolts. Journal of Fish Biology 49:1038–1040. Google Scholar


S. R. Baillie, J. H. Marchant, H. Q. P. Crick, D. G. Noble, D. E. Balmer, C. Barimore, R. H. Coombes, I. S. Downie, S. N. Freeman, A. C. Joys, D. I. Leech, M. J. Raven, R. A. Robinson, and R. M. Thewlis . 2007. Breeding Birds in the Wider Countryside: their conservation status 2006. BTO Research Report No. 470. BTO. Thetford. Available at: Scholar


G. Beaugrand and P. C. Reid . 2003. Long-term changes in phytoplankton, zooplankton and salmon related to climate. Global Change Biology 9:801–817. Google Scholar


M. Begon, J. L. Harper, and C. R. Townsend . 1990. Ecology: individuals, populations and communities. 2nd editionBlackwell Scientific Publications. Oxford. pp. Google Scholar


BirdLife International/Europe Bird Census Council 2000. European bird populations: estimates and trends. BirdLife Conservation Series No. 10. BirdLife International. Cambridge. pp. Google Scholar


P. R. Boyle, G. J. Pierce, and J. S. W. Diack . 1990. Sources of evidence for salmon in the diet of seals. Fisheries Research 10:137–150. Google Scholar


J. R. A. Butler 2002. Wild salmonids and sea louse infestations on the west coast of Scotland: sources of infection and implications for the management of marine salmon farms. Pest Management Science 58:595–608. Google Scholar


J. R. A. Butler, S. J. Middelmas, I. M. Graham, P. M. Thompson, and J. D. Armstrong . 2006. Modelling the impacts of removing seal predation from Atlantic salmon, Salmo salar, rivers in Scotland: a tool for targeting conflict resolution. Fisheries Management and Ecology 13:285–291. Google Scholar


D. N. Carss 1993. Grey heron, Ardea cinerea L., predation at cage fish farms in Argyll, western Scotland. Aquaculture and Fisheries Management 24:29–45. Google Scholar


D. N. Carss (Ed.). 2003. Reducing the conflict between cormorants and fisheries on a pan-European scale: REDCAFE Final Report to European Union DG Fish. pp. Google Scholar


D. N. Carss, R. M. Bevan, A. Bonetti, G. Cherubini, J. Davies, D. Doherty, A. El Hilli, M. J. Feltham, N. Grade, J. P. Granadeiro, D. Grémillet, J. Gromadzka, Y. N. R. A. Harari, T. Holden, T. Keller, G. Lariccia, R. Mantovani, T. M. McCarthy, M. Mellin, T. Menke, I. Mirowska-Ibron, W. Muller, P. Musil, T. Nazirides, W. Suter, J. F. G. Trauttmansdorff, S. Volponi, and B. Wilson . 1997a. Techniques for assessing Cormorant diet & food intake: towards a consensus view. In. Proceedings of the IVth European Conference on Cormorant. Bologna. pp. 197–230. Google Scholar


D. N. Carss and G. R. Ekins . 2002. Further European integration: Mixed sub-species colonies of Great Cormorants Phalacrocorax carbo in Britain: Colony establishment, diet, and implications for fisheries management. Ardea 90:23–41. Google Scholar


D. N. Carss, H. Kruuk, and J. W. H. Conroy . 1990. Predation on adult Atlantic salmon, Salmo salar L, by otters, Lutra lutra (L), within the River Dee system, Aberdeenshire, Scotland. Journal of Fish Biology 37:935–944. Google Scholar


D. N. Carss and M. Marquiss . 1992. Cormorants and the Loch Leven trout fishery. Report to Scottish Natural Heritage T135c1. pp. Google Scholar


D. N. Carss and M. Marquiss . 1994. The stomach contents of Cormorants from Loch Leven, 1992–1994. Report to Scottish Natural Heritage T02073c1. pp.  Google Scholar


D. N. Carss and M. Marquiss . 1996. The influence of a fish farm on grey heron Ardea cinerea breeding performance. In S. P. R. Greenstreet and M. L. Tasker , editors. (Eds.). Aquatic predators and their prey. Blackwell Scientific Publications. Oxford. pp. 133–142. Google Scholar


D. N. Carss and M. Marquiss . 1997. The diet of Cormorants Phalacrocorax carbo in Scottish freshwaters in relation to feeding habitats and fisheries. Ekologia Polska 45:207–222. Google Scholar


D. N. Carss and M. Marquiss . 1998. The numbers of salmon and brown trout consumed by fish eating birds. In M. Marquiss, D. N. Carrs, J. D. Armstrong, and R. Gardiner , editors. (Eds.). Fish Eating Birds and Salmonids in Scotland Report on fish-eating birds research (1990–97), to. The Scottish Office Agriculture, Environment and Fisheries Department. Available at: Scholar


D. N. Carss and M. Marquiss . 1999a. Fish eating birds and fisheries. Scottish Bird News 55:6–7. Google Scholar


D. N. Carss and M. Marquiss . 1999b. Skeletons in the cupboard? Quantifying bird predation on Atlantic salmon: atlas vertebra: fish length equations revisited. Journal of Zoology (London) 248:272–276. Google Scholar


D. N. Carss, M. Marquiss, and A. Lauder . 1997b. Cormorant Phalacrocorax carbo carbo predation at a major trout fishery in Scotland. In N. Baccetti and G. Cherubini , editors. (Eds.). Proceedings of the IVth European Conference on Cormorants pp. 281–294. Google Scholar


T. J. Carter, G. J. Pierce, J. R. G. Hislop, J. A. Houseman, and P. R. Boyle . 2001. Predation by seals on salmonids in two Scottish estuaries. Fisheries Management and Ecology 8:207–225. Google Scholar


K. Collis, D. D. Roby, D. P. Craig, S. Adamany, J. Y. Adkins, and D. E. Lyons . 2002. Colony size and diet composition of piscivorous waterbirds on the lower Columbia River: Implications for losses of juvenile salmonids to avian predation. Transactions of the American Fisheries Society 131:537–550. Google Scholar


K. Collis, D. D. Roby, D. P. Craig, B. A. Ryan, and R. D. Ledgerwood . 2001. Colonial waterbird predation on juvenile salmonids tagged with passive integrated transponders in the Columbia river estuary: Vulnerability of different salmonid species, stocks, and rearing types. Transactions of the American Fisheries Society 130:385–396. Google Scholar


P. D. Cunningham, L. J. Brown, and A. J. Harwood . 2002. Predation and scavenging of salmon carcasses along spawning streams in the Scottish Highlands. Final report for the Atlantic Salmon Trust. pp. Google Scholar


D. D. Dauble, T. P. Hanrahan, D. R. Geist, and M. J. Parsley . 2003. Impacts of the Columbia River hydroelectric system on main-stem habitats of fall Chinook salmon. North American Journal of Fisheries Management 23:641–659. Google Scholar


J. M. Davies and M. J. Feltham . 1994. Do Cormorants and anglers compete for the same resource. In I. J. Winfield , editor. (Ed.). Proceedings of the 25th Annual Institute of Fisheries Management. Study Course University of Lancaster. 13–15th September 1994. pp. 167–188. Google Scholar


J. M. Davies and M. J. Feltham . 1996. The diet of wintering Cormorants Phalacrocorax carbo L. in relation to angling catches on a coarse river fishery in north-west England. In S. P. R. Greenstreet and M. L. Tasker , editors. (Eds.). Aquatic predators and their prey. Blackwell Scientific Publishing. Oxford. pp. 106–110. Google Scholar


Department for Environment, Food and Rural Affairs (DEFRA) 2004. Fisheries and the presence of cormorants, goosanders and herons. Technical Advice Note 14. pp. Available at: Scholar


C. E. Derby and J. R. Lovvorn . 1997. Predation on fish by Cormorants and pelicans in a cold-water river: a field and modeling study. Canadian Journal of Fisheries and Aquatic Sciences 54:1480–1493. Google Scholar


D. Draulans 1987. The effectiveness of attempts to reduce predation by fish-eating birds - a review. Biological Conservation 41:219–232. Google Scholar


D. C. Duffy 1995. Why is the double-crested cormorant a problem? Insights from cormorant ecology and human socio-biology. Colonial Waterbirds 18:25–32. Google Scholar


M. J. Feltham 1990. The diet of red-breasted mergansers (Mergus serrator) during the smolt run in NE Scotland - the importance of salmon (Salmo salar) smolts and parr. Journal of Zoology 222:285–292. Google Scholar


M. J. Feltham 1995. Consumption of Atlantic salmon smelts and parr by goosanders - estimates from doubly-labeled water measurements of captive birds released on 2 Scottish rivers. Journal of Fish Biology 46:273–281. Google Scholar


M. J. Feltham, J. M. Davies, T. Holden, B. R. Wilson, I. G. Cowx, J. P. Harvey, and J. R. Britton . 1999. Case studies of the impact of fish-eating birds on inland fisheries in England and Wales. Ministry of Agriculture Fisheries and Food. UK. MAFF Project VC0106. pp. Google Scholar


M. J. Feltham and J. C. MacLean . 1996. Carlin tag recoveries as an indicator of predation on salmon smelts by goosanders and red-breasted mergansers. Journal of Fish Biology 48:270–282. Google Scholar


Fisheries Research Services 2003. Status of rainbow trout in Scotland: the results from a questionnaire survey. Scottish Fisheries Information Pamphlet No. 23. Scottish Executive Rural Affairs Department. pp. Available at: Scholar


Fisheries Research Services 2004. Scottish salmon and sea trout catches, 2003. Fisheries Series No. Fis/2004/1. Scottish Executive Rural Affairs Department. pp. Available at: Scholar


Fisheries Research Services 2006a. Marine mortality in Atlantic salmon. pp. Available at: Scholar


Fisheries Research Services 2006b. Scottish fish farms annual production survey 2005. A report prepared for the Scottish Executive. pp. Available at: Scholar


Fisheries Research Services 2006c. Scottish salmon and sea trout catches,. 2005. Fisheries Series No. Fis/2006/1. Scottish Executive Rural Affairs Department. pp. Available at: Scholar


I. A. Fleming, K. Hindar, I. B. Mjolnerod, B. Jonsson, T. Balstad, and A. Lamberg . 2000. Lifetime success and interactions of farm salmon invading a native population. Proceedings of the Royal Society of London Series B-Biological Sciences 267:1517–1523. Google Scholar


K. D. Friedland 1998. Ocean climate influences on critical Atlantic salmon (Salmo salar) life history events. Canadian Journal of Fisheries and Aquatic Sciences 55:119–130. Google Scholar


K. D. Friedland, L. P. Hansen, D. A. Dunkley, and J. C. MacLean . 2000. Linkage between ocean climate, post-smolt growth, and survival of Atlantic salmon (Salmo salar L.) in the North Sea area. Ices Journal of Marine Science 57:419–429. Google Scholar


D. W. Gibbons, J. B. Reid, and R. A. Chapman . 1993. The New Atlas of Breeding Birds in Britain and Ireland: 1988–1991. T. & A.D. Poyser. London. pp. Google Scholar


D. J. Gilvear, K. V. Heal, and A. Stephen . 2002. Hydrology and the ecological quality of Scottish river ecosystems. Science of the Total Environment 294:131–159. Google Scholar


S. P. R. Greenstreet, R. I. G. Morgan, S. Barnett, and P. Redhead . 1993. Variation in the numbers of shags Phalacrocorax aristotelis and common seals Phoca vitulina near the mouth of an Atlantic salmon Salmo salar river at the time of the smolt run. Journal of Animal Ecology 62:565–576. Google Scholar


R. D. Gregory, N. I. Wilkinson, D. G. Noble, J. A. Robinson, A. F. Brown, J. Hughes, D. A. Procter, D. W. Gibbons, and C. A. Galbraith . 2002. The population status of birds in the United Kingdom, Channel Islands and Isle of Man: an analysis of conservation concern 2002–2007. British Birds 95:410–448. Google Scholar


D. Gremillet, G. Wright, A. Lauder, D. N. Carss, and S. Wanless . 2003. Modelling the daily food requirements of wintering great Cormorants: a bioenergetics tool for wildlife management. Journal of Applied Ecology 40:266–277. Google Scholar


J. N. Henderson and B. H. Letcher . 2003. Predation on stocked Atlantic salmon (Salmo salar) fry. Canadian Journal of Fisheries and Aquatic Sciences 60:32–42. Google Scholar


M. M. Hennessy, L. Wilson, W. Struthers, and L. A. Kelly . 1996. Waste loadings from two freshwater Atlantic salmon juvenile farms in Scotland. Water Air and Soil Pollution 86:235–249. Google Scholar


B. Hughes, R. M. Bevan, J. M. Bowler, L. Still, D. N. Carss, M. Marquiss, R. D. Hearn, and J. H. Bruce . 1999. Feeding behaviour of fish-eating birds in Great Britain. London DETR. pp.  Google Scholar


P. Hyvarinen and T. Vehanen . 2004. Effect of brown trout body size on post-stocking survival and pike predation. Ecology of Freshwater Fish 13:77–84. Google Scholar


M. N. Jacobs, A. Covaci, and P. Schepens . 2002. Investigation of selected persistent organic pollutants in farmed Atlantic salmon (Salmo salar), salmon aquaculture feed, and fish oil components of the feed. Environmental Science & Technology 36:2797–2805. Google Scholar


T. Keller 1998. The food of cormorants (Phalacrocorax carbo sinensis) in Bavaria. Journal Fur Ornithologie 139:389–400. Google Scholar


G. J. A. Kennedy and J. E. Greer . 1988. Predation by cormorants, Phalacrocorax carbo (L.), on the salmonid populations of an Irish river. Aquaculture and Fisheries Management 19:159–170. Google Scholar


M. Kershaw and P. A. Cranswick . 2003. Numbers of wintering waterbirds in Great Britain, 1994/1995–1998/1999: I. Wildfowl and selected waterbirds. Biological Conservation 111:91–104. Google Scholar


J. S. Kirby, J. S. Holmes, and R. M. Sellers . 1996. Cormorants Phalacrocorax carbo as fish predators: An appraisal of their conservation and management in Great Britain. Biological Conservation 75:191–199. Google Scholar


J. S. Kirby, J. S. Holmes, and R. M. Sellers . 1997. Conservation and management of Cormorants Phalacrocorax carbo in Great Britain: the current situation. Ekologia Polska 45:295–302. Google Scholar


A. Koed, H. Baktoft, and B. D. Bak . 2006. Causes of mortality of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) smolts in a restored river and its estuary. River Research and Applications 22:69–78. Google Scholar


A. Koed, N. Jepsen, K. Aarestrup, and C. Nielsen . 2002. Initial mortality of radio-tagged Atlantic salmon (Salmo salar L.) smolts following release downstream of a hydropower station. Hydrobiologia 483:31–37. Google Scholar


P. Lack 1986. The Atlas of Wintering Birds in Britain and Ireland. T & A.D. Poyser. Calton. pp. Google Scholar


F. Last 2000. Predation of migratory salmonids. Atlantic Salmon Trust Workshop. Edinburgh. pp. Google Scholar


J. M. Lekuona and F. Campos . 1997. Foraging ecology of Cormorants (Phalacrocorax carbo) wintering in northern Spain. Folia Zoologica 46:243–252. Google Scholar


P. S. Levin and N. Tolimieri . 2001. Differences in the impacts of dams on the dynamics of salmon populations. Animal Conservation 4:291–299. Google Scholar


M. Marquiss 1998. Methods of control of sawbill duck predation. In M. Marquiss, D. N. Carrs, J. D. Armstrong, and R. Gardiner , editors. (Eds.). Fish Eating Birds and Salmonids in Scotland Report on fish-eating birds research (1990–97), to. The Scottish Office Agriculture, Environment and Fisheries Department. Available at: Scholar


M. Marquiss and D. N. Carss . 1994a. Avian piscivores: Basis for policy. A report produced under contract by the Institute of Terrestrial Ecology for the National Rivers Authority. R & D Project Record 461/8/N&Y. National Rivers Authority. pp. Google Scholar


M. Marquiss and D. N. Carss . 1994b. Fish-eating birds: assessing their impact on freshwater fisheries. R & D Report No. 15. National Rivers Authority. pp. Google Scholar


M. Marquiss and D. N. Carss . 1998. The diet of sawbill ducks and cormorants in Scotland. In M. Marquiss, D. N. Carrs, J. D. Armstrong, and R. Gardiner , editors. (Eds.). Fish Eating Birds and Salmonids in Scotland Report on fish-eating birds research (1990–97), to. The Scottish Office Agriculture, Environment and Fisheries Department. Available at: Scholar


M. Marquiss, D. N. Carrs, J. D. Armstrong, and R. Gardiner . (Eds.). 1998. Fish Eating Birds and Salmonids in Scotland. Report on fish-eating birds research (1990–97), to. The Scottish Office Agriculture, Environment and Fisheries Department. Available at: Scholar


M. Marquiss, M. J. Feltham, and K. Duncan . 1991. Sawbill ducks and salmon. Fisheries Research Services Report No. 18/9. Scottish Office Agriculture and Fisheries Department. Pitlochry. pp. Google Scholar


R. G. Mathers, M. De Carlos, K. Crowley, and D. Ó Teangana . 2002. A review of the potential effect of Irish hydroelectric installations on Atlantic salmon (Salmo salar) populations, with particular reference to the River Erne. Biology and Environment: Proceedings of the Royal Irish Academy 102B:269–79. Google Scholar


R. A. Mavor, M. Parsons, M. Heubeck, and S. Schmitt . 2006. Seabird numbers and breeding success in Britain and Ireland, 2005. UK Nature Conservation No. 30. Joint Nature Conservation Committee. Peterborough. pp. Available at: Scholar


P. McGinnity, C. Stone, J. B. Taggart, D. Cooke, D. Cotter, R. Hynes, C. McCamley, T. Cross, and A. Ferguson . 1997. Genetic impact of escaped farmed Atlantic salmon (Salmo salar L.) on native populations: use of DNA profiling to assess freshwater performance of wild, farmed, and hybrid progeny in a natural river environment. Ices Journal of Marine Science 54:998–1008. Google Scholar


J. McIntosh 1978. Distribution and food of the cormorant on the lower reaches of the River Tweed. Fish Management 9:107–113. Google Scholar


S. J. Middlemas and J. D. Armstrong . 2002. Reconstructing the lengths of juvenile Atlantic salmon from atlas bones: estimating the parr to smolt ratio from regression analyses revisited. Journal of Fish Biology 60:134–138. Google Scholar


N. J. Milner, J. M. Elliott, J. D. Armstrong, R. Gardiner, J. S. Welton, and M. Ladle . 2003. The natural control of salmon and trout populations in streams. Fisheries Research 62:111–125. Google Scholar


P. I. Mitchell, S. F. Newton, N. Ratcliffe, and T. E. Dunn . 2004. Seabird Populations of Britain and Ireland. T. & A.D. Poyser. London. pp. Google Scholar


B. R. S. Morrison 1988. The use of rotenone in fisheries management Scottish Fisheries Information Pamphlet No. 15. Department of Agriculture and Fisheries for Scotland. pp. Available at: Scholar


I. Newton 1998. Population limitation in birds. Academic Press. London. pp.  Google Scholar


R. Noordhuis, E. C. L. Marteijn, R. Noordhuis, S. Dirksen, and T. J. Boudewijn . 1997. The trophic role of cormorants Phalacrocorax carbo in freshwater ecosystems in the Netherlands during the non-breeding season. Ekologia Polska 45:249–262. Google Scholar


K. J. Park, J. R. Calladine, K. E. Graham, C. M. Stephenson, and C. V. Wernham . 2005. The impacts of predatory birds on waders, songbirds, gamebirds and fisheries interests. A report to Scotland's Moorland Forum. Research contract FO4AC20. pp. Available at: Scholar


G. J. Pierce, P. R. Boyle, and J. S. W. Diack . 1991. Digestive-tract contents of seals in Scottish waters - comparison of samples from salmon nets and elsewhere. Journal of Zoology 225:670–676. Google Scholar


N. J. Quick, S. J. Middlemas, and J. D. Armstrong . 2004. A survey of antipredator controls at marine salmon farms in Scotland. Aquaculture 230:169–180. Google Scholar


A. Radford, G. Riddington, J. Anderson, and H. Gibson . 2004. The economic impact of game and coarse angling in Scotland. Report commissioned by the Scottish Executive Environment and Rural Affairs Department. pp. Google Scholar


M. M. Rehfisch, C. V. Wernham, and J. H. Marchant . 1999. Population, distribution, movements and survival of fish-eating birds in Great Britain. DETR. London. pp. Google Scholar


G. Riddington, A. Radford, J. Anderson, and P. Higgins . 2004. An assessment of the economic impact of water-related recreation and tourism in the Spey catchment in 2003. Glasgow Caledonian University. Research Report No. IH02035ST. pp. Available at: Scholar


I. C. Russell, P. J. Dare, D. R. Eaton, and J. D. Armstrong . 1996. Assessment of the problem of fish-eating birds in inland fisheries in England and Wales. Ministry of Agriculture, Fisheries and Food. London. pp. Google Scholar


Scottish Executive Rural Affairs Department (SEERAD) 2001a. Protecting and promoting Scotland's freshwater fish and fisheries: a review. Available at: Scholar


Scottish Executive Rural Affairs Department (SEERAD) 2001b. Protecting and promoting Scotland's freshwater fish and fisheries - summary of responses. pp. Available at: Scholar


W. M. Shearer, R. M. Cook, D. A. Dunkley, J. C. MacLean, and R. G. J. Shelton . 1987. A model to assess the effect of predation by sawbill ducks on the salmon stock of the river North Esk. Scottish Fisheries Research Report 37. HMSO Edinburgh. pp. Available at: Scholar


M. E. Solomon 1949. The natural control of animal populations. Journal of Animal Ecology 18:1–35. Google Scholar


Special Committee on Seals (SCOS) 2005. Scientific advice on matters related to the management of seal populations: 2005. pp. Available at: Scholar


S. O. Stefansson, P. McGinnity, B. T. Bjornsson, C. B. Schreck, and S. D. McCormick . 2003. The importance of smolt development to salmon conservation, culture, and management: perspectives from the 6th International Workshop on Salmonid Smoltification. Aquaculture 222:1–14. Google Scholar


J. Steinmetz, S. L. Kohler, and D. A. Soluk . 2003. Birds are overlooked top predators in aquatic food webs. Ecology 84:1324–1328. Google Scholar


D. C. Stewart, S. J. Middlemas, W. R. Gardiner, S. Mackay, and J. D. Armstrong . 2005. Diet and prey selection of cormorants (Phalacrocorax carbo) at Loch Leven, a major stocked trout fishery. Journal of Zoology (London) 267:191–201. Google Scholar


E. Swansburg, G. Chaput, D. Moore, D. Caissie, and N. El-Jabi . 2002. Size variability of juvenile Atlantic salmon: links to environmental conditions. Journal of Fish Biology 61:661–683. Google Scholar


V. M. Thom 1986. Birds in Scotland. T. & A.D. Poyser. Calton. pp. Google Scholar


C. V. Wernham, M. P. Toms, J. H. Marchant, J. A. Clark, G. M. Siriwardena, and S. R. Baillie . (Eds). 2002. The Migration Atlas: movements of the birds of Britain and Ireland. T. & A.D. Poyser. London. pp.  Google Scholar


B. R. Wilson, M. J. Feltham, J. M. Davies, T. Holden, J. R. Britton, J. P. Harvey, and I. G. Cowx . 2003. Increasing confidence in impact estimates - the Monte Carlo approach. In T. M. Keller and D. N. Carss , editors. (Eds.). Cormorants: Ecology and Management at the Start of the 21st Century, Freising, Germany. Blackwell Publishing. Oxford. pp. 375–387. Google Scholar


L. R. Wires, D. N. Carss, F. J. Cuthbert, and J. J. Hatch . 2003. Transcontinental connections in relation to Cormorant-fisheries conflicts: perceptions and realities of a “bete noire” (black beast) on both sides of the Atlantic. In T. M. Keller and D. N. Carss , editors. (Eds.). Cormorants: Ecology and Management at the Start of the 21st Century. Freising, Germany Blackwell Publishing. Oxford. pp. 389–397. Google Scholar


G. A. Wright 2003. Impact of cormorants on the Loch Leven trout fishery and the effectiveness of shooting as mitigation. In I. G. Cowx , editor. (Ed.). Interactions between fish and birds: implications for management. Blackwell Science. Oxford. pp. 288–297. Google Scholar


P. Yodzis 2001. Must top predators be culled for the sake of fisheries. Trends in Ecology and Evolution 16:278–84. Google Scholar


A. F. Youngson, J. C. MacLean, and R. J. Fryer . 2002. Rod catch trends for early-running MSW salmon in Scottish rivers (1952–1997): divergence among stock components. ICES Journal of Marine Science 59:836–849. Google Scholar
Catriona M. Harris, John R. Calladine, Chris V. Wernham, and Kirsty J. Park "Impacts of piscivorous birds on salmonid populations and game fisheries in Scotland: a review," Wildlife Biology 14(4), 395-411, (1 December 2008).
Received: 26 October 2007; Accepted: 1 May 2008; Published: 1 December 2008

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