Although the main theoretical framework determining how to exploit populations was derived almost 50 years ago, overexploitation is common. I review 10 major concepts underlying the regulation of exploitation: population increase can be exploited; density dependence is essential; quantifying density dependence is exceedingly difficult; sustainable exploitation involves reducing population size; population growth rate is usually mismeasured; sustainability has many conflicting definitions and the choice depends upon the objectives; it is better to monitor the population than the harvest; quotas are unstable; increasing effort is simple, reducing it is painful; exploit conservatively. I then give a brief account of each of the nine main methods that are used to determine sustainable exploitation and the uses, advantages and limitations of each. The nine techniques are: surplus production models, yield per recruit models, Robinson and Redford model, linking yield to recruitment and mortality, adjusting to population changes, comparing demography across sites, reducing to a fixed fraction of unexploited population size, full population models and adaptive management.

The sustainability of exploitation is based on density-dependent renewal of populations: when population density decreases as some individuals are taken, the remaining individuals compensate by surviving or reproducing better. In general there is a trade-off between two desired outcomes: a high yield and a high remaining population size. A hunting strategy is Pareto optimal if it balances this trade-off without wasting possibilities of improving the performance in either aspect. Lack of knowledge concerning the age structure, mating system or density dependence operating in a population will very easily cause suboptimality in this sense, whereas utilising knowledge of density dependence may, in some cases, even overcome the conflict between the goals, so that harvesting can increase rather than decrease population sizes. Suboptimal timing of harvesting is an example which not only causes unnecessary harm to a population, but also hampers estimation of the compensatory or additive nature of mortality. A bias towards additivity will be found if hunting and natural mortality overlap in time, and even ‘superadditive’ results are possible. A mortality pattern that appears additive cannot, therefore, be used to deduce that overwinter survival is density independent. These results have consequences to harvest planning. Adjusting the length of the open season is a tool frequently used to regulate the harvest. Since estimated slopes of compensation cannot be assumed to remain constant if the timing of the open season is changed, the effect of a prolonged season will be more drastic than a mere change in kill rates would predict. Such factors are likely to have the strongest effects in species with long harvest seasons, such as many migratory European waterfowl.

Virtual population analysis (cohort analysis) was used to reconstruct past dynamics of a harvested population of martens Martes americana in the Bracebridge District of southern Ontario. Harvests in the Bracebridge District were managed using a quota system set by regional authorities. Quotas changed from year to year, apparently on the basis of past trapping success and variation in the proportion of young-of-the-year among harvested animals. The proportion of young in the harvest was a sensitive indicator of the annual rate of increase, whereas trapping success tended to be linked most strongly, in inverse fashion, with marten harvesting quotas. The proportion of martens harvested each year was constant, averaging 34%, despite 3-fold variation in marten abundance. This proportion was very close to the maximum sustainable yield (36%) for the population, suggesting that the management policy in the administrative unit was effective in the past in sustaining the source population as well as yielding high trapping returns. Monte Carlo simulation showed that proportionate harvesting, such as the policy in the Bracebridge District during 1972–1991, should be considerably less likely to lead to overharvesting than a constant quota policy, particularly at high average yields.

An array of models for sustainable harvesting has been proposed, and most are in some way dependent on a density-dependent response. I tested a subset of these models by manipulating three North American deer populations to reveal density dependence and determine population response to harvests. These were a white-tailed deer Odocoileus virginianus population on the George Reserve (GR) in southeastern Michigan, and black-tailed deer O. hemionus columbianus populations on Hopland Research and Extension Center (HREC) in Mendocino County, California, and on Fort Hunter Liggett (FHL) in Monterey County, California. The study areas represented a gradation in size, productivity, management control, and environmental stochasticity. The GR population showed a nearly linear density-dependent relationship of r on N. A mean maximum sustainable yield (MS Y) of 49 deer/year was obtained at an N of 56% of K carrying capacity (KCC). A fixed harvest with stochasticity, however, reduced the sustainable MSY to 43 deer/year. A second population growth experiment 50 years after the initial introduction showed an equivalent growth rate. Analysis of harvest data from FHL showed buck harvest to be positively related to size of previous female harvests. From these results, a ‘linked-sex harvest strategy’ (LSHS) was proposed in which female harvest is sequentially incremented so long as buck harvest continues to increase, up to a presumed ‘safe’ female to buck ratio. At HREC, bucks were harvested in public bucks-only seasons. Buck harvest was monitored for 6-year pre- and posttreatment periods without female removals, and for a 7-year treatment period during which 20 females/year were removed for three years and 30 females/year for four years. There was no significant difference in pre- and post-treatment period buck harvests, so they were combined as a ‘control’. There was a significant (25%) increase in buck harvest during the treatment period despite its coinciding with six consecutive years of drought. The combined-sex harvest was more than double that of the buck harvest alone during the control periods. The relevance of these studies to deer harvest management is discussed.

We analysed sex- and age-specific harvesting strategies of moose using an age-structured population model that includes density dependence as well as environmental stochasticity. In order to find the strategy that maximises the mean annual yield we simulated the process over a large number of years. The mean annual yield is a function of the three parameters (number of harvested individuals of calves, adult (≥ ½ years old) bulls and adult females) that are involved in the definitions of the strategies. We compare, by numerical maximisation of a function in several variables, two harvest strategies: proportional harvesting, i.e. removal of a certain proportion of individuals in a given age-and sex-class, and threshold harvesting, i.e. all individuals of a given sex- and age-class are harvested when the size of this subpopulation exceeds a certain threshold. In general, proportional harvest gives a smaller mean annual yield than threshold harvesting. The variance in the annual yield is, however, larger for threshold than for proportional harvesting. These differences between the two harvest strategies increase when the annual survival of calves is low, when there is high environmental stochasticity and when there is strong density regulation operating on survival. For both harvest strategies, the optimal harvest strategy involves high harvest of calves and adult bulls, whereas adult females should hardly be harvested.

The moose Alces alces population in Finland has been managed for sustained harvest since 1970 by regulating annual hunting quotas. However, against all expectations the population size declined in the 1990s. An ecological risk analysis approach was used to build a growth model with annual harvest for the moose population. In the model there is stochasticity in the parameters representing population dynamics. We shall address: 1) whether the population decline could be due to a mismatch between harvest and anticipated population growth rate, and 2) to what extent hunting the moose population down to a much lower target size succeeds. A central element in this is the assumption that the estimate of the pre-hunting population size errs. First, the probability of a population decline due to hunting increases from values close to 0% up to 100% in a very narrow range (15–25%) of harvest rates. Even with high birth rates the risk of a population decline was substantial when the hunting rate exceeded 25% for cows and 37.5% for calves and bulls. The 1974–1994 moose harvest rate was, on average, ca 45% of the population size in autumn. The high rate suggests that the harvest might have been too intense in that period to keep the population stable. Second, we set the target to reduce the moose population drastically (to say 50% of the existing population size). Assuming that the estimates of the population size may err, our analysis shows that the achieved population size after the severe harvest is far below the size we aimed at.

The optimal harvesting strategies for unstable populations are explored using first discrete time models and second a continuous time model specifically applied to the destabilising effects of the caecal nematode Trichostrongylus tenuis on the dynamics of red grouse Lagopus lagopus scoticus. In discrete time models, with overcompensation generating either cyclic or chaotic fluctuations in abundance harvesting can act as both a stabilising and a destabilising process. Maximum yields occur at the harvesting rate that coincides with the point where the harvesting stabilises the overcompensation. Optimal harvesting rates increase with the degree of overcompensation although these are more vulnerable to overharvesting. Harvesting in the continuous time model provides similar results, although observed hunting records do not appear to be stabilised by harvesting. Empirical data on the mortality caused by other natural enemies of red grouse, the hen harrier Circus cyaenus and the louping ill virus, show that these mortalities do stabilise grouse dynamics. One explanation is that both hen harriers and louping ill virus cause significant mortality to chicks before the infective stages of T. tenuis are laid down on the ground, whereas shooting takes place after the infective stages are laid down and thus do not stabilise the populations.

Spatial structure has a paramount influence on population dynamics. This has until recently been neglected in harvesting theory. In this paper, we demonstrate how source-sink and habitat selection theory can provide guidance for harvesting spatially structured populations. We also show how harvesting can affect the spatial distribution of the exploited resource, which has consequences for the design of protected areas. This implicit treatment of space is complemented by a spatially explicit predator-prey model. It turns out that harvesting of the prey and/or the predator species in one patch in space sometimes has effects on the other species outside the harvested patch. We stress the importance of considering how realistic the representation of the spatial dimension has to be in population management.

Willow ptarmigan Lagopus lagopus is considered a popular small game species by many hunters in Scandinavia. A simple harvest strategy would be to prohibit harvest in parts of the total area. We used a spatial model of a fluctuating population of willow ptarmigan, divided into 25 subareas to investigate the possible advantages of buffer zones in managing harvest. We let the breeding success be the source of annual environmental stochasticity but without any spatial variation. Survival was assumed to be density dependent over the total area, whereas dispersal was modelled as density independent. We then compared four major scenarios in which we let dispersal and harvest vary. About 75% of the area could be left open to hunting even if the level of harvest was close to the extinction level if executed in all grids. This harvest strategy would be particularly advantageous if the goal is to provide as many hunting opportunities as possible, rather than to harvest a maximum sustainable yield. Furthermore, it is quite simple and does not need a resource-demanding control system. We believe that a harvest strategy which sets aside a part of the area as a buffer, and places a limit to the harvest effort in the grids that are open for hunting, would be a cost-efficient system with only a small risk of overharvesting.

Since 1994, goose shooting in Denmark has only been allowed from 1½ hours before sunrise to 10 a.m. (since 1997 until 11 a.m.). The aim of the diurnal regulation was to provide autumn-staging and wintering geese with more undisturbed feeding opportunities, and hence to extend the length of their stay in Danish haunts. A field study was carried out during 1994–1997 to investigate the effects of the regulation on the behaviour and site use by geese, focused on greylag geese Anser anser and pink-footed geese Anser brachyrhynchus at three important Danish sites. Data from earlier studies and monitoring schemes provided baseline information. In one study area with low shooting intensity, greylag geese did not change the timing of their morning departure from the roost to the feeding areas. In two sites with higher shooting intensities, they gradually delayed their morning departure from the roosts over the years. In the two sites with intensive shooting, greylag geese redistributed themselves during the daytime, albeit in small numbers. In the site with low shooting intensity, greylag geese depleted the waste grain resources, the preferred food. In the two sites with higher shooting intensities, the geese left while food was still plentiful. Pink-footed geese did not change their roost flight departure and only marginally redistributed themselves during the daytime. In sites where shooting-free areas were established, numbers of greylag and pink-footed geese immediately increased. The weak reaction by the geese to diurnal regulation was not due to a lack of behavioural flexibility in response, but reflected the fact that staying and adjusting to the diurnal regulation was a less attractive option than moving on to less disturbed sites. In conclusion, the diurnal shooting regulation did not achieve the intended management objectives.

Waterfowl management in the United States is one of the more visible conservation success stories in the United States. It is authorized and supported by appropriate legislative authorities, based on large-scale monitoring programs, and widely accepted by the public. The process is one of only a limited number of large-scale examples of effective collaboration between research and management, integrating scientific information with management in a coherent framework for regulatory decision-making. However, harvest management continues to face some serious technical problems, many of which focus on sequential identification of the resource system in a context of optimal decision-making. The objective of this paper is to provide a theoretical foundation of adaptive harvest management, the approach currently in use in the United States for regulatory decision-making. We lay out the legal and institutional framework for adaptive harvest management and provide a formal description of regulatory decision-making in terms of adaptive optimization. We discuss some technical and institutional challenges in applying adaptive harvest management and focus specifically on methods of estimating resource states for linear resource systems.

Hunting Eurasian beaver Castor fiber with firearms during late spring is the dominating harvest form in Norway but may violate the Norwegian wildlife management principle of not hunting during the breeding season. In particular, shooting mothers from newborn young would be considered cruel. As beaver cannot be sexed or effectively aged under spring hunting conditions, selective harvesting at this time is impossible. We examined 32 pregnant beaver shot between 27 March and 12 May 1997–1999 in southeast Norway. No post-parturition females were shot, despite a 15-day extension of the normal hunting season to 15 May. A regression model predicted a mean birth date for the population of 13 May, with most births between 7 and 18 May and few before 30 April. Post-parturition females were not shot primarily because most births occur after hunting has ceased. Additionally, reduced activity of mothers outside the lodge may limit exposure to hunters. Terminating hunting a month earlier would eliminate the shooting of pregnant females with well-developed foetuses. However, as few watersheds in Norway are ice-free before mid-April, and most beaver are presently bagged in late April, this would likely result in a major reduction in beaver harvests and an increase in damage complaints.