This article summarizes catch data for sharks collected by fishery observers during two periods (1995–2000 and 2004–2006) in the Hawaii-based pelagic longline fishery, which targets swordfish Xiphias gladius in the shallow-set sector and bigeye tuna Thunnus obesus in the deep-set sector. The blue shark Prionace glauca was the predominant shark species caught throughout the study period (84.5% of all sharks). Five other species (bigeye thresher Alopias superciliosus, oceanic whitetip shark Carcharhinus longimanus, shortfin mako Isurus oxyrinchus, silky shark C. falciformis, and crocodile shark Pseudocarcharias kamoharai) were relatively common (1.0–4.1%). Two major developments affected shark catches in this fishery during the study period. The first was the prohibition in 2000 of shark finning under most circumstances. The second development was that management measures were taken in 2000 and 2001 to protect sea turtles (leatherback sea turtles Dermochelys coriacea and loggerhead sea turtles Caretta caretta) and these measures included a closure of the shallow-set (swordfish-targeting) sector for more than 3 years. The closure caused decreases in shark catches because the shallow-set sector was typically characterized by high catch rates. The shallow-set sector was reopened in 2004. Comparisons of nominal catch per unit effort (number of sharks/1,000 hooks) revealed significant differences in catch rates between the two fishery sectors and the two periods. Blue shark and shortfin mako catch rates were significantly greater in the shallow-set sector than in the deep-set sector of the fishery, whereas the opposite was true for the deeper-dwelling bigeye threshers and crocodile sharks. Catch rates for the blue shark, oceanic whitetip shark, bigeye thresher, and crocodile shark were significantly lower in 2004–2006 than in 1995–2000. For the blue shark in particular, the combination of reduced catch rates, the finning ban, and an apparent capacity to resist the stress of capture on longline gear resulted in low (4%–5.7%) minimum mortality estimates. Therefore, we conclude that the Hawaii-based pelagic longline fishery has made substantial progress in reducing shark mortality.
Sharks are of considerable interest from at least three important scientific or practical perspectives. First, many aspects of the biology of these fishes have not yet been studied in detail (Carrier et al. 2004; Grogan and Lund 2004). Second, they are ecologically important as predators in pelagic food webs (Kitchell et al. 2002; Schindler et al. 2002; Compagno 2008). Third, sharks comprise much of the nontarget catch in many commercial fisheries (Beerkircher et al. 2004; Gilman 2007a; Erickson and Berkeley 2008; Pikitch et al. 2008) and are vulnerable to overfishing because their typical life history traits include slow growth, relatively late maturation, and low fecundity (Smith et al. 1998; Cortés 2004). Moreover, because most sharks are of low value, they may be under-reported or not reported at all in logbooks (Walsh et al. 2002; Nakano and Clarke 2006), which can increase the difficulty of discerning population trends and can introduce uncertainty into stock assessment models.
The Hawaii-based pelagic longline fishery, which targets bigeye tuna Thunnus obesus in the deep-set sector and swordfish Xiphias gladius in the shallow-set sector, takes sharks, especially blue sharks Prionace glauca, in substantial numbers (Kleiber et al. 2001; Walsh et al. 2002; Dalzell et al. 2008). Unlike most other fisheries that take sharks, the Hawaii-based longline fishery is very well suited to analyses of shark catches because detailed operational and catch data are gathered by the Pacific Islands Regional Observer Program (PIROP). This program was established as the Hawaii Longline Observer Program in March 1994 for the purpose of monitoring interactions between fishing vessels and protected or endangered sea turtles (leatherback sea turtles Dermochelys coriacea and loggerhead sea turtles Caretta caretta) (DiNardo 1993). At the present time, PIROP observers monitor interactions with protected or endangered species, record a large suite of operational details (e.g., position, number of hooks, set and haul times, target species, bait type), obtain species-specific tallies of the catch, including the animal's condition on retrieval (i.e., live or dead) and subsequent disposition (i.e., retained or discarded), and measure fish (Pacific Islands Regional Office 2006). The PIROP is now the largest pelagic observer program for longline fisheries in the Pacific Ocean, representing at least 59% of such observer effort (P. Williams, Secretariat of the Pacific Community, personal communication).
This article provides a detailed quantitative description and analysis of catch data for sharks from the Hawaii-based pelagic longline fishery as reported by PIROP observers from January 1995 to December 2006. These detailed catch and operational data are particularly important to the interpretation of trends because management decisions taken by the Western Pacific Regional Fishery Management Council (one of eight regional fishery management councils established in 1976 by the Magnuson-Stevens Fishery Conservation and Management Act, also known as the Magnuson-Stevens Act [MSA]) during this period probably affected shark catches in several ways. Specifically, regulations promulgated in 2001 in response to high interaction rates between the shallow-set sector and leatherback sea turtles and loggerhead sea turtles led to a closure of the swordfish fishery for more than 3 years. The closure caused the Hawaii-based longline fleet to begin targeting bigeye tuna almost exclusively by using deep-set gear (Walsh et al. 2005). This entailed a southward shift in effort away from the main swordfish and blue shark habitat in temperate waters toward tropical and semitropical regions, where other shark species (e.g., oceanic whitetip shark Carcharhinus longimanus) might be expected to be more common than the blue shark (Nakano and Seki 2003; Bonfil et al. 2008). The swordfish sector was reopened in 2004 but with operational requirements intended to protect sea turtles (e.g., 100% observer coverage; use of circle hooks and thawed, dyed bait); some of these requirements may also have affected catches of sharks and swordfish. Finally, Hawaii Revised Statute 188-40.5 and the federal Shark Finning Prohibition Act (U.S. Public Law 106-557), both enacted in 2000, affected the disposition of shark catches by prohibiting finning in most circumstances unless the carcass was retained.
The dual objectives of this article are to provide quantitative information needed for management of sharks in the Hawaii-based longline fishery and other pelagic fisheries and to contribute to the fundamental knowledge of these fishes. We met these goals by presenting results that include a description of the shark catch composition, species-specific catch statistics (e.g., nominal catch rates and minimum mortality estimates), and biological and distributional information, including the catches of sharks and target species in 2005. Moreover, these data were collected in a fishery that operates throughout a vast region from near the equator to the North Pacific transition zone in pelagic habitats and near seamounts or insular areas; the fishery uses two distinct types of operational techniques according to the species that is targeted. These results are also directly relevant to efforts to reduce the mortality of sharks taken as nontarget species by U.S. pelagic longline fisheries (Beerkircher et al. 2004) as mandated by the MSA (“bycatch” is defined herein according to the MSA as sharks that are caught but not retained for sale or consumption; “incidental catch” follows prior usage [Walsh et al. 2002] as sharks retained for sale or processing, including finning). This article summarizes progress attained by the Hawaii-based longline fishery relative to the MSA mandate from January 1995 through December 2006.
Methods
Data source and observer effort
Catch data were gathered by PIROP observers on 26,507 longline sets during 2,121 commercial fishing trips from January 1995 through December 2006. Observer coverage rates were computed on a fleetwide basis and by set types defined according to gear configuration (Department of Commerce 2004).
Deep sets used either a monofilament mainline and at least 15 hooks/float or basket gear with any number of hooks per float (one vessel); shallow sets used a monofilament mainline and less than 15 hooks/float. In addition, the most commonly used bait on deep sets (47% of all deep sets) was sauries Cololabis spp., whereas squid Illex spp. were used as bait on 90% of shallow sets in 1995 to 1999 and mackerel-like (Scombridae; Clupeidae; Carangidae) fish were used on 93% of shallow sets after 2004. The mean time at which sets began also differed: 0752 hours for deep sets and 1837 hours for shallow sets. The median depth of the deepest hook on 266 deep sets was 248 m, whereas that on 333 shallow sets was 60 m (Bigelow et al. 2006).
The initial quality control of the catch data was conducted by PIROP. The data were first entered and checked by the observer and then re-checked by a debriefer. Logical and numerical tests were computed and a final data examination was performed by a third individual. Additional detailed evaluations based on observer notes, photographs, and published distributional accounts, which focused on species identifications and catch sizes, were later conducted at the Pacific Islands Fisheries Science Center (PIFSC). All catch statistics were computed with these corrected data. Detailed descriptions of the use of observer data are presented by Walsh et al. (2002, 2005, 2007).
Categorization of shark condition and collection of size measurements
Observers categorized sharks as live or dead at the time of release. Any responsiveness led to categorization as alive.
Observers measured the fork lengths (FLs; cm) of sharks after they were caught and brought aboard fishing vessels; FL data were collected according to the PIROP observer field manual (Pacific Islands Regional Office 2006). From January 1995 through January 2006, the protocol called for as many intact sharks, tuna, swordfish, and istiophorid billfish to be measured as possible, subject to time and safety constraints. As of February 2006, every third fish brought aboard was measured, regardless of species.
Statistical methods
Descriptive statistics on species composition, catch per set, nominal catch per unit effort (CPUE; i.e., number of sharks/1,000 hooks), and catch frequencies were initially tabulated for all species from all years. Statistics for those shark species that constituted at least 1% of the total shark catch were next tabulated by set type (shallow-set and deep-set sectors) and by two time intervals (1995–2000 and 2004–2006) representing the periods before the closure and after the reopening of the shallow-set sector. (The duration of shallow-set activity varied each year after the reopening. In 2004, shallow-set activity began in June and continued through December. This sector remained open throughout 2005. It was closed after March 2006 for the remainder of the year because the fleet reached the limit on incidental takes of loggerhead sea turtles. The effects of set type and time period on nominal CPUE of six species were tested by two-way analysis of variance (ANOVA) with interaction by using loge transformed annual means as observations. Shark FLs were also log transformed and tested for the main effects (time period, set type, and sex) and the three linear interactions by using three-way ANOVA. Comparisons of means were performed by Bonferroni t-tests. Sex ratios and various other proportions were evaluated with χ2 tests. Relationships between blue shark sizes and latitude were assessed in terms of their correlations. All statistical procedures were conducted in S-PLUS version 6.2.1 (Insightful Corporation; Seattle, Washington). The significance criterion for statistical tests was P < 0.05 or P ≤ 0.05 for the Bonferroni t-tests.
The minimum mortality of these species was estimated as the proportion of sharks caught that were not released alive. Hence, for the 1995–2000 period, the estimates reflect the effects of finning. Another source of mortality throughout the study was retention for sale or consumption (primarily makos and threshers). The tabulated values represent minimum estimates because any responsiveness by the shark led to categorization as alive at release and because sharks could not be monitored for postrelease mortality. No significance tests were attempted because of this uncertainty. Sample sizes for minimum mortality estimates were at least 50 sharks.
Results
Observer Coverage
Levels of observer effort increased greatly during the 12-year study period from an initial rate of 4.7% in 1995 followed by four more years at similar levels (Table 1). Observer effort nearly tripled in 2000 and averaged 24.4% coverage thereafter (2001–2006).
The allocation of observer effort changed markedly during the study period. The initial emphasis was on the shallow-set sector, in keeping with the monitoring objective. The closure led to a near-total allocation of observers (96.7–99.9%) to the deep-set sector during 2002–2004.
The geographic distribution of observer coverage also changed during the study period. Shallow sets were deployed east of 130°W in 1996, 1998, and 2000, but there was no shallow-set activity in these waters during 2004–2006. Deep sets were deployed across 23° of longitude in 1995–2000 but 33° of longitude in 2004–2006.
Catch Composition
Sharks made up 15.6% of the observed catch, with at least one species taken on 95.1% of the observed sets (Table 2). The shark catch included 20 or 21 species from seven families in three orders. Carcharhinus, with seven species, was the most speciose genus. The bignose shark and sandbar shark were combined because of uncertainty about these species identifications. The blue shark ranked third in the observed catch (13.2%) behind bigeye tuna (16.4%) and the longnose lancetfish Alepisaurus ferox (15.9%), a teleost bycatch species.
The blue shark was predominant in the catch (84.5% of all sharks), taken on 90.7% of the observed sets. Five other species (oceanic whitetip shark, silky shark, shortfin mako, crocodile shark, and bigeye thresher) were relatively common (1.0%–4.1%). Most sets that caught sharks (77.2%) took either blue sharks or blue sharks and one other common species. All other sharks constituted less than 1% of the total catch.
Several uncommon species were taken primarily in the peripheral areas of this fishery, and occasionally with relatively large catches. For example, certain requiem sharks (e.g., the gray reef shark and blacktip shark) were caught exclusively in tropical waters (1°S–8°N). Most (53.3%) salmon sharks were taken on five sets at relatively high latitudes (29–34°N).
Figure 1 depicts the catch rates for the common shark species for each sector and time period. In the shallow-set sector (Figure 1A), the pooled CPUE for all sharks in 1995–2000 was 19.9 fish/1,000 hooks (Figure 1A), and blue sharks represented 92.5% of the pooled value. For the shallow-set sector in 1995–2000, the ranking of shark species from highest to lowest CPUE was (1) blue shark, (2) oceanic whitetip shark, (3) shortfin mako, and (4) bigeye thresher. By 2004–2006, the pooled shark CPUE had decreased to 14.5 fish/1,000 hooks, and blue sharks contributed 90.3% of the total. The shortfin mako CPUE was greater than that for the oceanic whitetip shark in 2004–2006, reflecting the 389% increase between periods. Only 3.4% of the shallow sets were deployed south of 20°N, but a change in the species composition was apparent, with 71.5% blue sharks and 17.0% oceanic whitetip sharks.
In the deep-set sector during 1995–2000, the mean blue shark nominal CPUE was 3.753 fish/1,000 hooks, and those of five other common shark species were 0.064–0.272 fish/1,000 hooks (Figure 1B). The CPUE ranking (highest to lowest CPUE) in this sector during 1995–2000 was (1) blue shark, (2) oceanic whitetip shark, (3) bigeye thresher, (4) silky shark, (5) shortfin mako, and (6) crocodile shark. The pooled shark CPUE decreased by 43.7% between the 1995–2000 period and the 2004–2006 period; the mean nominal CPUE values of the individual species decreased by 12.5–77.9%. The CPUE ranking in 2004–2006 was (1) blue shark, (2) bigeye thresher, (3) shortfin mako, (4) oceanic whitetip shark, (5) silky shark, and (6) crocodile shark.
The species composition varied latitudinally within the deep-set sector. Above 20°N, blue sharks and shortfin makos constituted 92.5% of the shark catch. Blue sharks remained predominant between 10°N and 20°N (81.6% of all sharks); the remainder of the shark catch consisted primarily of bigeye threshers (7.7%), oceanic whitetip sharks (3.5%), and crocodile sharks (1.9%). Silky sharks made up the greatest percentage (29.3%) of the shark catch from tropical and insular areas (i.e., south of 7°N).
Effects of Fishery Sectors and Time Periods on Nominal CPUE, Disposition, and Mortality
Nominal CPUE of five species (Table 3) differed significantly between set types, time periods, or both. The nominal CPUE values for the blue shark and the shortfin mako were significantly greater in the shallow-set sector than the deep-set sector (both tests: P < 0.001) and greater during 1995–2000 than during 2004–2006 (blue sharks: P < 0.05; shortfin makos: P < 0.005). The differences between sectors for shortfin makos were 3.25-fold in 1995–2000 and 14.5-fold in 2004–2006. The shortfin mako CPUE also differed significantly (P < 0.01) between the two time periods, reflecting high catch rates in the shallow-set sector during 2004–2006. The nominal CPUE values for both the bigeye thresher and the crocodile shark were significantly greater in the deep-set sector than in the shallow-set sector (bigeye threshers: P < 0.001; crocodile sharks: P < 0.01) and were greater during 1995–2000 than during 2004–2006 (bigeye threshers: P < 0.01; crocodile sharks: P < 0.01). The oceanic whitetip shark nominal CPUE was also significantly greater (P < 0.001) in 1995–2000 than in 2004–2006.
The disposition of sharks (Table 3) varied between sectors and among species in 1995–2000. A significantly greater percentage of blue sharks were finned in the deep-set sector than in the shallow-set sector (χ2 test: P < 0.0001). Both oceanic whitetip sharks and silky sharks were finned at high rates (45–100%). Bigeye threshers and shortfin makos were usually kept for consumption or sale or else released.
Minimum mortality estimates decreased substantially for several species in both fishery sectors after the finning prohibition. The estimates for blue sharks decreased from 51.1% to 5.7% in the shallow-set sector and from 61.9% to 4.0% in the deep-set sector. These minimum estimates included sharks that were released dead. The range for blue sharks (4.0–8.5%) was considerably less than those of the other species, all of which exceeded 20% in either sector or period.
Effects of Fishery Sector, Time Period, and Sex on Shark Sizes
The blue shark mean FLs (Table 4) differed significantly between periods and sexes (both tests: P < 0.001). The mean FL in 1995–2000 (177.9 cm), including both sexes, was significantly greater than the mean FL in 2004–2006 (170.8 cm). The mean FL of males (180.7 cm), including both time periods, was significantly greater than that of females (173.2 cm).
The sizes of blue sharks also exhibited spatial, seasonal, and sexual variation. Blue shark FLs were negatively correlated with latitude in the shallow-set sector above 35°N (r = −0.408, df = 163, P < 0.001). The sex ratio (male : female) from these waters was 6.8:1.0. In temperate waters (20–35°N), 46.0% of the measured blue sharks were longer than 180 cm FL. Bivariate regressions based on data from sharks with measurements of both FL and total length (TL) predicted a mean TL of 215 cm for a female of 180 cm FL (N = 102, R2 = 0.998) and a mean TL of 212 cm for a male of 180 cm FL (N = 74, R2 = 0.906). Most (69.1%) of these large sharks were caught in the first or second quarter during 1996–1999, with a 1.6:1.0 sex ratio. In contrast to the shallow-set sector, the FLs of blue sharks of both sexes caught in the deep-set sector were positively correlated with latitude (females: r = 0.308, df = 1,511, P < 0.001; males: r = 0.093, df = 1,714, P < 0.001). Above 20°N, 58.1% of the blue sharks were longer than 180 cm FL; the sex ratio among these large sharks was 1.9:1.0. South of 20°N, 43.0% of the blue sharks were longer than 180 cm FL, with a 2:1 sex ratio.
Shortfin mako FLs varied significantly between periods, set types, and sexes (three tests: all P < 0.01). Shortfin makos from the deep-set sector were significantly larger than those caught on shallow sets. Shortfin makos caught during 1995–2000 were significantly larger than those caught during 2004–2006. The significant effect of sexes actually reflected an interaction with sectors; females were larger than males in the deep-set sector, whereas males were larger than females in the shallow-set sector.
Bigeye threshers, silky sharks, and crocodile sharks were caught primarily on deep sets, and only five FL measurements of oceanic whitetip sharks were obtained from the shallow-set sector in 2004–2006. Therefore, FLs of these species were not tested for differences between sectors. Male oceanic whitetip sharks were significantly larger than females, and sharks caught in 1995–2000 were significantly larger than those caught in 2004–2006 (both tests: P < 0.01). Silky sharks caught in 1995–2000 were also significantly larger than those caught in 2004–2006 (P < 0.01). The FLs of bigeye threshers caught in 1995–2000, however, were smaller and significantly different from the FLs of those caught in 2004–2006. Crocodile shark FLs did not differ significantly between periods or sexes (both tests: P > 0.50); the pooled mean FL of crocodile sharks was 85.0 cm.
There were three patterns in the sex ratios of these species. Males constituted the majority of the shortfin makos caught by the shallow-set sector in 1995–2000 and 2004–2006 (two χ2 tests: both P < 0.01). Males also dominated the catches of bigeye threshers (two χ2 tests: both P < 0.0001) and crocodile sharks (two χ2 tests: both P < 0.01) from the deep-set sector. Crocodile sharks exhibited the greatest sexual segregation of any species.
Distributions of the Catches of Sharks and Target Species
Figure 2 depicts the distribution and species composition of longline catches in 2005, when both fishery sectors remained open throughout the year. In the shallow-set sector (Figure 2A) during the first quarter, the swordfish CPUE (19.2 fish/1,000 hooks) in the most heavily fished region (30–35°N; 150–160°W) was less than the combined CPUE for the blue shark (18.8 fish/1,000 hooks) and the shortfin mako (1.3 fish/1,000 hooks). A much higher ratio of swordfish CPUE (22.0 fish/1,000 hooks) to blue shark CPUE (9.4 fish/1,000 hooks) was attained to the southwest (25–30°N; 165–170°W). During the second quarter, the swordfish nominal CPUE (16.3 fish/1,000 hooks) was again more than double that for blue sharks (7.7 fish/1,000 hooks). The mean sea surface temperature (SST) on these shallow sets during the first two quarters was 20.9°C. There was very little shallow-set activity in the third quarter; in the fourth quarter (35–40°N; 140–150°W), the swordfish CPUE (12.4 fish/1,000 hooks) was less than the blue shark CPUE (15.7 fish/1,000 hooks). The shortfin mako CPUE reached its annual maximum, 1.4 fish/1,000 hooks, in the fourth quarter.
Substantial catches of bigeye tuna were taken across 10° of latitude and 20° of longitude (15–25°N; 150–170°W) in the deep-set sector (Figure 2B) during the first quarter. The ratio of bigeye tuna CPUE (5.5 fish/1,000 hooks) to blue shark CPUE (1.4 fish/1,000 hooks) was 3.9:1.0. Second-quarter activity was concentrated in two areas. The ratio of bigeye tuna CPUE to blue shark CPUE in the more northerly area (25–30°N; 150–155°W) was 3.6:1.0, versus 1.4:1.0 in the more southerly area (15–20°N; 150–165°W). The largest fraction of the bigeye thresher catch (45.5%) was also taken in the second quarter, primarily within 10–20°N and 155–165°W. Third-quarter activity was concentrated within 20–25°N and 145–160°W. The ratio of bigeye tuna CPUE to blue shark CPUE remained low at 1.6:1.0. During the fourth quarter between 15°N and 20°N and between 155°W and 165°W, more blue sharks were caught than the target species. Other bycatch and incidentally caught species contributed most (73.9%) of the total catch in this sector.
Discussion
Shark Catch Composition
The predominance of the blue shark was expected (Walsh et al. 2002) and consistent with general accounts of its distribution and abundance (Compagno 1988; Nakano and Stevens 2008) and published descriptions of the Hawaii-based longline fishery (He et al. 1997; Gilman 2007b; Dalzell et al. 2008). The observed shark catch in this fishery could still be aptly described by Strasburg's (1958) statement that “the great blue shark is wide ranging throughout the area considered, whereas certain of the other species live within rather narrowly circumscribed limits.” The predominance of the blue shark in this fishery is usually so great that in addition to under-reporting and nonreporting biases (Walsh et al. 2002), logbooks from observed trips are sometimes inaccurate, with all sharks logged as blue sharks when observers report multiple species, apparently because captains are accustomed to the shark catch consisting entirely of blue sharks (W.A.W., unpublished data).
Oceanic whitetip sharks and silky sharks were among the common species, but the percentages of sets with catches of these species were very low. The oceanic whitetip shark is an abundant, epipelagic oceanic species with a circumglobal distribution in tropical waters, usually above 20°C (Bonfil et al. 2008). The silky shark is one of the most common semipelagic sharks in all tropical oceans, usually found at ambient temperatures above 23°C (Bonfil 2008). The mean SST during shallow-set activity in 2005 (20.9°C) indicates that much of the activity of this sector occurred at times and in locales outside the thermal ranges of these species, especially the silky shark. Thus, decreases in nominal CPUE for these species in 2004–2006 in the shallow-set sector appear to reflect to some unknown degree the timing and location of fishing. If so, and if this sector continues to operate primarily in the same general areas during the first half of the year, then catch rates for these tropical carcharhinids will probably remain low.
The Hawaii-based longline fishery catches both species of makos (shortfin mako and longfin mako) and all three species of threshers (bigeye thresher, pelagic thresher, and thresher shark). The shortfin mako catch was 25-fold greater than the longfin mako catch, and the bigeye thresher catch was 11 times greater than the pelagic thresher catch. The individual catches of the pelagic thresher and the thresher shark are uncertain. The occurrence of the thresher shark in Hawaiian waters is enigmatic (Mundy 2005), and some thresher shark identifications from the early years of the PIROP were later deemed to be uncertain. Therefore, these catches were combined, assuming that any misidentifications would only involve the thresher shark and pelagic thresher because the bigeye thresher is so distinctive in appearance.
Most shark species (10 species) represented less than 0.1% of the shark catch. These species were caught occasionally to very rarely and were apparently minimally affected by the longline fishery.
Blue Shark
The blue shark size data and sex ratios appeared to be consistent with hypotheses about the life history and distribution of this species in the North Pacific (Nakano 1994; Nakano and Seki 2003; Nakano and Stevens 2008) in at least one major respect. The highly skewed sex ratio above 35°N and the significant negative correlation between size and latitude support the suggestion that latitudes from 35°N to 40°N are important in the early life history of males.
The relatively large blue sharks (i.e., >180 cm FL) caught from 20°N to 35°N in the first and second quarters during 1996–1999 were probably mature because 200 cm TL is considered the approximate size at maturity for both sexes in the North Pacific (Nakano and Stevens 2008). These catches may have reflected seasonal movements because many of the measured blue sharks were caught in or near the North Pacific Transition Zone (Roden 1991). This would also be consistent with the results of Nakano and Stevens (2008), who described seasonal movements to higher latitudes and into highly productive oceanic convergence or boundary zones.
Effects of Fishery Sectors
The two fishery sectors were characterized by qualitative differences in the species composition of the shark catch and quantitative differences in CPUE, sizes, and sex ratios. The signs of the correlations between size and latitude also differed between sectors for blue sharks, reflecting the preponderance of small males taken above 35°N in the shallow-set sector and possibly reflecting movements of large sharks to feeding areas in the deep-set sector. Such effects can be complex, as in shortfin makos, which exhibited sector-specific size differences between sexes.
Sector-specific effects and their associated complexities may create opportunities for fishery managers. If, for example, it is deemed important to conserve adult female shortfin makos, the focus should be on the deep-set sector. If the intention is to reduce shark bycatch in the aggregate, emphasis should be placed on the shallow-set sector early in the year at high latitudes, where large numbers of small male blue sharks are likely to be caught.
Nominal CPUE of Common Sharks
Nominal CPUE values for five species exhibited significant decreases between 1995–2000 and 2004–2006. Interpretation of the shallow-set results from the 2004–2006 period is complicated by the fact that the changes in hook and bait types were confounded. The months with the greatest activity and the geographic distribution of sets also differed between time periods. The distributional changes in particular would have introduced sampling variation. Nonetheless, it appears that the switch to mackerel-like fish as bait probably contributed to the reduced blue shark catch rates in this sector. In the Atlantic, Watson et al. (2005) employed a two-way experimental design and determined that circle hooks affected catch rates for swordfish and blue sharks positively, whereas Atlantic mackerel Scomber scombrus bait did so negatively. Changes in bait types may also have contributed to the decrease in blue shark nominal CPUE in the deep-set sector. Although not mandatory, use of sauries decreased over time, from 76–100% in 1995–1999 to 48% in 2000 and 49–55% in 2004–2006.
The most serious possible explanation for a decrease in nominal CPUE of one or more species would be population decline(s). The most recent North Pacific blue shark assessment (Kleiber et al. 2009) indicated that the population increased by 6.5% from 1995 to 2002. Because the duration of this study is greater than that of the assessment, however, the latter cannot be used to address the possibility that the blue shark may be or may have been undergoing population decline.
Shortfin makos exhibited a large increase in nominal CPUE between the 1995–2000 period and the 2004–2006 period; this was the only increase observed among the common species. The increase may also have been related to the switch to mackerel-like bait, though in a manner opposite to blue sharks. Stillwell and Kohler (1982) examined gut contents of 399 shortfin makos caught in fishing tournaments and on longlines in the Northwest Atlantic and identified Atlantic mackerel in 2.2% of the samples. The bluefish Pomatomus saltatrix was the only teleost identified from more samples (43.8%). Scott and Scott (1988) described the shortfin mako as feeding mostly on fishes, especially mackerels and other scombrids, in Canadian Atlantic waters. It appears that bait types may have strong, species-specific effects on shark bycatch rates. In practical terms, however, any treatment or technique that reduces the blue shark CPUE significantly in fisheries that catch both species would probably yield a large net reduction in bycatch because shortfin mako catches are usually about 3% to 13% of the blue shark catches (Stevens 2008).
Shark Conservation
The very large reductions in the minimum mortality estimates for 2004–2006 after the finning ban are critically important from the perspective of shark conservation. Because the blue shark was by far the predominant species and a major bycatch species in many high-seas longline fisheries (Nakano and Stevens 2008), it appears that shark mortality from fishing could be reduced considerably if finning prohibitions were adopted elsewhere.
Bycatch mortality in the longline fishery now consists primarily of sharks caught and subsequently released dead. The percentage of sharks that were caught and released dead was very low for blue sharks in 2004–2006 (4.0–5.7%), whereas those for all other common species exceeded 20% in each sector or period. This suggests that blue sharks are much less sensitive to the stress associated with capture by longline gear than the other common shark species.
It must be emphasized that these mortality estimates were minimal because the postrelease fate of sharks could not be monitored, but high survival rates among longline-caught blue sharks have been reported previously. Moyes et al. (2006) estimated that 90% to 95% of all blue sharks and up to 100% of apparently healthy blue sharks could survive capture. Kerstetter and Graves (2006) reported a 7.4% mortality rate for blue sharks caught on longlines with circle hooks in the western North Atlantic. Thus, even if the minimum mortality estimates are low by an order of magnitude, about half of all released blue sharks would be expected to survive.
Sizes of Sharks
Several differences in mean sizes between sectors or periods were statistically significant but probably not biologically important. For example, the mean sizes indicate that about half of all blue sharks of both sexes were mature in both sectors during 1995–2000 and in the deep-set sector during 2004–2006. The relatively small mean sizes of male blue sharks and female shortfin makos during 2004–2006 in the shallow-set sector were influenced by catches in restricted locales during short intervals. It is also likely that decreases in mean sizes reflected sampling bias because there was little incentive to bring large sharks aboard fishing vessels after the finning prohibition. The decreases in oceanic whitetip sharks and silky sharks cannot yet be explained. It would be useful to assess whether changes in the distribution of fishing effort underlie these decreases.
Distributions of Catches of Target Species and Sharks
Distributional information on shark catch may help fishers increase the ratio of target species catch to shark bycatch. Although from only a single year, the 2005 data suggest that shifting the shallow-set operations during the first quarter from the most heavily fished area (30–35°N; 155–160°W) toward the southwest (25–30°N; 165–170°W) might increase this ratio. In the deep-set sector, fishers might wish to remain above 20°N during the third and fourth quarters because blue shark catches exceeded those for bigeye tuna south of this latitude. A second possible use is to permit informed conjecture about how management measures intended for other purposes (e.g., time-area closures to protect endangered species) will affect sharks. Such management measures could cause spillover effects on the distribution of fishing effort, which in turn might influence bycatch rates positively or negatively, depending upon the final location of the redirected effort.
Conclusions
The shark catch in the Hawaii-based pelagic longline fishery was aspeciose, with the blue shark as the predominant species. Management efforts in this fishery can therefore be directed toward blue sharks and a few other common species.
The estimates of minimum mortality for the blue shark were very low in 2004–2006 (4.0–5.7%). The combination of reduced catch rates, the finning ban, and the apparent capacity of this species to resist the stress of capture on longline gear contributed to these low estimates. By reducing mortality of blue sharks in particular, the Hawaii-based pelagic longline fishery has made substantial progress in reducing shark mortality, in keeping with the mandates of the MSA.
All other common sharks exhibited greater sensitivity to the stress of capture or handling than blue sharks. As such, reductions in bycatch mortality attained by finning prohibitions would probably be species specific and, for most species, would be smaller than those attained for blue sharks.
Shark bycatch in the two fishery sectors differed both qualitatively and quantitatively. Higher nominal CPUE values for blue sharks and shortfin makos in the shallow-set sector and for bigeye threshers and crocodile sharks in the deep-set sector indicate that set depth is highly influential on shark catch rates. Deep-setting of longline gear may prove to be an effective bycatch mitigation technique for epipelagic species. The extent to which deep-setting is adopted commercially will probably depend upon whether catch rates for target species can be maintained. Manipulation of bait types and comparison of target species CPUE to bycatch CPUE ratios may also be potentially useful mitigation techniques.
The mean nominal CPUE values for oceanic whitetip sharks and silky sharks were negatively biased, probably to a considerable degree, because these species are not distributed throughout the area exploited by this fishery. Nominal catch rates for the other common species, except blue sharks and possibly shortfin makos, were probably similarly biased and would not accurately reflect relative abundance. Indices of relative abundance could be improved by standardizing CPUE with appropriate predictor variables (e.g., time, latitude, longitude, bait types). We (W.A.W. and K.A.B.) are currently engaged in this research.
Acknowledgments
S. Joseph Arceneaux, Eric Forney, Dawn Golden, and Thomas Swenarton of PIROP graciously assisted with data acquisition and verification. Francine Fiust and Audrey Rivero prepared the originally submitted version of this article, Diosdado Gonzales provided computing assistance, and Kurt Kawamoto and Walter Machado participated in useful discussions of this fishery. Russell Ito, Pierre Kleiber, Joseph O'Malley, Robert Skillman, and Jerry Wetherall of PIFSC and Suzanne Kohin of the Southwest Fisheries Science Center reviewed earlier versions of the article. Three anonymous reviewers provided comments that improved the revised article considerably. This project was funded by Cooperative Agreement NA17RJ1230 between the Joint Institute for Marine and Atmospheric Research and the National Oceanic and Atmospheric Administration (NOAA). The views expressed herein are those of the authors and do not necessarily reflect the views of NOAA or any of its subdivisions.
References
Figure 1.
Catch per unit effort (CPUE; number of sharks/1,000 hooks) plotted on a logarithmic scale for common shark species caught in the (A) shallow-set sector and (B) deep-set sector of the Hawaii-based longline fishery. Solid bars represent mean CPUE values from the period 1995–2000; cross-hatched bars represent mean CPUE values from the period 2004–2006. The percentages represent the changes in CPUE between the two periods.
Figure 2.
Species composition of catches in the (A) shallow-set sector and (B) deep-set sector of the Hawaii-based longline fishery in each quarter of 2005 (5° latitude × 5° longitude squares; nonconfidential data). The sizes of the circles are scaled by the number of fish caught; the slices represent percentages of the catch. “Other” denotes all other bycatch and incidentally caught species (i.e., sharks and teleosts).
Table 1.
Summary of observed effort in the Hawaii-based pelagic longline fishery, January 1995 to December 2006 (the sum of the annual trip totals exceeds the overall total because 61 trips that began in December ended in January; tabulations are based on haul dates).
Table 2.
Summary statistics for observed shark catches in the Hawaii-based longline fishery, January 1995 to December 2006. Catch per unit effort (CPUE) (nominal) is defined as number of fish per 1,000 hooks.
Table 3.
Summary of sector- and period-specific observed catches of common sharks taken by the Hawaii-based longline fishery. Under percent released, percentages of sharks that were released dead are given in parentheses (CPUE = catch per unit effort).
Table 4.
Summary of mean fork lengths (FL) and sex ratios of common sharks caught in the Hawaii-based longline fishery by sector (deep and shallow sets) and time period. Sample sizes are in parentheses.
