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Two types of depredation mitigation devices (DMD) were designed and tested aboard pelagic longline fishing vessels targeting tunas and swordfish off the Seychelles. The DMDs were designed to reduce the damage and removal of fish from the fishing gear by predators such as toothed whales and sharks. The first DMD was termed "spider" and was made up of a 100 mm diameter plastic disc with sixteen holes in its outer range and a 37 mm diameter central hole.  Eight 1200 mm long hanging legs were created by placing four polyester strands into the outer holes and the entire system is triggered by a biting fish.  The second DMD, "sock", was designed to cover the fish to hide it from predators through either a net made up of fiberglass mosquito netting or of propylene fiber net.  Shark depredation was significantly higher throughout the surveys than toothed whale depredation, but toothed whales caused more damaged to fish through depredation events. The sock DMD did not affect the catch rates of targeted species or depredation events by either sharks or toothed whales.  These results for the spider DMD were not presented. Trigger rates for the two DMDs were up to 87.3% for the spider and 69.2% for the sock.  The protection rate was as high as 80% for spider but only 15.5% for the sock. Entanglement rates of the DMDs with the fishing gear were low for the spider (3.6%) and higher for the sock (17.8%).  In addition, the use of these DMDs significantly slowed down the fishing process, and therefore additional work into the design and application of DMDs needs to be explored.

Tests were conducted aboard pelagic longline vessels in the Bay of Bengal to determine if there were differences in the catch composition, catch rate, hooking position or length frequency of target and bycatch species caught with circle and J-hooks.  Circle hooks caught similar percentages of both target (46.7%) and bycatch (53.3%) species, while J-hooks caught a higher percentage of bycatch (74.5%) species.  Catch rates for the target species were higher for circle hooks (2.2 individuals/1,000 hooks) compared to J-hooks (1.9 individuals/1,000 hooks).  In contrast, catch rates for the bycatch species were higher on J-hooks (5.6 individuals/1,000 hooks) than circle hooks (2.6 individuals/1,000 hooks).  For the target species, swordfish (Xiphias gladius) had the highest catch rates for both hook types but catch rates were slightly higher on circle hooks.  Close to three quarters (73.3%) of fish were hooked in the mouth when circle hooks were used while only half (53%) were hooked in the mouth, and 38% in the digestive system, when J-hooks were used.  Swordfish caught with J-hooks were slightly larger (mode of 250-269 cm) than those caught with circle hooks (mode 210-229 cm). 

From 2001 to 2006, 71 pop-up satellite archival tags (PSATs) were deployed on five species of pelagic shark (blue shark [Prionace glauca]; shortfin mako [Isurus oxyrinchus]; silky shark [Carcharhinus falciformis]; oceanic whitetip shark [C. longimanus]; and bigeye thresher [Alopias superciliosus]) in the central Pacific Ocean to determine species-specific movement patterns and survival rates after release from longline fishing gear. Only a single post-release mortality could be unequivocally documented: a male blue shark, which succumbed seven days after release. Meta-analysis of published reports and the current study (n=78 reporting PSATs) indicated that the summary effect of post-release mortality for blue sharks was 15% (95% CI, 8.5–25.1%) and suggested that catch-and-release in longline fisheries can be a viable management tool to protect parental biomass in shark populations. Pelagic sharks displayed species-specific depth and temperature ranges, although with significant individual temporal and spatial variability in vertical movement patterns, which were also punctuated by stochastic events (e.g., El Niño-Southern Oscillation). Pelagic species can be separated into three broad groups based on daytime temperature preferences by using the unweighted pair-group method with arithmetic averaging clustering on a Kolmogorov-Smirnov Dmax distance matrix: 1) epipelagic species (silky and oceanic whitetip sharks), which spent >95% of their time at temperatures within 2°C of sea surface temperature; 2) mesopelagic-I species (blue sharks and shortfin makos, which spent 95% of their time at temperatures from 9.7° to 26.9°C and from 9.4° to 25.0°C, respectively; and 3) mesopelagic-II species (bigeye threshers), which spent 95% of their time at temperatures from 6.7° to 21.2°C. Distinct thermal niche partitioning based on body size and latitude was also evident within epipelagic species.

Sharks, turtles, billfish, and marine mammals are frequently caught accidentally in commercial fisheries. Although conservationists and fisheries managers encourage the release of these nontarget species, the long-term outcome of released animals is uncertain. Using blue sharks (Prionace glauca), we developed a model to predict the long-term survival of released animals based on analysis of small blood samples. About 5% of the sharks were landed in obviously poor condition (lethargic and unresponsive to handling); these moribund sharks were sampled and euthanized. A subset of the remaining sharks was sampled and tagged with pop-up satellite archival tags (PSATs). Each of the PSATs that reported data (11 tags) showed that the sharks roamed at sea for at least 3 weeks post-release. Five variables differentiated moribund sharks from survivors: plasma Mg2þ (moribund, 1.57 6 0.08 mM; survivor, 0.98 6 0.05 mM; P , 0.00001), plasma lactate (moribund, 27.7 6 4.1 mM; survivor, 5.80 6 2.96 mM; P , 0.001), erythrocyte heat shock protein 70 (Hsp70) mRNA (relative levels: moribund, 3.96 6 0.53; survivor, 1.00 6 0.29; P , 0.005), plasma Ca2þ (moribund, 3.70 6 0.14 mM; survivor, 3.13 6 0.11; P , 0.005), and plasma Kþ (moribund, 7.01 6 0.66 mM; survivor, 5.12 6 0.44 mM; P , 0.05). These analyses were used to develop logistic regression models that could ‘‘predict’’ the long-term survival of captured sharks, including a larger group of sharks that we sampled but did not tag. The best logistic model, which incorporated Mg2þ and lactate, successfully categorized 95% of fish of known outcome (19 of 20). These analyses suggest that sharks landed in an apparently healthy condition are likely to survive long term if released (95% survival based on biochemical analyses; 100% based on PSATs). 

Key pelagic shark species are caught in the North Pacific by Japanese longline fisheries as bycatch. These species include blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus). In this study, fishery-independent data was leveraged by utilizing a generalized linear mixed model to standardize these data and identify changes in spatio-temporal catch rates. For blue shark, catch was recorded from temperate waters of the western and central North Pacific. Shortfin mako were distributed in coastal and offshore waters along the Kuroshio-Oyashio transition zone and the Mixed Water Region. The yearly changes in the catch per unit effort (CPUE) of blue shark illustrated a downward trend until 2008 and an upward trend thereafter, whereas those of shortfin mako exhibited a flat trend with large fluctuations from 1994 to 2008, and a down-and-up trend thereafter. Successfully leveraging this otherwise unreliable dataset suggest it can now be utilized to improve the stock assessment and management of these shark species.

A structured literature review of 103 papers was conducted to characterize trends in shark bycatch research.  Gear modifications, such as hook size/type and net size, were the most commonly evaluated technique to reduce shark bycatch.  Early studies tended to focus on trawl fisheries while more current studies focus on longline fisheries, particularly tuna fisheries. Overall, longline fishery studies made up the majority (44.6%) of published papers and typically were aimed at reducing bycatch.  Later studies also tended to explore potential solutions to bycatch issues.  In terms of trawl fisheries, net size and mesh design were most commonly suggested to reduce shark bycatch, while in longline fisheries, hooks size and type were the most common options suggested. The majority of published papers (40%) focus on the North Atlantic Ocean followed by the North Pacific Ocean (14.4%).  The lack of information on post-release survivorship of sharks made it difficult for the authors to quantify the true amount of bycatch or to recommend handling techniques to fishermen.

Large pelagic sharks are caught incidentally in the swordfish and tuna fisheries of the Mediterranean Sea. In this study, twelve shark species were documented as bycatch over three years from 1998 to 2000. Blue shark (Prionace glauca) was the predominant species in all gears and areas examined. Shortfin mako (Isurus oxyrinchus), common thresher shark (Alopias vulpinus), and tope shark (Galeorhinus galeus) were the next most abundant shark species found in more than half of the areas sampled. Catch composition varied both in the areas and gears investigated. Sharks represented 34.3% in weight of total catches sampled in the Alboran Sea and 0.9% in the Straits of Sicily. Higher shark catches were observed in the swordfish (Xiphias gladius) longline fishery, where a nominal CPUE value reached 3.8 sharks/1000 hooks in the Alboran Sea. Size distribution by fishing gear varied significantly. Albacore (Thunnus alalunga) longline catches consisted mainly of juvenile sharks, whereas subadult and adult specimens were more frequent in the swordfish longline and driftnet fishery. The percentage of sharks brought onboard alive was exceptionally high; only 5.1% of the specimens died. Few discards (seven blue shark) were recorded in the Greek longline fleet during onboard sampling in the Aegean Sea.

A study was conducted from 1998 to 2001 on blue shark (Prionace glauca) bycatch in the Italian and Greek surface drift swordfish (Xiphias gladius)  longline fisheries in the Mediterranean Sea to examine whether catches are related to some environmental, spatial, temporal or operational parameters and to what extent. Spatial and temporal factors were the most influential regarding blue shark distribution and abundance, with a considerable interaction between them; the modeled environmental factors were of minor importance. Spatial distribution revealed a strong longitudinal gradient whereby blue shark occurrences increased in an east to west direction, whereas catches by latitude were higher in southern- and northern-most regions. Blue sharks were more frequently encountered in autumn and in distant open waters; however, the likelihood of making a larger catch peaked in late spring–early summer and in the vicinity of land. Catch rates differed significantly depending on the fishing gear configuration. Deeper settings (>20 m), more resilient lines and use of fish attractants increased the probability of P. glauca capture.

During the period 1998-2001, a survey to study the incidence of the drift longline fishery on shark populations was carried out in the eastern Mediterranean Sea. A total of 8 different pelagic shark species were recorded in the catches of the Greek swordfish (Xiphias gladius) and albacore (Thunnus alalunga) longline fisheries. Among them, the blue shark, Prionace glauca, was the most common species in the catches while the bigeyed sixgill shark, Hexanchus nakamurai, was reported for the first time in the area. Shark bycatches were noteworthy in the swordfish fishery reaching a 3.8% in number and 3.6% in biomass of the total catch. The higher abundance indices were observed in the Levantine Basin (CPUE = 0.74 sharks/1,000 hooks). Catches in this area consisted of much larger specimens than those measured in other areas of the Mediterranean Sea.

An analysis is made of the catch, effort and catch per unit effort (CPUE) of the incidental capture of billfish and other species by longline vessels in eastern Venezuela. The information was based on fishing logbooks from 1986 to 2000. The fishing grounds were divided into two sectors according to the different hydrological regimes: the Caribbean Sea (11º17ºN, 61º75W) and the western Atlantic Ocean (5º17ºN, 50º60ºW). The number of vessels in the fleet increased from 9 to 41 units between 1986 and 1996. Tunas were the dominant group in the landings, representing on average 73.2% in the Caribbean Sea and 78.8% in the Atlantic Ocean. Yellowfin tuna (Thunnus albacares) was the most important species, accounting for 67.4% and 56% of the total weight captured in the Caribbean Sea and Atlantic Ocean, respectively. The incidental catch was composed of white marlin (Tetrapturus albidus), sail fish (Istiophorus albicans), blue marlin (Makaira nigricans), swordfish (Xiphias gladius), dolphinfish (Coryphaena hippurus), and sharks (mainly Carcharhinus spp. and Isurus spp.). White marlin and blue marlin presented a similar pattern of variation and a sustained decrease in average yearly CPUE during the study period. For both species, CPUE seasonality was strong and more evident in the Caribbean, with a minimum in June and August. Blue marlin showed the highest yields per unit of effort (1.7 kg/100 hooks/day) in both sectors; dolphinfish showed similar CPUE values in both sectors, while the yields of swordfish were higher in the Caribbean Sea and those of sharks were higher in the Atlantic Ocean. The average annual CPUE of white marlin and several shark species showed significant differences between the Caribbean Sea and Atlantic Ocean sectors. The changes in abundance of the species seem to be associated with feeding migratory movements.