Search The Database

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.

Although counting nesting females and nests has been widely used to assess sea turtle abundance, surveys of in-water populations can provide a more effective means of measuring the success of recovery actions. Here, the bycatch of turtles recorded in observer data from fishing trips in the Pamlico-Albemarle Estuarine Complex, North Carolina (United States), from 1995−1997, 2001−2003, and 2007−2009 was leveraged to identify changes in species composition and overall abundance throughout the three different fishing periods. A significant increase in loggerhead (Caretta caretta) captures was detected among the three sampling periods, while the addition of sampling in 2007−2009 revealed significant increases in capture rates of green (Chelonia mydas) and Kemp’s ridley (Lepidochelys kempii) sea turtles. Loggerheads also dominated in the early sampling periods to an equivalent proportion of green and loggerhead sea turtles in 2007−2009. Long-term in-water studies such as this are critical for monitoring trends in sea turtle populations and implementing effective conservation plans.

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.

The study compared the efficiency of five models of pots designed to catch Atlantic cod (Gadus morhua) for use in Newfoundland and Labrador (Canada)‘s cod fishery. Catch per unit effort (CPUE), total lengths of cod, and bycatch rates were compared across each pot type. All pot types were successful at catching cod, but two models (a modified Newfoundland-style pot, which used 58mm clear monofilament entrance funnels, a mesh separator panel, and two bait bags; and a four-entrance pot) had the highest CPUE. Modifying the Newfoundland pot increased its CPUE by 145% without a corresponding increase in bycatch. 

The use of light tori lines (made with nylon monofilament branch lines and streamers made from other light material) on pelagic longlines reduced the incidental capture of seabirds by 64%. The capture of target species, including swordfish (Xiphias gladius) and blue sharks (Prionace glauca), was increased by 32% and 15.1% respectively when light tori lines were used.

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.

Tests with lanthanide metals were conducted to determine their potential application as a shark repellent in longline fisheries.  Six lanthanide metals were tested (cerium (Ce), neodymium (Nd), praseodymium (Pr), cerium-lanthanum mischmetal (CeLa), praseodymium neodymium metal alloy (PrNdA) and praseodymium neodymium mischmetal (PrNdM)) to quantify their voltage and compare dissolution rates in seawater. A behavioral study was used to determine their efficacy in deterring bonnethead (Sphyrna tiburo) and lemon sharks (Negaprion brevirostris).  There were no differences in the voltages produced by the metals and their voltages decreased with distance from the samples.  Freshwater produced the greatest voltages, which decreased logarithmically as salinity increased to 10 ppt.  The metals had significantly different dissolution rates, except Nd and Pr.  The time to dissolution ranged from 15.6 hrs for PrDnA to 107.6 for CeLa (lot 2). Behavioral tests were only conducted with Nd, which was not effective in repelling either shark species.

This study examined demersal trawl gear used to catch Greenland halibut (Reinhardtius hippoglossoides) in west Greenland. A towed video sled recorded trawling impacts on seafloor fauna. Trawling effort was significantly linked with community composition, with a negative association between trawling effort and abundance of some taxa, including some vulnerable marine ecosystem (VME) indicators, as well as regionally rare species (Balticina finmarchica).