Shark Catch Rates Research Articles

Artisanal fisheries catch highlights hotspot for threatened sharks and rays in the Republic of the Congo

AbstractGlobal catch rates of sharks and rays from artisanal fisheries are underreported, leading to a lack of data on population status. This forms a major barrier to developing effective management plans, such is the case in Central and West Africa. Over 3 years, we undertook the first systematic quantitative assessment of sharks and rays landed by an artisanal fishery in the Republic of the Congo. During 507 sampling days (mean 14 surveys per month), we recorded 73,268 individuals. These comprised 42 species, of which 81% are considered at an elevated risk of extinction. Landings were dominated by immature individuals, especially for species of conservation concern. Presence of species thought to have largely disappeared from the region such as the African wedgefish (Rhynchobatus luebberti) and smoothback angelshark (Squatina oculata) suggest Congolese waters are a potential stronghold for these species—warranting increased protection. We identified seasonality of catch within years, but not across years. Both inter‐ and intra‐annual trends varied by species, signifying annual fluctuations in catch of each species but consistent catch of all species year‐on‐year. Analysis showed increased catch between the short‐wet and the long‐wet, and the long‐dry seasons (January–February and August–September). Lowest catch was shown to occur during the short‐wet and the short‐dry seasons (October–December), which may provide an opportunity for seasonal closures or gear restrictions.

OCCURRENCE OF IMMATURE SHARKS IN ARTISANAL FISHERIES OF THE SOUTHERN GULF OF MEXICO

A common feature of Mexican artisanal fisheries is the high proportion of immature sharks in catches. Immature sharks have been historically caught in the southern Gulf of Mexico; however, there is a lack of catch rate estimates. Fishery-dependent sampling in the states of Tabasco (2007-2010 and 2012-2014) and Campeche (2011-2014) was conducted to estimate catch rates of immature sharks of large shark species (species reaching > 150 cm of total length). The catch rate of the 12 recorded species was < 1 shark/fishing day. Immature individuals accounted for a high percentage of the sharks caught in Tabasco (94.5%) and Campeche (90%). We suggest the coast of both states is a juvenile shark habitat for the 12 species. Sharks with predominantly coastal habitats are probably more vulnerable to anthropogenic impacts in the study area, including artisanal fisheries, the oil industry, and coastal development.Keywords: shark nurseries, juvenile shark habitat, fishery management, Campeche Bank.

The effectiveness of Shark‐Management‐Alert‐in‐Real‐Time (SMART) drumlines as a tool for catching white sharks, Carcharodon carcharias, off coastal New South Wales, Australia

AbstractWhite (Carcharodon carcharias L.), bull (Carcharhinus leucas, Müller & Henle) and tiger (Galeocerdo cuvier, Péron & Lesueur) sharks are the primary species responsible for unprovoked shark bites. Historically, management practices were based on culling “target” shark species (i.e. white, bull and tiger sharks), which resulted in high levels of bycatch and mortality. Shark‐Management‐Alert‐in‐Real‐Time (SMART) drumlines were trialled in New South Wales, Australia, aiming to optimise the capture of target shark species while minimising bycatch and mortality. Target shark species accounted for 70% of the total catch, with white sharks contributing 298 of the 350 sharks that were caught. Four animals died, and bycatch consisted of 13 species including two threatened species. Generalised linear mixed models (GLMMs) revealed a significant spatial, temporal, environmental and gear influence on white shark catch rates. SMART drumlines are a useful tool for catching target shark species with low bycatch and mortality relative to historical bather protection methods.

Effect of pelagic longline bait type on species selectivity: a global synthesis of evidence

Fisheries can profoundly affect bycatch species with ‘slow’ life history traits. Managing bait type offers one tool to control species selectivity. Different species and sizes of marine predators have different prey, and hence bait, preferences. This preference is a function of a bait’s chemical, visual, acoustic and textural characteristics and size, and for seabirds the effect on hook sink rate is also important. We conducted a global meta-analysis of existing estimates of the relative risk of capture on different pelagic longline baits. We applied a Bayesian random effects meta-analytic regression modelling approach to estimate overall expected bait-specific catch rates. For blue shark and marine turtles, there were 34% (95% HDI: 4–59%) and 60% (95% HDI: 44–76%) significantly lower relative risks of capture on forage fish bait than squid bait, respectively. Overall estimates of bait-specific relative risk were not significantly different for seven other assessed taxa. The lack of a significant overall estimate of relative capture risk for pelagic shark species combined but significant effect for blue sharks suggests there is species-specific variability in bait-specific catch risk within this group. A qualitative literature review suggests that tunas and istiophorid billfishes may have higher catch rates on squid than fish bait, which conflicts with reducing marine turtle and blue shark catch rates. The findings from this synthesis of quantitative and qualitative evidence support identifying economically viable bycatch management measures with acceptable tradeoffs when multispecies conflicts are unavoidable, and highlight research priorities for global pelagic longline fisheries.

Shark-catch composition and seasonality in the data-poor small-scale fisheries of the southern Gulf of Mexico

Regional ecological information is relevant to the international management of vulnerable species such as sharks. On the basis of monthly fishery data (2007–2010) from two small-scale multispecies fleets, namely, the bottom longline fleet (MLL) and vertical line+shark-specific longline fleet (VL+SSL) operating on the Campeche Bank, we recorded 19 shark species. The occurrence of immature stages of 15 species, gravid females of Carcharhinus leucas, three Endangered species (Sphyrna mokarran, S. lewini, Isurus oxyrinchus) and three Vulnerable and six Near-threatened species emphasises the importance of this area. Rhizoprionodon terraenovae (42.01% of total abundance), S. lewini (24.82%) and S. tiburo (14.57%) were the most abundant species. The commercial logbooks (2007–2012) for one fleet recorded 6517 for MLL and 1617 for VL+SSL fishery trips. The general additive model indicated a maximum of shark catch rate (CR) for MLL and SSL during 2011 (mean±s.e., 49.94±8.44 and 91.38±16.07kg per 100 hooks respectively). Monthly CR for MLL was highly variable; the maximum was observed in February (15.45±21.43kg per 100 hooks), the maximum monthly CR for SSL was in January (88.04±14.85kg per 100 hooks). For MLL, a positive relation was observed between effort and CR, whereas, for SSL, no clear relation was observed.

Spatio-temporal changes in catch rates of pelagic sharks caught by Japanese research and training vessels in the western and central North Pacific

Blue shark and shortfin mako are key pelagic shark species distributed throughout the temperate and tropical waters of the world’s oceans. These species are caught in the North Pacific by Japanese longline fisheries as bycatch and occasionally targeting blue shark. Japanese research and training vessels (JRTVs) have been collecting fishery-independent data on these sharks in the western and central North Pacific since 1992. However, the catch per unit of effort (CPUE) estimated from the fishery-independent data had not been used for stock assessments due to an issue concerning reporting after the 2000s. In this study, a statistical filtering method was used to remove unreliable set-by-set data collected by JRTVs. The nominal CPUE of the JRTVs was then standardized using the spatio-temporal generalized linear mixed model to provide the spatial and temporal changes in the CPUEs of these sharks in the North Pacific. The hotspots of blue shark appeared in temperate waters of the western and central North Pacific, whereas those of 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 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. The information predicted from the fishery-independent data is very useful for improving the stock assessment and management of these sharks as it provides an understanding of spatio-temporal changes in the hotspots and temporal changes in the catch rates.

Spatiotemporal variation in size-structured populations using fishery data: an application to shortfin mako (Isurus oxyrinchus) in the Pacific Ocean

We develop a length-disaggregated, spatiotemporal, delta-generalized linear mixed model (GLMM) and apply the method to fishery-dependent catch rates of shortfin mako sharks (Isurus oxyrinchus) in the North Pacific. The spatiotemporal model may provide an improvement over conventional time-series and spatially stratified models by yielding more precise and biologically interpretable estimates of abundance. Including length data may provide additional information to better understand life history and habitat partitioning for marine species. Nominal catch rates were standardized using a GLMM framework with spatiotemporal and length composition data. The best-fitting model showed that most hotspots for “immature” shortfin mako occurred in the coastal waters of Japan, while hotspots for “subadult and adult” occurred in the offshore or coastal waters of Japan. We also found that size-specific catch rates provide an indication that there has been a recent increasing trend in stock abundance since 2008.

Local indicators of abundance and demographics for the coastal shark assemblage of Bimini, Bahamas

Abstract Understanding population dynamics is essential for implementing effective conservation and management of coastal sharks. Fisheries-independent surveys can offer valuable information for such data-limited situations. A 12-year (2004–2015) standardized, shallow water longline survey was conducted monthly in the coastal waters of Bimini, Bahamas. Each monthly survey comprised five longline sets, totaling 75 hooks, with a soak time of 24 h. A total of 770 sharks from nine species were caught over the course of the study, with tiger ( Galeocerdo cuvier ), nurse ( Ginglymostoma cirratum ), blacktip ( Carcharhinus limbatus ) and lemon ( Negaprion brevirostris ) sharks comprising 95% of the catch. The majority of tiger (87%), nurse (62%), blacktip (67%), and lemon (82%) sharks were of immature lengths. A greater number of captured tiger (77%) and blacktip (66%) sharks were female, while nurse (55%) and lemon sharks (73%) were predominantly male. Poisson generalized additive models were used to analyze local abundance trends and examine how catch rates were influenced by year, month, location, tide, hour of capture, and lunar cycle. Seasonal trends indicate greater catches of the nurse, blacktip and lemon sharks during the summer months. Annual trends indicated relatively stable catch rates for the tiger, blacktip and lemon shark. Nurse shark catch rates were highly variable during the survey. Results from this study improve our understanding of the coastal shark assemblage in Bimini, Bahamas, and provide important local abundance trend information that could be beneficial for conservation and regional assessments.

Elasmobranch captures in the Fijian pelagic longline fishery

Abstract Pelagic longline fisheries for relatively fecund tuna and tuna‐like species can have large adverse effects on incidentally caught species with low‐fecundity, including elasmobranchs. Analyses of observer programme data from the Fiji longline fishery from 2011 to 2014 were conducted to characterize the shark and ray catch composition and identify factors that significantly explained standardized catch rates. Catch data were fitted to generalized linear models to identify potentially significant explanatory variables. With a nominal catch rate of 0.610 elasmobranchs per 1000 hooks, a total of 27 species of elasmobranchs were captured, 48% of which are categorized as Threatened under the IUCN Red List. Sharks and rays made up 2.4% and 1.4%, respectively, of total fish catch. Blue sharks and pelagic stingrays accounted for 51% and 99% of caught sharks and rays, respectively. There was near elimination of ‘shark lines’, branchlines set at or near the sea surface via attachment directly to floats, after 2011. Of caught elasmobranchs, 35% were finned, 11% had the entire carcass retained, and the remainder was released alive or discarded dead. Finning of elasmobranchs listed in CITES Appendix II was not observed in 2014. There were significantly higher standardized shark and ray catch rates on narrower J‐shaped hooks than on wider circle hooks. Based on findings from previous studies on single factor effects of hook width and shape, the smaller minimum width of the J‐shaped hooks may have caused the higher shark and ray catch rates. For sharks, the effect of hook width may have exceeded the effect of hook shape, where small increases in shark catch rates have been observed on circle vs J‐shaped hooks. Shark and ray standardized catch rates were lowest in the latter half of the year. Focusing effort during the second half of the year could reduce elasmobranch catch rates. Copyright © 2016 John Wiley & Sons, Ltd.

Trends in the exploitation of South Atlantic shark populations.

Approximately 25% of globally reported shark catches occur in Atlantic pelagic longline fisheries. Strong declines in shark populations have been detected in the North Atlantic, whereas in the South Atlantic the situation is less clear, although fishing effort has been increasing in this region since the late 1970s. We synthesized information on shark catch rates (based on 871,177 sharks caught on 86,492 longline sets) for the major species caught by multiple fleets in the South Atlantic between 1979 and 2011. We complied records from fishing logbooks of fishing companies, fishers, and onboard observers that were supplied to Brazilian institutions. By using exploratory data analysis and literature sources, we identified 3 phases of exploitation in these data (Supporting Information). From 1979 to 1997 (phase A), 5 fleets (40 vessels) fished mainly for tunas. From 1998 to 2008 (phase B), 20 fleets (100 vessels) fished for tunas, swordfishes, and sharks. From 2008 to 2011 (phase C), 3 fleets (30 vessels) fished for multiple species, but restrictive measures were implemented. We used generalized linear models to standardize catch rates and identify trends in each of these phases. Shark catch rates increased from 1979 to 1997, when fishing effort was low, decreased from 1998 to 2008, when fishing effort increased substantially, and remained stable or increased from 2008 to 2011, when fishing effort was again low. Our results indicate that most shark populations affected by longlines in the South Atlantic are currently depleted, but these populations may recover if fishing effort is reduced accordingly. In this context, it is problematic that comprehensive data collection, monitoring, and management of these fisheries ceased after 2012. Concurrently with the fact that Brazil is newly identified by FAO among the largest (and in fastest expansion) shark sub-products consumer market worldwide.

A cross‐taxa assessment of pelagic longline by‐catch mitigation measures: conflicts and mutual benefits to elasmobranchs

AbstractElasmobranch mortality in pelagic longline fisheries poses a risk to some populations, alters the distribution of abundance between sympatric competitors, changing ecosystem structure, processes and stability. Individual and synergistic effects on elasmobranch catch and survival from pelagic longline gear factors, including methods prescribed to mitigate bycatch of other vulnerable taxa, were determined. Overall relative risk of higher circle vs. J‐shaped hook shark catch rates conditioned on potentially informative moderators, from 30 studies, was estimated using an inverse‐precision weighted mixed‐effects meta‐regression modelling approach. Sharks had a 1.20 times (95% CI: 1.03–1.39) significantly higher pooled relative risk of capture on circle hooks, with two significant moderators. The pooled relative risk estimate of ray circle hook catch from 15 studies was not significant (RR = 1.22, 95% CI: 0.89–1.66) with no significant moderators. From a literature review, wire leaders had higher shark catch and haulback mortality than monofilament. Interacting effects of hook, bait and leader affect shark catch rates: hook shape and width and bait type determine hooking position and ability to sever monofilament leaders. Circle hooks increased elasmobranch catch, but reduced haulback mortality and deep hooking relative to J‐shaped hooks of the same or narrower width. Using fish vs. squid for bait increased shark catch and deep hooking. Pelagic stingray (Pteroplatytrygon violacea) catch and mortality were lower on wider hooks. Using circle instead of J‐shaped hooks and fish instead of squid for bait, while benefitting sea turtles, odontocetes and possibly seabirds, exacerbates elasmobranch catch and injury, therefore warranting fishery‐specific assessments to determine relative risks.

Influence of hook type on catch of commercial and bycatch species in an Atlantic tuna fishery

Experimental sets were conducted on a Taiwanese deep set longline fishing vessel operating in the tropical Atlantic Ocean to evaluate the effects of relatively wide circle hooks vs. Japanese tuna hooks with respect to catch rates of both target and incidental species. On circle hooks there were significantly higher catch rates of bigeye tuna (Thunnus obesus), yellowfin tuna (T. albacares), swordfish (Xiphias gladius) and blue sharks (Prionace glauca) as compared to tuna hooks. Significantly higher rates of albacore (T. alalunga) and longbill spearfish (Tetrapterus pfluegeri) were caught on Japanese tuna hooks as compared to circle hooks. Overall, 55 sea turtles were incidentally captured, most (n=47) of which were leatherback turtles (Dermochelys coriacea), and capture rates were similar between hook type. Immediate survival rates (percentage alive) when landed were statistically similar for all major target fish species and sea turtles independent of hook type. Most (64%) sea turtles were hooked on the first and second branchlines closest to the float, which are the shallowest hooks deployed on a longline. Lengths of six retained species were compared between hook types. Of these, swordfish was the only species to show a significant difference in length by hook type, which were significantly larger on circle hooks compared to tuna hooks. Additional incentives to use circle hooks would be the increased catch rate in targeted bigeye tuna over traditional Japanese tuna hooks. This international collaboration was initiated in direct response to regional fisheries management organization recommendations that encourage member countries to conduct experiments aimed to identify means to reduce bycatch in longline fishing gear. Information presented may be useful for managers in developing international fisheries policies that aim to balance increases in commercial fishery revenue and endangered species protection.

Standardized catch and survival rates, and effect of a ban on shark retention, Palau pelagic longline fishery

Abstract Pelagic longline fisheries affect both market and vulnerable bycatch species and can have broad effects on community structure and processes. Observer data from the Palau longline fishery were analysed to identify opportunities to mitigate vulnerable species bycatch, determine temporal trends in local abundance, and assess changes following a ban on shark retention and wire leaders. Catch and haulback condition data for bigeye and yellowfin tunas, blue and silky sharks and pelagic stingrays were fitted to standardized catch and survival rate models. The fishery caught silky and blue sharks, olive ridley sea turtles and other species of conservation concern. Changing from shallow sets to deep daytime sets might reduce shark and sea turtle catch rates but increase turtle haulback mortality rates, maintain economically viable tuna catch rates, but increase catch rates of pelagic stingrays, a lower conservation concern than main caught species of sharks and turtles. Focusing fishing effort during the middle of the calendar year would maximize yellowfin tuna and minimize silky shark standardized catch rates, but maximize blue shark catch rates. A large decline in shark fishing mortality rate very likely occurred following a ban on shark retention and wire leaders. This was due to large reductions in the nominal shark catch rate and shark retention, partially offset by decreases in the shark haulback survival rate and pre‐catch survival rate. Significantly higher blue shark and lower pelagic stingray nominal catch rates occurred on wire vs. monofilament leaders. Significantly higher blue shark and lower yellowfin tuna nominal catch rates occurred on sets using shallow ‘shark lines’. It is a research priority to compare the probability of shark pre‐catch survival after escaping from monofilament leaders with an ingested hook and trailing line to the survival probability when captured on wire leaders. Copyright © 2015 John Wiley & Sons, Ltd.

Hooks equipped with magnets can increase catches of blue shark (Prionace glauca) by longline fishery

Blue shark (Prionace glauca) populations are decreasing worldwide and the species is currently classified as near threatened. However, it is the main species caught by the Spanish and Portuguese longline fisheries; and blue shark is specifically targeted by a part of these fleets in the northeastern Atlantic Ocean. Sharks are well known to be able to detect electric fields in the microvolt range and this sense has been proposed to provide a mechanism to detect the earth's magnetic field. As a result, the use of magnets has been proposed as a method to reduce shark interactions with fishing gear. We therefore tested two models of high field strength neodymium magnets to effect shark catch rates during commercial longline fishing operations. Our results show that magnets do not reduce blue shark catch rates and can even have an attractive effect. This effect was significantly higher for the larger magnet model tested (26mm×11mm×12mm, 0.885T) compared to the smaller one (20mm×13mm×15mm, 0.464T). We also noted that hooks remain magnetized after removal of the magnets and are even slightly magnetized without any previous contact with a magnet.

Long-term trajectory of some elasmobranch species off the Tuscany coasts (NW Mediterranean) from 50 years of catch data

The time series of elasmobranch catch rates off the Tuscany coasts (NW Mediterranean) were investigated by means of min/max auto-correlation factor analysis in order to estimate variations in population abundance and evaluate the influence of environmental and anthropogenic factors. The analyses highlighted a general decreasing trend in the catch rates of sharks and skates from 1961 to the mid-1990s, mainly influenced by the increase in fishing effort. Since the 1990s, the EU Common Fishery Policy for the Mediterranean has promoted the reduction of fishing fleets through incentives to vessel demolition. The Porto S. Stefano trawl fleet has decreased by about 50%, leading to a decrease in fishing effort which seemed to be the most relevant factor affecting the increasing trend shown by the catch rates of Galeus melastomus, Scyliorhinus canicula and skates from 1991 to 2009. The elasmobranch assemblage did not undergo major shifts but the weighted frequency of occurrence shows that elasmobranchs were more frequent in the past. Particular caution should be paid in interpreting the recent rebound of some species as an early sign of recovery: trawl survey data and landing data show that over the last 50 years elasmobranch fauna have undergone a drastic decline and that recent rebounds are still far from a recovery to historical levels.

The effects of a lanthanide metal alloy on shark catch rates

Bycatch of sharks in longline fisheries has contributed to declines in shark populations and prompted the need for exploring novel technologies to reduce the incidental capture of sharks. One potential strategy is to exploit the unique electrosensory system of sharks, used to detect weak electric fields. Metals from the lanthanide series, made up of neodymium (Nd) and praseodymium (Pr), produce strong electric fields in water. In this study, we tested the effects of an Nd/Pr alloy on shark catch rates. Using longline fishing gear, we compared the catch rates of baited hooks affixed with either a block of the metal alloy (experimental) or a lead weight (control). Four experiments were conducted in different regions of the Pacific Ocean. Two bottom longline experiments were conducted inside and offshore of Kaneohe Bay, Hawaii. One of these experiments targeted young of the year scalloped hammerhead sharks (Sphyrna lewini), while the other targeted sandbar (Carcharhinus plumbeus) and tiger sharks (Galeocerdo cuvier). In the Southern California Bight (SCB), pelagic longlines were deployed to target mako (Isurus oxyrinchus) and blue sharks (Prionace glauca) and longlines targeting pelagic sharks were set in the Eastern Tropical Pacific (ETP) off Ecuador. There was a significant reduction in juvenile hammerhead sharks caught on hooks with the lanthanide metal compared to the controls. In contrast, there was no difference in the catch rates for experiments targeting sandbar sharks in Hawaii or those conducted in the SCB and Ecuador. These results suggest that there are inter-specific differences regarding the effects of lanthanide metals on catch rates. This may reflect the diverse feeding strategies and sensory modalities used by shark species for detecting and attacking prey.

Shark bycatch and mortality and hook bite-offs in pelagic longlines: Interactions between hook types and leader materials

This study addressed the influence of hook type (circle vs J-hook) and leader material (nylon vs wire) on longline catch and mortality rates of target and bycatch species in a pelagic longline fishery targeting swordfish, Xiphias gladius, and tunas. A total of 603 individuals (53% classified as bycatch) were caught on 17,000 hooks. Sharks constituted 45% of the bycatch. Bite-offs (i.e. missing hooks) corresponded to ∼33% of the shark catch and occurred mostly on nylon leaders (97%). Hook type had no significant effect on catchability or mortality of any species or groups. However, nylon leaders caught more bigeye tuna, Thunnus obesus and all target species combined, while wire leaders caught more blue shark, Prionace glauca and all sharks combined. If bite-offs were assumed to be undetected sharks, differences in shark catchability between leader types disappear. Moreover, significant differences in blue shark catch rate between leader types was found in J-hook treatments only. Higher proportions of live sharks were found on wire leaders. The catch and mortality rates of sharks in longline fisheries may be underestimated when monofilament leaders are used. This study highlights the need for understanding the role of every longline component in gear performance analysis.

Population structure, distribution and relative abundance of the blue shark (Prionace glauca) in the Caribbean Sea and adjacent waters of the North Atlantic

Catch and fishing effort data from the Venezuelan Pelagic Longline Observer Program (VPLOP) in the Caribbean Sea and adjacent waters of the Atlantic were used to investigate the population structure, distribution and relative abundance of the blue shark, Prionace glauca. A total of 959 blue sharks were sampled by trained observers during the period of January 1994 to December 2007. Fork length (FL) in males ranged from 79 to 355cm and from 97 to 349cm in females. Sexual segregation at the spatial and temporal scale was observed, males dominated January–June and during October–December across all areas; while females dominated during July–September with a higher magnitude in the Atlantic side (Guyana-Amazon area). Sex ratio for mature (>180cm FL) blue shark favored females in the Caribbean Sea from July to December, and in the Guyana-Amazon area in July–September; while immature blue sharks were dominated by males across all areas. Thirty-four pregnant females were caught in the Caribbean Sea and three in the Guyana-Amazon area. Pregnant females displayed a seasonal trend which increased from September through January, where >50% of the pregnant females occurred during November–January; of the pregnant females caught in the Caribbean Sea two had embryos close to the size-at-birth [>35cm total length (TL)]. The spatial distribution of blue sharks in the Caribbean Sea was marked by high concentrations around the offshore islands of Venezuela and the Netherland Antilles year round; while in the Guyana-Amazon area the high concentrations were more seasonally localized off northeastern coast of South America but diminished progressively and by the end of the year was absent in the area. Relative abundance of blue shark was estimated from the Venezuelan pelagic longline (VPL) fleet using a General Linear Model Approach with a delta lognormal model distribution to account for the high proportion of zero observations. The variability within vessel groups was partitioned into each vessel variance and the auto-correlation within a vessel was evaluated by using a variance–covariance matrix model. Diagnostic checks and deviance table analysis were performed to identify the set of model factors and interactions that adequately explained the observed variability. The results indicated that variability of catch rates within vessels is as large as the variability among vessel groups, where smaller and medium size vessels show higher catch rates of blue sharks in the VPL fleet. Standardized catch rates indicated in general a lower relative abundance in the most recent years followed by a slight recovery that requires caution due to the predicted high variance. Fishing activities can have uneven effects in the blue shark population due to its spatial and temporal segregation in the study area; potential changes in the fishing operations of the VPL fleet can have negative effects in the population structure of blue sharks in the Caribbean Sea and adjacent waters if habitat for blue sharks are increasingly used to target other valuable species (e.g., large tunas and swordfish).

The Effect of Circle Hooks on Shark Catchability and At-Vessel Mortality Rates in Longlines Fisheries

Circle hooks have gained recent attention as a cost-effective bycatch mitigation tool in pelagic longline fisheries, particularly for marine turtles. Over the last few years, a growing number of studies have investigated the use of circle hooks and their effects on other species, including elasmobranchs. To elucidate the potential value of circle hook use as a tool for shark conservation and management in pelagic longline fisheries, we conducted a quantitative review of all available studies to date. We compiled 15 published and eight gray literature studies and where possible used random effects meta-analysis and analysis of covariance to test the effects of circle hooks on catchability and at-vessel mortality rates. Overall, results suggest that using circle hooks on pelagic longlines do not have a major effect on shark catch rates, but do reduce at-vessel mortality compared to J-hooks. Thus circle hooks should be seen as one potential tool to help reduce bycatch mortality of sharks in longline fisheries. However, the high level of heterogeneity found between studies highlights the need for shark-specific controlled experiments to provide more definitive results.

Spatial predictions of blue shark (Prionace glauca) catch rate and catch probability of juveniles in the Southwest Atlantic

AbstractCarvalho, F. C., Murie, D. J., Hazin, F. H. V., Hazin, H. G., Leite-Mourato, B., and Burgess, G. H. 2011. Spatial predictions of blue shark (Prionace glauca) catch rate and catch probability of juveniles in the Southwest Atlantic. – ICES Journal of Marine Science, 68: 890–900. Generalized regression analysis and spatial prediction was applied to catch per unit effort (cpue) data for blue shark (Prionace glauca) caught by the Brazilian tuna longline fleet between 1997 and 2008 (43 546 longline sets) to predict the effect of environmental, spatial, and temporal factors on catch distribution. In addition, the size distribution of blue sharks measured by on-board observers during the years 2006–2008 was used to model the proportion of juvenile blue sharks in the catches from a spatial perspective. Latitude was the most important factor influencing blue shark cpue in the Southwest Atlantic, with cpue spatial predictions suggesting two areas of higher catch probabilities. Latitude was also the most important factor influencing the proportion of juveniles in the catches. The spatial prediction map showed that juveniles were more frequently caught south of 35°S (∼38°S). This information can assist in the design of management strategies either to exploit this predictable spatial distribution of the catch or to manage the fisheries in a spatially explicit manner if one component (i.e. juveniles) requires protective measures.