Tuna Longline Fleet Research Articles

Opportunity to Leverage Tactics Used by Skilled Fishers to Address Persistent Bycatch Challenges

ABSTRACTEffective management of shark bycatch is urgently needed to reverse widespread population declines, especially in longline fisheries that are estimated to be responsible for half of global shark catch. Management of shark catch typically focuses on the safe release of landed sharks, with limited attention to reducing the initial catch. Where controls on fishing effort or catch do exist, management frameworks tend to treat fishing fleets as homogeneous units. The underlying assumption is that fishers have similar abilities to catch target species and avoid bycatch. We test this assumption by analysing variability in shark bycatch rates among individual vessels in an industrial tuna longline fleet operating in the Western Pacific. Controlling for factors such as geographic location, time of day and gear depth, we find that individual vessels drive highly variable bycatch rates of blue (Prionace glauca) and silky sharks (Carcharhinus falciformis) – two shark species with the highest global catch volumes. Additionally, we found that the operating company can influence fisher performance. As countries and regional organisations increasingly adopt shark conservation plans and make international conservation commitments, it is crucial to identify viable new strategies that do not unduly burden the industry or penalise good actors. Tailoring management actions to individual fishers and companies – holding high‐bycatch fishers accountable and incentivising low‐bycatch fishers to continuously improve – presents a crucial opportunity to address the overfishing of sharks and other global bycatch challenges.

Blue shark (Prionace glauca) occurrence and relative abundance in the western South Atlantic Ocean influenced by spatiotemporal variability, environmental variables, and oceanographic processes

The aim of this study was to evaluate spatiotemporal patterns in the distribution and relative abundance of blue sharks and their relationship with environmental variables and oceanographic processes in the southwestern Atlantic Ocean. We modeled data on catch records from the Brazilian pelagic tuna longline fleet using generalized additive models (GAMs). The distribution of Prionace glauca was influenced by salinity, chlorophyll-a and temperature. Our models showed that both the catch per unit of fishing effort (CPUE) and the probability of presence increased mainly between March and August. The CPUE was also influenced by ocean depth and sea surface height, and the probability of occurrence by ocean fronts and slope. The highest CPUE values and probability of occurrence of blue sharks tended to occur mainly in parts of the continental slope off Brazil and at the Rio Grande Rise, characterized by the presence of seamounts. Such hotspots for P. glauca could potentially be considered in fisheries management plans.

Bycatch Estimates From a Pacific Tuna Longline Fishery Provide a Baseline for Understanding the Long-Term Benefits of a Large, Blue Water Marine Sanctuary

Bycatch on pelagic tuna longlines has contributed to population declines in several far-ranging, oceanic species and presents a conservation challenge that area-based management tools are increasingly promoted to address. In January 2020 the Republic of Palau, concerned about the impacts of longline fishing in its waters, closed 80% of its exclusive economic zone to all extractive activities, reserving the remaining 20% for a domestic fishing zone (DFZ). One of a growing number of very large marine protected areas, the Palau National Marine Sanctuary (PNMS) spans ∼500,000 km2 and was established inter alia to allow for the recovery of fish stocks adversely impacted by tuna longline fleets. Given that the main tuna stocks targeted in the western Pacific are not overexploited, the benefits of protection potentially afforded by the sanctuary are likely greater for vulnerable bycatch species. Evaluations of the sanctuary’s performance require, in part, a baseline of historical catch rates and effort distribution in the distant-water fleet (DWF) and locally based fleet (LBF) operating in Palau prior to sanctuary implementation. We describe the fishing effort, catch rates, catch estimates and fishing mortality in Palau’s longline fishery based on logbook, observer and electronic monitoring data. We defined bycatch as any species, retained or discarded, other than targeted tunas. Between 2010 and 2020, 104.8 million hooks were deployed, catching over 2 million individuals from 117 taxa at an overall target:bycatch ratio of 1:1, with a retention rate of ∼62%. Pronounced differences in fishing strategies and spatial distribution of effort between fleets were associated with large variations in catch rates and composition. The LBF had a larger effect on populations of at-risk species relative to the DWF, with higher catch rates and magnitudes for several vulnerable species and higher observable fishing mortality rates (64% vs 50% in the DWF). The sanctuary reshaped Palau’s longline fishery, contracting the fishery’s area and capacity. The relocation of the DFZ eliminated the LBF and constrained the DWF to an area where the fleet’s total catch rates and those of a number of vulnerable species were historically lower relative to former fishing grounds now closed by the sanctuary. Our results highlight the importance of consistent bycatch monitoring and emphasize the need for regional area-based approaches for managing longline fisheries.

The discards and bycatch of Chinese tuna longline fleets in the Pacific Ocean from 2010 to 2018

A substantial level of non-target species are often caught in tuna longline fisheries, which can therefore impose a heavy ecological burden on pelagic fish communities. However, limited studies have quantitatively examined the scale of bycatch varying across fishing fleets and areas in regional tuna longline fisheries, as data are often sparse. We used scientific observer data to analyze the discard and bycatch patterns of Chinese tuna longline fleets in the Pacific Ocean by applying general statistical analysis and spatial hotspots analysis. A total of 9971 sets were observed, representing 1.56% of the total efforts. Of 435,156 individuals, 30.4% were recorded as bycatch, and 15.8% were discarded. The majority of bycatch was other bony fishes (15.5%) and sharks (7.3%). Bycatch and discard rates of the bigeye tuna (Thunnus obesus, BET) fleet were larger than that of the albacore (Thunnus alalunga, ALB) fleet. The highest bycatch rates of other bony fishes and elasmobranchs most often occurred in the western and central temperate Pacific and the eastern tropical Pacific, respectively. Most marine turtles and mammals were caught by the BET fleet in the Tropical Pacific, and the bycatch of seabirds often occurred in the Temperate Pacific. Low bycatch rates but high discard mortality of marine mammals, turtles, and sea birds were found. Bycatch rates were estimated by fleet, seasons, and areas, with the rates of marine turtles ranging from 0 to 0.024 per 1000 hooks, and those of sea birds ranging from 0 to 0.228 per 1000 hooks. Six bycatch hotspots, identified in this study, should be considered in future management and conservation planning. Implementing measures such as live releasing bycatch, reducing the fishing efforts in the area with high bycatch rate, optimizing monitoring programs, and allocating observers more widely in areas where there is more bycatch are necessary.

Sustain or phase out: Transformation of Taiwan’s management scheme on distant water tuna longline fisheries

Fishing nations worldwide have introduced various measures to manage their fisheries. However, many have failed including Taiwan. Taiwan operated one of the world’s largest and most productive distant water tuna longline fleets but overcapacity and an incommensurate fisheries management scheme (FMS) resulted in illegal, unreported, and unregulated (IUU) fishing activities by the fleet. As a result, Taiwan received serious punitive sanction by the International Commission for the Conservation of Atlantic Tunas (ICCAT) in 2005 for illegal “fish laundering” activities. Ten years later, it also received a “yellow card” warning from the European Union (EU) under the EU IUU Regulation. Taiwan struggled to transform its FMS to meet the requirements imposed under the two events through three FMS transformation projects. The ICCAT sanction was lifted in 2006, the EU yellow card was lifted in mid-2019, and the resulting FMS is considered close to completion and a useful example of global best practice. This study reviews the failure of Taiwan’s FMS for tuna longline fisheries and identifies the potential drivers: low policy priority, weak institutional arrangements, and insufficient enforcement resources. It documents the evolution of the FMS which was painful but could offer valuable lessons for fisheries managers and scholars worldwide. Key effects of the transformation projects and some recommendations for the future are also provided.

THE IMPACT OF ENVIRONMENTAL CHANGES ON BLACK MARLIN, Makaira indica (Cuvier, 1832) ABUNDANCES IN THE EASTERN INDIAN OCEAN

Black marlin (Makaira indica) is commonly caught as frozen by-catch from Indonesian tuna longline fleets. Its contribution estimated 18% (~2,500 tons) from total catch in Indian Ocean. Relative abundance indices as calculated based on commercial catches are the input data for several to run stock assessment analyses that provide models to gather information useful information for decision making and fishery management, however, little are known about the influence of environmental factors to its abundance. In this paper, the abundance was represented as standardized index in order to eliminate any bias on other factors which might influence it. Data were collected from August 2005 to December 2017 through scientific observer program (2005-2017) and national observer program (2016-2017). Most of the vessels monitored were based in Benoa Port, Bali. Overall, time trends of abundance was fluctuated, although, there was increasing trend since 2010 then dropped significantly into relatively similar figure in 2005. Even though, Sea Surface Temperature (SST) and Sea Surface Height (SSH) were statistically significant when incorporating into the models, but it allegedly wasn’t the main driver in determining the abundance of black marlin. Instead, it was more likely driven by spatio-temporal factors (year and area) effect rather than environmental changes.

Age and growth estimates of the blue shark Prionace glauca in the central South Pacific Ocean

The age and growth of the blue shark Prionace glauca in the central South Pacific is described based on 267 specimens that were collected by scientific observers on board Taiwanese large-scale tuna longline fleets between May 2009 and May 2011. Growth band pairs (identified as translucent and opaque bands) were counted on images photographed from X-ray films of the vertebrae from the caudal peduncle region. The marginal increment ratio and centrum edge analysis indicated that a growth band pair was formed on the vertebral centrum once per year. The band pairs after the birthmark were counted from 2 to 11 for females and from 2 to 15 for males. The bias corrected Akaike information criterion indicated that the von Bertalanffy growth function (VBGF) best fitted the observed total length (TL)-at-age data. The parameters of sex-specific VBGF were estimated as follows: for females, theoretical maximum length (L∞) mean±s.d.=330.4±46.6cm TL, growth coefficient k=0.164±0.057year–1 and theoretical age at length 0 (t0)=–1.29±0.78 years; for males, L∞=376.6±32.6cm TL, k=0.128±0.022year–1 and t0=–1.48±0.54 years. The longevities were estimated to be at least 16.8 and 21.6 years for females and males respectively.

COMPARISON OF INDONESIAN TUNA LONGLINE FISHING PERFORMANCE WITHIN AND OUTSIDE INDONESIA EXCLUSIVE ECONOMIC ZONE (EEZ)

Indonesian tuna longline fleets have been fishing in the Exclusive Economic Zone (EEZ) and high seas of the Indian Ocean for quite some time. However, effort has never been made to separate catch from the EEZ and the high seas as it important for fisheries management. A total of 2,430 set-by-set longline fishing data had been collected by scientific observers based in the Research Institute of Tuna Fishery in Bali since August 2005 to December 2014 on which present analysis was made. The research aims to compare between trend of tuna catch of the EEZ and of the high seas of Indian Ocean. The results show that the mean hook rate of both catches of big eye tuna (BET) and southern Bluefin tuna (SBT) caught in the high seas was significantly higher than that the EEZ (two sample t-test, p<0.05), while for yellow fin tuna (YFT) it was in the opposite direction (two sample t-test, p<0.05). As for albacore (ALB), the mean hook rate value was statistically similar in both fishing grounds (two sample t-test, p>0.05).

Vulnerability of teleosts caught by the pelagic tuna longline fleets in South Atlantic and Western Indian Oceans

Productivity and Susceptibility Analysis (PSA) is a methodology for evaluating the vulnerability of a stock based on its biological productivity and susceptibility to fishing. In this study, we evaluated the vulnerability of 60 stocks of tuna, billfishes and other teleosts caught by the tuna longline fleets operating in the South Atlantic and Indian Ocean using a semi-quantitative PSA. We (a) evaluated the vulnerability of the species in the study areas; (b) compared the vulnerability of target and non-target species and oceans; (c) analyzed the sensitivity of data entry; and (d) compared the results of the PSA to other fully quantitative assessment methods. Istiophoridae exhibited the highest scores for vulnerability. The top 10 species at risk were: Atlantic Istiophorus albicans; Indian Ocean Istiompax indica; Atlantic Makaira nigricans and Thunnus alalunga; Indian Ocean Xiphias gladius; Atlantic T. albacares, Gempylus serpens, Ranzania laevis and X. gladius; and Indian Ocean T. alalunga. All species considered at high risk were targeted or were commercialized bycatch, except for the Atlantic G. serpens and R. laevis which were discarded, and may be considered as a false positive. Those species and others at high risk should be prioritized for further assessment and/or data collection. Most species at moderate risk were bycatch species kept for sale. Conversely, species classified at low risk were mostly discarded. Overall, species at high risk were overfished and/or subjected to overfishing. Moreover, all species considered to be within extinction risk (Critically Endangered, Endangered and Vulnerable) were in the high-risk category. The good concordance between approaches corroborates the results of our analysis. PSA is not a replacement for traditional stock assessments, where a stock is assessed at regular intervals to provide management advice. It is of importance, however, where there is uncertainty about catches and life history parameters, since it can identify species at risk, and where management action and data collection is required, e.g. for many species at high and most at moderate risk in the South Atlantic and Indian oceans.

Vulnerability of the Oceanic Whitetip Shark to Pelagic Longline Fisheries.

A combination of fisheries dependent and independent data was used to assess the vulnerability of the oceanic whitetip shark to pelagic longline fisheries. The Brazilian tuna longline fleet, operating in the equatorial and southwestern Atlantic, is used as a case study. Fisheries dependent data include information from logbooks (from 1999 to 2011) and on-board observers (2004 to 2010), totaling 65,277 pelagic longline sets. Fisheries independent data were obtained from 8 oceanic whitetip sharks tagged with pop-up satellite archival tags in the area where longline fleet operated. Deployment periods varied from 60 to 178 days between 2010 and 2012. Tagging and pop-up sites were relatively close to each other, although individuals tended to travel long distances before returning to the tagging area. Some degree of site fidelity was observed. High utilization hotspots of tagged sharks fell inside the area under strongest fishing pressure. Despite the small sample size, a positive correlation between tag recorded information and catch data was detected. All sharks exhibited a strong preference for the warm and shallow waters of the mixed layer, spending on average more than 70% of the time above the thermocline and 95% above 120 m. Results indicate that the removal of shallow hooks on longline gear might be an efficient mitigation measure to reduce the bycatch of this pelagic shark species. The work also highlights the potential of tagging experiments to provide essential information for the development of spatio-temporal management measures.

Incidental catch of seabirds and sea turtles by Taiwanese longline fleets in the Pacific Ocean

To understand the sea turtle and seabird bycatch of Taiwanese tuna longline fleets for conservation purposes, this research analyzed the data collected by onboard observers between 2008 and 2013. In total, data from 149 trips and 24.3 million hooks were analyzed, including 50 albacore large-scale tuna longline vessel (LTLVs) trips, 72 bigeye LTLVs trips, and 27 small-scale tuna longline vessel (STLVs) trips. Seabird bycatch was mostly from the albacore LTLVs. The highest bycatch rate was 0.320 bird per thousand hooks in the southwest Pacific Ocean in the first quarter, followed by the same area in the second quarter (0.046 bird per thousand hooks) by the albacore LTLVs. For seabird bycatch species, 81.7% were albatrosses, including wandering albatross (Diomedea exulans), Laysan albatross (Phoebastria immutabilis), white-capped albatross (Thalassarche steadi), black-footed albatross (Phoebastria nigripes), and black-browed albatrosses (Diomedea melanophris); other seabird species included white-chinned petrel (Procellaria aequinoctialis), flesh-footed shearwater (Puffinus carneipes), frigate bird and booby. Regarding sea turtles, the bycatch rate peaked in the second quarter in the western tropical Pacific Ocean by STLVs (0.034 turtle per thousand hooks), followed by albacore LTLVs (0.028 turtle per thousand hooks) during the same time period in the same region. The major bycatch species included olive ridley (Lepidochelys olivacea), followed by green (Chelonia mydas), and leatherback (Dermochelys coriacea). Observer training for seabird species identification and detailed information collection for mitigation measures should be implemented to ensure better data quality. This will help implement mitigation measures in areas and fisheries where a large number of birds are taken as bycatch.

Relationship between temporal–spatial distribution of fishing grounds of bigeye tuna (Thunnus obesus) and thermocline characteristics in the Atlantic Ocean

We evaluated the isoline distribution of thermocline characteristics (the upper and lower boundary temperatures and depths) in the bigeye tuna (Thunnus obesus) fishing grounds in the tropical Atlantic Ocean. We plotted the thermocline characteristics contour on a spatial overlay map using data collected on a monthly basis from Argo buoys and monthly CPUE (catch per unit effort) from bigeye tuna long-lines from the International Commission for the Conservation of Atlantic Tunas (ICCAT). In addition, frequency analysis and the empirical cumulative distribution function (ECDF) were used to calculate the optimum ranges of thermocline characteristics of the central fishing grounds. Our analysis suggested that there were significant seasonal variations in the upper boundary temperature and depth of thermocline in the central fishing grounds, which significantly influenced the temporal and spatial distribution of the bigeye tuna population. The overlay maps suggest that the central fishing ground of the bigeye tuna is located in the enclosed curve region between the higher value (70 m) and the lower value (40 m) of the upper boundary depth, from December to March of each year. After that, the fishing grounds spread to the north and south on both sides. CPUE is highest in areas where the upper boundary depths of the thermocline are between 40 and 60 m to the north of the equator. This extends to between 60 and 100 m to the south of the equator. The CPUE is highest in areas where the upper boundary temperature of the thermocline is between 24 and 26.9 °C in the low-latitude Atlantic Ocean, but decreases to 18 °C offshore of Namibia during July to October. The overlay maps showed that the lower boundary temperature and depth of thermocline have little seasonal variation. The lower boundary depth of the thermocline isoline depth showed that the higher depth area (>250 m) was shaped like the letter ‘W’. Similarly, the lower boundary temperature of thermocline isoline depth suggests that the temperature is higher besides the equator and lower around the equator area. The lower boundary water depth in the central fishing ground of the bigeye tuna ranged between 150 m and 250 m in the low-latitude Atlantic Ocean over the year, whereas the CPUE was low when the depth exceeded 250 m. In addition, the lower boundary water depth in the central fishing ground of the bigeye tuna decreased to between 140 and 160 m offshore of Namibia from April to October. The lower boundary temperature in the central fishing ground was about 12 °C in the low-latitude Atlantic Ocean and 14 °C offshore of Namibia. Conversely, if the temperature is >15 °C, the CPUE tends to be low. The numerical computation results suggest that the optimum ranges for the upper boundary thermocline temperature and depth were between 23 and 26.9 °C, and 30 and 70 m, respectively. The optimum ranges were 12–14.9 °C and 150–230 m for the lower boundary thermocline temperature and depth, respectively. We documented the distribution interval range of thermocline characteristics for the central fishing ground of the bigeye tuna in the Atlantic Ocean. Our results provide a reference for improving the efficiency of the bigeye tuna long-line fleet and aids tuna resource management in the Atlantic Ocean.

Sharks caught by the Brazilian tuna longline fleet: an overview

Large pelagic sharks are distributed throughout all of the oceans and are caught as bycatch in pelagic longline fisheries worldwide. In the southern Atlantic Ocean, more than a dozen shark species are caught by the Brazilian tuna longline fleet. This study compiles information of the main shark species caught by the Brazilian tuna longline fishery in the southwestern and equatorial Atlantic Ocean. Catch and effort data of 14,860 longline sets from the Brazilian chartered tuna longline fleet, between 2004 and 2010, were analyzed. The blue shark Prionace glauca was the main shark species captured by this fishery. Shark catches showed contrasting trends during the study period: the silky (Carcharhinus falciformis) and the oceanic whitetip (C. longimanus) sharks catch increased up to 2008 and then declined, while mako sharks (Isurus spp.) showed an opposite trend. Effort for the Brazilian longline fishery had a higher concentration from 10°N to 30°S and from 20°W to 40°W. High values of catch per unit effort of southwestern and equatorial Atlantic Ocean sharks were heterogeneously distributed and, although elasmobranchs were caught over most of the longline fishing range, only blue sharks were caught in all areas. In the southern Atlantic Ocean, high fishing effort zones overlap significantly with some nursery areas, especially for the oceanic whitetip shark, indicating that these areas are at a direct risk from the industrial longline fishery.

Size, distribution and catch rates of the oceanic whitetip shark caught by the Brazilian tuna longline fleet

Catch and effort data from 14,835 longline sets conducted by foreign tuna longline vessels chartered by Brazil, from 2004 to 2010, were analyzed aiming at assessing the size, distribution and the relative abundance of the oceanic whitetip shark (Carcharhinus longimanus) in the southwestern and equatorial Atlantic Ocean. The nominal catch per unit of effort (CPUE) exhibited a gradual increase, from 0.04 sharks/1000 hooks, in 2004, the first year of the time series, up to 0.13, in 2007. In 2008, however, the CPUE increased sharply, reaching 0.43, dropping, then, back to 0.15, in 2010. A CPUE standardization was performed using a delta-GLM approach, but the standardized index of abundance did not differ significantly from the nominal CPUE. The models indicated that the catches of oceanic whitetip sharks are higher for the Spanish fishing strategy, which is characterized by the deployment of hooks at shallower depths. These results indicate that the use of deep longline hooks (>100m) may help to mitigate the bycatch of this species.

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.

Effects of circle hooks on pelagic catches in the Hawaii-based tuna longline fishery

Sixteen vessels within the deep-set Hawaii-based tuna longline fleet tested the catch efficacy, fish size selectivity and survival on longline retrieval of large-size 18/0 circle hooks vs. Japanese style tuna hooks, size 3.6 sun and vs. size 9/0 “J” hooks. Vessels alternated hook types throughout the longline gear and maintained a 1:1 ratio of circle hooks to their existing tuna or J-hooks. Observers monitored a total of 1393 sets; 1182 sets were circle hooks vs. tuna hooks and 211 sets were circle hooks vs. J-hooks. The 18 most-caught species were analyzed representing 97.6% of the total catch by number. Two statistical methods were used to assess differences in catch (randomization test) or catch rate (generalized linear mixed models (GLMMs)). There were no significant catch or catch rate (catchability) differences among hook types for bigeye tuna ( Thunnus obesus), the primary target species, with either statistical method. However, GLMMs indicated that catch rates on circle hooks were significantly lower for 16 and 8 species compared to tuna and J-hooks, respectively. There were no significant differences in mean length of bigeye tuna among hook comparisons. Caught condition at retrieval varied considerably among the 18 species. Large circle hooks had greater effects on catch rates than on fish size selectivity and fish survival. We contend that reduced catch rates are a function of 18/0 circle hook shape, where the minimum width (4.9 cm) was 57% and 25% wider than the Japanese tuna (3.1 cm) and J-hook (3.9 cm), respectively. In contrast to tuna hooks, large circle hooks have conservation potential for use in the world's pelagic tuna longline fleets for some highly migratory species, with catch rate reductions of 29.2–48.3% for billfish species and 17.1–27.5% for sharks.

Catch rates and size composition of blue sharks (Prionace glauca) caught by the Brazilian pelagic longline fleet in the southwestern Atlantic Ocean

Distribution and relative abundance of blue sharks ( Prionace glauca ) in the southwestern Atlantic Ocean was modeled based on catch-per-unit-effort (CPUE) per 1000 hooks and length frequencies of blue sharks caught by the Brazilian pelagic tuna longline fleet. As a measure of relative abundance, CPUE of blue sharks caught in 58 238 fishing sets by the Brazilian pelagic tuna longline fleet (national and chartered), from 1978 to 2009, was standardized by a Generalized Linear Model (GLM) using three different approaches: i) a negative binomial error structure (log link); ii) the traditional delta lognormal model; and iii) the Tweedie distribution, recently proposed to adjust models with a high proportion of zeros. A cluster analysis using the K-means method was used to identify target species and incorporate it as a factor into the GLM. Cluster analysis grouped the data into six different fishing clusters according to the percentage of target species. Target factor (cluster) was the most important factor explaining the variance in all three CPUE models. The Tweedie model showed a relatively better fit compared to the other models. Blue shark nominal and standardized CPUE showed a relatively stable trend from 1978 to 1995. From 1995 onwards, however, there was an increasing trend in the standardized CPUE, up to a maximum value in 2008. In general, nominal CPUE and standardized CPUE tracked well up until 2000, after which standardized CPUE’s values were at a noticeably lower level than nominal CPUE. Length frequency data were analyzed for 11 932 blue sharks measured as part of the Brazilian onboard observer program operating on the pelagic tuna longline fleet between 2006 and 2008, with sizes ranging from 91 to 224 cm fork length. Overall, blue shark size data showed clear spatial and seasonal distributions for males and females in the southwestern Atlantic Ocean, with juveniles predominantly concentrated in the most southerly latitudes.

Bycatch and discards by Taiwanese large-scale tuna longline fleets in the Indian Ocean

Conservation of ecologically related species and understanding the discard composition of fisheries are major concerns for marine ecosystem conservation. However, high sea longline fisheries data are insufficient because of difficulties in deploying observers for data collection. Observer data collected from 77 trips on Taiwanese large-scale longline fishing vessels in the Indian Ocean from June 2004 to March 2008 were used to estimate the scale of the bycatch. At least 40 species were recorded. Albacore, bigeye, yellowfin, and southern bluefin tuna were the major species recorded and comprised over 73.30% of the total retained catch. Major bycatch species were swordfish, blue shark, sailfish, pomfret, and escolar. The average discard rate was 14.09%, ranging from 3.20% for the yellowfin tuna fleet to 18.09% for the bigeye tuna fleet. In total, 0.80% of the catch of the albacore, 4.74% of the bigeye, and 2.32% of the yellowfin tuna were discarded. There were significant differences among seasons and areas for the discard rates of the bigeye and yellowfin tuna. The discard rates of the bigeye and southern bluefin tuna were positively correlated to the catch per unit effort. The depredation percentage of tuna by cetaceans was from 0.7% to 12.3% of total discards for the different fleets. The high discard and cetacean depredation rates showed that major possible reasons for discards are depredation by cetaceans, economic factors, and quota limitations. Regarding other species, 61 seabirds and 84 sea turtles were a part of the bycatch. The major species were Indian Yellow-nosed Albatross in the southern Indian Ocean and Olive Ridley turtles in tropical areas. The estimated annual incidental catch numbers were 715 to 311 seabirds and 1856 to 1127 sea turtles from 2004 to 2007. For conservation, this discard information could be used to assess tuna stocks. Mitigation measures, including the live release of small-sized fish, and the use of bird-scaring lines and circle hooks, are required to minimize the bycatch.

Stomach contents of the pelagic stingray (Pteroplatytrygon violacea) (elasmobranchii: dasyatidae) from the tropical atlantic

Universidade Federal Rural de Pernambuco - LEMAR/DEPAq (Av. Dom Manuel de Medeiros s/n, Dois Irmaos, 52171-900 Recife, PE, Brasil) E-mail: p.travassos@depaq.ufrpe.br; mari.trrr@gmail.com The pelagic stingray Pteroplatytrygon violacea (Bonaparte, 1832) is the only pelagic dasyatid ray occurring in tropical and subtropical waters of the Atlantic, Indian and Pacific Oceans (BIGELOW; SCHROEDER, 1953; MOLLET, 2002; ELLIS, 2007). This poorly known species was originally described from the Mediterranean Sea, and was first reported from the Atlantic by Bigelow & Schroeder (1962). The species has no commercial value, but is fairly abundant off the Southeastern United States representing about 2.5% in number of the catches by pelagic longlines between 1992-2000 (BEERKIRSHER et al., 2004). The first record in southern Brazilian waters was reported by Sadowski; Amorim (1977) and Mazzoleni; Schwingel (2002) subsequently recorded the pelagic stingray as a bycatch species regularly caught by tuna longliners off southern Brazil. Menni et al. (1995) reported on the presence of the pelagic stingray in northeastern Brazilian waters. Although the pelagic stingray is caught regularly by tuna longliners operating along the Brazilian coast, few biological data are available on the species. In this context, the stomach contents of the pelagic stingray were analyzed to provide more specific information on its feeding habits in the southwestern equatorial Atlantic Ocean. The sampled area was located between 40o- 25oW and 5oN-20oS (Fig. 1). All specimens were caught by the Research Vessel Riobaldo (CEPENE-IBAMA), in the years 1993, 1994 and 1995 through the Ecotuna Project, and by the Brazilian tuna longline fleet, in the years 2005 and 2006 (SEAP, Onboard Observer Program), in waters of 2000 to 5000 m local depth, with hook depth between 50 and 250 m along the longline. The specimens were stored on ice onboard, and at the laboratory, they were sexed and had their disc width measured to the nearest centimeter.

Maximizing Resource Rent From the Western and Central Pacific Tuna Fisheries

Rent generated by the tuna fisheries occurring in the waters of Pacific Islands Nations is estimated for various levels and combinations of purse-seine, pole-and-line, frozen tuna longline, and fresh tuna longline fishing effort, using a multi-species, multi-fleet bioeconomic model. The underlying population model integrates available information on the population dynamics of skipjack, yellowfin, bigeye, and Southern albacore tunas in the Pacific Ocean. The economic model utilizes the most recent data on fishing effort costs for the purse seine, pole-and-line, and longline fleets operating in the western and central Pacific Ocean, along with recent estimates of prices by species, method of capture and market, and estimates of demand elasticities. The results of the model indicate that fishery rent could be increased substantially above the current level by decreasing the size of all fleets, with the possible exception of the tuna longline fleet. The results also suggest that the countries of the region co...