Longline Effort Research Articles

Condition of the international fisheries, catch and effort trends, and fishery data gaps for dolphinfish (Coryphaena hippurus) from 1950 to 2018 in the Western Central Atlantic Ocean

We conducted a scientific literature review, and a comprehensive analysis based on international fisheries databases, for dolphinfish (Coryphaena hippurus) from the Western Central Atlantic Ocean (WCA) from 1950 to 2018. This analysis updated the dolphinfish catch and efforts trends in comparison to those calculated in Mahon (1999), the first regional catch review for the species that was conducted with data from the 1950s through the mid-1990s. Results showed that the commercial pelagic longline effort doubled within, and quadrupled outside, of national jurisdictions. Commercial landings increased nearly three-fold, but 23 nations still do not report explicit dolphinfish landings to the FAO yet are known to catch dolphinfish. In the WCA, the US Atlantic recreational fishery represents the largest reporting sector by two-fold. When combined with reported commercial landings for 2016, total direct dolphinfish catch was 14,110 metric tons, of which 62 % was estimated to be recreational catch. Since the first regional fishery analysis of dolphinfish, the uncertainty of the status of the fishery has increased with several nations reporting higher landings of unidentified marine fish species. Also, new burgeoning social (e.g., FAD programs) and environmental processes (e.g., Sargassum blooms) lead to the presumption that higher amounts of juvenile dolphinfish are caught throughout the region. First reports of consequential amounts of dolphinfish bycatch have been documented in the pelagic longline fisheries, as well as the first modeled and anecdotal evidence of stock decline has been suggested. Results stress the immediate need for WCA nations to adopt a precautionary approach for proper fishery management of dolphinfish throughout the WCA, not only to increase spawning biomass but also for overall stock health and its conservation.

Sex-specific effects of fisheries and climate on the demography of sexually dimorphic seabirds.

Many animal taxa exhibit sex-specific variation in ecological traits, such as foraging and distribution. These differences could result in sex-specific responses to change, but such demographic effects are poorly understood. Here, we test for sex-specific differences in the demography of northern (NGP, Macronectes halli) and southern (SGP, M.giganteus) giant petrels - strongly sexually size-dimorphic birds that breed sympatrically at South Georgia, South Atlantic Ocean. Both species feed at sea or on carrion on land, but larger males (30% heavier) are more reliant on terrestrial foraging than the more pelagic females. Using multi-event mark-recapture models, we examine the impacts of long-term changes in environmental conditions and commercial fishing on annual adult survival and use two-sex matrix population models to forecast future trends. As expected, survival of male NGP was positively affected by carrion availability, but negatively affected by zonal winds. Female survival was positively affected by meridional winds and El Niño-Southern Oscillation (ENSO), and negatively affected by sea ice concentration and pelagic longline effort. Survival of SGPs did not differ between sexes; however, survival of males only was positively correlated with the Southern Annular Mode (SAM). Two-sex population projections indicate that future environmental conditions are likely to benefit giant petrels. However, any potential increase in pelagic longline fisheries could reduce female survival and population growth. Our study reveals that sex-specific ecological differences can lead to divergent responses to environmental drivers (i.e. climate and fisheries). Moreover, because such effects may not be apparent when all individuals are considered together, ignoring sex differences could underestimate the relative influence of a changing environment on demography.

Trade-offs for the southern longline fishery in achieving a candidate South Pacific albacore target reference point

South Pacific albacore is a species of primary importance in the longline fishery of a number of Small Island Developing States in the Western and Central Pacific Ocean. Despite the fact that the stock is assessed as not being subject to overfishing and not overfished, economic returns have declined significantly over the past decade. This has led to calls for management intervention. Given stated biological and economic objectives for the fishery, members of the Pacific Islands Forum Fisheries Agency proposed an interim stock target reference point to the Western and Central Pacific Fisheries Commission that would imply a larger stock size, higher catch rates and a more profitable fishery (FFA Members, 2015). The purpose of this study is to examine the biological and economic consequences along the trajectories of two distinct longline effort reduction regimes that achieve the proposed target reference point within 20 years and review the trade-offs in terms of forgone catch or effort and forgone revenue. The two effort regimes examined are a one-off reduction implemented immediately, and a phased reduction under which effort is reduced by a fixed percent each year. The results are discussed in the light of wider Pacific Island objectives for fishery production and fleet profitability and highlights the importance of moving beyond a purely biological stock-based focus when providing management advice.

Spatiotemporal patterns of overlap between short-finned pilot whales and the U.S. pelagic longline fishery in the Mid-Atlantic Bight: An assessment to inform the management of fisheries bycatch

Short-finned pilot whales (Globicephala macrorhynchus) depredate pelagic longlines along the shelf break of the Mid-Atlantic Bight. The mortality and serious injury of short-finned pilot whales in the U.S. pelagic longline fishery recently exceeded Potential Biological Removal levels defined under the U.S. Marine Mammal Protection Act, and bycatch mitigation techniques developed to date have been unsuccessful. We examine the spatial and temporal characteristics of pilot whale habitat use and longline fishing effort, quantify spatiotemporal patterns of pilot whale bycatch based on environmental factors, and assess the potential for a spatial management approach to mitigate pilot whale bycatch. We assess patterns of overlap and bycatch of pilot whales and longlines by applying Area Under the Curve and Williamson’s Spatial Overlap Index analyses to telemetry data from short-finned pilot whales, along with longline fishing effort and Pelagic Observer Program (POP) fisheries observer data from 2014 and 2015. We found that proximity to the 1000 m isobath, season, and sea surface temperature (SST) were important variables influencing pilot whale-longline overlap and POP bycatch rates. Pilot whale density was consistently highest immediately inshore of the 1000 m isobath, but longline effort varied seasonally relative to the 1000 m isobath. Resultant seasonal patterns in pilot whale-longline overlap relative to the 1000 m isobath were strongly and significantly correlated with POP bycatch rates; the highest bycatch rates primarily occurred in fall and winter months, when longline effort shifted inshore near the 1000 m isobath. We observed differences in the distribution of logbook and POP longline sets relative to the 1000 m isobath; POP sets were more dispersed relative to this feature while the overall distribution of longline effort was typically focused at the 1000 m isobath. Since bycatch primarily occurred close to the 1000 m isobath, more bycatch might be observed if the observer effort better reflected the overall distribution of longline effort. In winter months, POP bycatch occurred in cooler waters than most observations of tagged pilot whales, and therefore the relationship between bycatch and SST during winter months requires further exploration. Together, our results suggest that a spatial management approach could be effective in reducing pilot whale bycatch in the pelagic longline fishery, and an improved understanding of the relationships between pilot whale bycatch and dynamic variables might allow high-risk regions for pilot whale bycatch to be further delineated.

Factors related to the decline and rebuilding of billfish stocks in the Atlantic and Indian oceans

Abstract The article examines factors related to the decline and rebuilding of billfish stocks in the Atlantic and Indian oceans. Longline effort has declined over the last 10–15 years in both oceans. This decline in fishing pressure has led to the recovery of some stocks, but some species that are caught incidentally in industrial longline fisheries remain overexploited. Using a simple moving average technique on fishing mortality trajectories, we estimated a threshold effort size of 240 million hooks for the Atlantic Ocean and 364 million hooks for the Indian Ocean where stocks start experiencing overfishing. In addition, we highlight differences in the economic characteristics of the major fleets catching billfish in the two oceans and discuss how this may be associated with differences in management, enforcement, and stock rebuilding.

The distribution of breeding Sooty Albatrosses from the three most important breeding sites: Gough, Tristan and the Prince Edward Islands

ABSTRACTSooty Albatrosses (Phoebetria fusca; Endangered) breed only on sub-Antarctic islands in the South Atlantic and south-west Indian Oceans, with most of the population at Gough Island (≈37%), the Prince Edward Islands (≈24%) and the Tristan da Cunha archipelago (≈20%). Breeding Sooty Albatrosses from all three of these populations were tracked during the incubation and brood-guard periods. Birds from Marion Island (Prince Edwards) ranged farther north, despite being the most southerly of the three study sites. Tristan-Gough Sooty Albatrosses concentrated mostly around the Sub-Antarctic Front (SAF) in the southern Atlantic Ocean, whereas Marion birds were associated with both the SAF and the Sub-Tropical Front (STF) in the southern Indian Ocean. Our tracking data describe where 80% of breeding Sooty Albatrosses forage during the incubation and brood-guard period, including the first records of birds from Marion and Tristan. Such data are important to identify key areas where these threatened birds need protection from mortality on long-line fishing gear. Overlap with the distribution of tuna long-line effort was greater for Sooty Albatrosses from Tristan da Cunha and Gough Island than for Marion birds, suggesting that birds breeding at Atlantic colonies might be at greater risk of bycatch mortality in this fishery.

ROLE OF SUB SURFACE TEMPERATURE, SALINITY AND CHLOROPHYLL TO ALBACORE TUNA ABUNDANCE IN INDIAN OCEAN

The swimming layer is one of the important factors to get maximum catches, especially on tuna longline effort. The vertical abundance of the albacore tuna was investigated based on catch data and 3-DINDESO Ocean Model data, such as sub-surface conditions of sea water potential temperature (Temp), salinity (Sal) and mass concentration of diatoms and flagellates expressed as chlorophyll (Chl) in the Eastern Indian Ocean period 2014-2015. Combining he statistical method of generalized additive model (GAM) was performed to analysis in this study. There were seven GAM models that generated with the number of ALB vertical abundance as a response variable, and Temp, Sal, and Chl as predictor variables. Sal has highly significant (P < 0.001) while Chl and Temp significant (P < 0.01) to ALB vertical abundance. Deduced from GAMs, indicated that a negative effect of Sal on the number of ALB was observed at salinity >34.52 psu. There was a positive effect of salinity on the number of ALB, which was from 34.30 to 34.47 psu and Chl showed a positive effect of this variable on the number of ALB caught occurred between 0.01 mg/m3 and 0.12 mg/m3 in the region of high confidence level where negative effect on > 0.13 mg/m3. While ALB catches abundance varied in the temperature range with the highest frequency at 24.0-24.9 °C. Sal was the most important environmental variable to the number of ALB vertically caught, followed by Chl and Temp.

Sex‐related variation in the vulnerability of wandering albatrosses to pelagic longline fleets

AbstractThe population of wandering albatrosses Diomedea exulans at South Georgia is decreasing because of bycatch in longline fisheries. Until at least the early 1990s, the survival rate of females was lower than males, consistent with the adult female‐biased bycatch reported for fisheries operating around the Brazil‐Falklands Confluence (BFC). Here we use extensive tracking data (1990–2012) from breeding birds at South Georgia to investigate overlap with longline fishing effort reported to the International Commission for the Conservation of Atlantic Tunas (ICCAT). Using data from multiple years, we conclude that breeding females are at higher risk than males from all the main pelagic longline fleets in the south‐west Atlantic. Our overlap index (based on fishing effort and bird distributions) correlated positively with numbers of ringed birds reported dead on longliners, indicating that the metric was a good proxy of bycatch risk. The consistent sex bias in overlap across years, and the likely resulting sex‐biased mortality, could account for lower adult female survival rate at the colony. The risk from fisheries changed seasonally; both sexes overlapped with pelagic longline effort during incubation (January–March), and particularly during post‐brood chick‐rearing (May–December), whereas overlap was negligible during brooding (April). The highest percentage of overlap was with the Taiwanese fleet, then vessels flagged to Brazil, Uruguay, Spain, Japan and Portugal. Females were consistently at greatest risk in the BFC region, whereas males showed lower and more variable levels of overlap with fisheries from 35 to 45°S. Our results have important implications for management of ICCAT longline fisheries and conservation of this highly threatened albatross population.

Environmental associations with broad‐scale Japanese and Taiwanese pelagic longline effort in the southern Indian and Atlantic Oceans

AbstractTunas are globally important to fisheries because of their broad distribution and high market value. However, these characteristics also complicate their management, owing to difficulties estimating population size and sustainable harvest levels. Fishing effort data can be difficult to interpret, as the magnitude and distribution of effort are the result of multiple factors, including environmental variation and targeting different species. Yet, resolving patterns in fishing effort may provide information enabling the setting of sustainable harvest levels, the management of bycatch and the projection of potential responses to environmental change. To better understand the allocation of fishing effort, we evaluated Japanese and Taiwanese longline effort in the southern Indian and Atlantic Oceans and their relationships to the environment using generalized additive models. The variables evaluated included sea surface temperature (SST), sea ice extent, climate indices, and seasonal cycles, and the results were interpreted in physical, ecological, and management contexts. Our findings indicated hierarchical relationships are impacting fishing effort, with a seasonal cycle an important component of most models, which we hypothesize relates to variation in species targeting. This was often followed by finer‐scale environmental indices, including isotherms and sea ice, indicating region‐specific dynamics impacting the magnitude and distribution of fishing effort. Additionally, effort in some regions was associated with climate indices, highlighting interannual variation. This first quantitative description of the environmental associations of multi‐species tuna fleets in the Southern Ocean is a step towards an improved understanding of fleet behaviour.

PRODUCTIVITY AND ECONOMIC ANALYSIS OF THE INDIAN OCEAN LONGLINE FISHERY LANDED AT BENOA PORT BALI INDONESIA

This study highlighted the occurrence of productivity and economic analysis of Indian Ocean longline fishery which was landed in Benoa port Bali Indonesia. The aim of this study is to determine feasibility of tuna longline effort based on business analysis and current condition. The data used in this study based on the Research Institute for Tuna Fisheries (RITF) observer program in Benoa from 2010-2011. This paper presents the current information on Catch per Unit of Effort (CPUE) and feasibility analysis based on the recent economic parameters. The CPUEs of tuna longline vessel in 2010 and 2011 respectively were 288.35 kg/effort and 281.97 kg/effort. The feasibility analysis of Indian Ocean tuna longline effort showed that tuna longline efforts remains profitable<br />and feasible with payback periods (year-3, month - 2 and day- 18), internal rate of return (53%), average rate of return (61.24%) and net present value between Rp 1.709.897.950,- (first year) and Rp 85.331.099.211,- (at the end of 25 years).

Comparative Influence of Ocean Conditions on Yellowfin and Atlantic Bluefin Tuna Catch from Longlines in the Gulf of Mexico

Directed fishing effort for Atlantic bluefin tuna in the Gulf of Mexico (GOM), their primary spawning grounds in the western Atlantic, has been prohibited since the 1980s due to a precipitous decline of the spawning stock biomass. However, pelagic longlines targeted at other species, primarily yellowfin tuna and swordfish, continue to catch Atlantic bluefin tuna in the GOM as bycatch. Spatial and temporal management measures minimizing bluefin tuna bycatch in the GOM will likely become important in rebuilding the western Atlantic bluefin stock. In order to help inform management policy and understand the relative distribution of target and bycatch species in the GOM, we compared the spatiotemporal variability and environmental influences on the catch per unit effort (CPUE) of yellowfin (target) and bluefin tuna (bycatch). Catch and effort data from pelagic longline fisheries observers (1993–2005) and scientific tagging cruises (1998–2002) were coupled with environmental and biological data. Negative binomial models were used to fit the data for both species and Akaike's Information Criterion (corrected for small sample size) was used to determine the best model. Our results indicate that bluefin CPUE had higher spatiotemporal variability as compared to yellowfin CPUE. Bluefin CPUE increased substantially during the breeding months (March-June) and peaked in April and May, while yellowfin CPUE remained relatively high throughout the year. In addition, bluefin CPUE was significantly higher in areas with negative sea surface height anomalies and cooler sea surface temperatures, which are characteristic of mesoscale cyclonic eddies. In contrast, yellowfin CPUE was less sensitive to environmental variability. These differences in seasonal variability and sensitivity to environmental influences suggest that bluefin tuna bycatch in the GOM can be reduced substantially by managing the spatial and temporal distribution of the pelagic longline effort without substantially impacting yellowfin tuna catches.

Use of shark fin trade data to estimate historic total shark removals in the Atlantic Ocean

In order to address critical gaps arising from limited available data on historic shark catches in the Atlantic Ocean, a method was developed to estimate shark removals using shark fin trade data. A characterization of the global fin trade as of 2000, including number and biomass by shark species, was used as the basis of the methodology. A first step involved scaling Hong Kong trade-derived estimates for 2000 to annual global values for 1980-2006 based on the observed quantity of imports to Hong Kong and an approximation of Hong Kong's share of the global trade in each year. The resulting global fin trade figures for each year were then scaled to Atlantic-specific values using three different factors: (1) area of the Atlantic range relative to the global range of pelagic sharks; (2) Atlantic catches of tunas and billfishes relative to global catches of tunas and billfishes; and (3) Atlantic longline effort relative to global longline effort. The strengths and weaknesses of each scaling factor and the assumptions inherent in the methodology are discussed. These estimates are not intended to replace reliable fisheries dependent catch data compiled by ICCAT from submissions of members and cooperators, but can serve as one of a variety of useful cross-validation tools when historic catch data are missing or uncertain.

Bycatch of loggerhead sea turtles: insights from 14 years of stranding data

We present a detailed analysis of sea turtle strandings (n = 619) over a 14 yr period (1993 to 2006) from the Valencian Community (eastern Spain). Loggerhead turtles Caretta caretta accounted for 98.1% of recorded strandings. Although we detected an increasing trend in the annual number of strandings over the study period, we expect that this was driven primarily by the increase in detection effort. Loggerhead turtles were mainly juveniles (mean ± SD curved carapace length from notch to tip (CCL) = 53.6 ± 12.6; range: 16 to 80.2 cm, n = 312) and strandings were far more fre- quent in summer months (69.6% June to September). We believe that interaction with longline fish- eries was by far the main cause of observed stranding (>28% of all cases, 43.5% of 393 turtles with likely mortality cause identified). Turtles showing signs of interaction with longlines were, on aver- age, larger (CCL = 57.5 ± 10.4; range: 29.8 to 80.2 cm, n = 116) than turtles stranded due to other causes (CCL = 51.4 ± 13.3; range: 16 to 79 cm, n = 196; t-test: t = -4.49, p < 0.001) and were more fre- quent in summer months, when longline fishing effort off the coast was highest. Recent reductions in longline effort may have led to a decrease in recent years in the proportion of stranded turtles with evidence of longline interaction. Although inferences from stranding data must be subject to a num- ber of caveats, when considered over wide spatio-temporal extents and in conjunction with other data sources, they can offer useful insights into the geographic ranges, seasonal distribution and life history of marine species of conservation concern.

Sea turtle by-catch in pelagic longline sets off southern Brazil

Results are presented on sea turtle by-catch observed in pelagic longline sets during research cruises in the south Atlantic in the vicinity of the shelf edge break in southern Brazil. The longline gear was set in a similar manner to that employed by the commercial longline fleet operating out of Santos, Brazil that targets swordfish, Xiphias gladius. Nineteen sea turtles were caught on 14 of 41 sets, which constituted a catch rate of 1.5 turtles per 1000 hooks. Thirty four percentage of all sets captured at least one turtle. The catch included loggerhead, Caretta caretta, leatherback, Dermochelys coriacea, and most likely olive ridley, Lepidochelys olivacea. All turtles, except one loggerhead, were captured live and released. Captures occurred during spring, summer and winter suggesting a year around presence and vulnerability. Given increased levels of longline effort directed at swordfish in the southeast Atlantic, substantial turtle by-catch can be expected. There is a critical need for additional information to be able to assess the potential impact on swordfish longline fisheries on sea turtle populations.

Spatio-temporal trends of longline fishing effort in the Southern Ocean and implications for seabird bycatch

Longline fisheries have expanded throughout the world's oceans since major commercial distant-water pelagic fleets began fishing for tuna and tuna-like species in the early 1950s. Along with the more recent development and expansion of demersal longline fleets for species such as Patagonian toothfish, these vessels are a major source of mortality to several species of seabird. Vessels can set many thousands of baited hooks in a day across many kilometres of water. These waters are often used as foraging areas by wide-ranging seabirds. Attracted by baits and offal, the birds can be caught on the baited hooks and subsequently drown. To provide a greater understanding of the potential impact of the Southern Ocean's longline fleets on seabird populations, this paper describes the trends in longline effort of the major pelagic and demersal fisheries in southern waters. The total reported effort from all longline fleets south of 30°S has been well over 250 million hooks per year since the early 1990s. However the spatial and temporal distribution of this effort has not been constant. While effort from the Japanese pelagic distant-water longline fleet declined through the 1990s, the Taiwanese fleet expanded dramatically. Likewise demersal fishing for toothfish increased markedly during the mid-1990s. These fisheries, along with substantial illegal longline fisheries, may be placing the long-term viability of many Southern Ocean species of seabird in jeopardy.

Use of charter boat data to infer the annual availability of black marlin, Makaira indica, to the recreational fishery off Cairns, Australia

Catch data, recorded in daily diaries kept by the captains of charter boats in the recreational fishery that targets black marlin (Makaira indica) in the Cairns/Lizard Island region off north-eastern Australia, were used to calculate annual indices of fishing success (catch per day). Generalized Linear Models were used to investigate factors likely to influence catch rates, such as captain, area fished, water temperature, moon phase and the strength and direction of prevailing currents. Despite a high degree of inter-annual variability in the catch, results indicate that there has been a long-term decline of between 20–30% in estimated availability between 1970 and 1997. Models fitted to the data for the period 1987–1996 indicate that several environmental factors can have a significant influence on availability in any given year, with higher catch rates occurring close to a half moon and associated with water temperatures around 26.0–26.5°C. Catch rates for the period 1980–1997 were also found to be negatively correlated with longline effort levels within the inshore region close to the recreational fishery, though this correlation was not found to be significant.

Stock assessment of blue marlin (Makaira nigricans) in the Pacific using MULTIFAN-CL

In the Pacific, blue marlin are an incidental catch of longline fisheries and an important resource for big game recreational fishing. Over the past two decades, blue marlin assessments by different techniques have yielded results ranging from an indication of declining stock to a state of sustained yield at approximately the maximum average level. Longline fishing practices have changed over the years since the 1950s in response to changes in principal target species and to gear developments. Despite increasingly sophisticated attempts to standardize fishing effort with changing fishing practices, the stock assessments to date are likely confounded to a greater or lesser degree by changes in catchability for blue marlin. Yet, only data from commercial longline fisheries targeting tuna provide sufficient spatial and temporal coverage to allow assessment of this resource. To re-assess the blue marlin stocks in the Pacific and also to assess the efficacy of a habitat-based standardization of longline effort, a collaborative analysis was conducted involving scientists at the National Research Institute of Far Seas Fisheries, Shimizu, Japan, the Inter-American Tropical Tuna Commission, La Jolla, California, and the NOAA Fisheries Honolulu Laboratory, Honolulu, Hawaii. Using MULTIFAN-CL as an assessment tool, there was considerable uncertainty in quantifying the fishing effort levels that would produce a maximum sustainable yield. However, it was found that, at worst, blue marlin in the Pacific are close to a fully exploited state, that is the population and the fishery are somewhere near the top of the yield curve. Furthermore, it was found that effort standardization using a habitat-based model allowed estimation of parameters within reasonable bounds and with reduced confidence intervals about those values.

Application of a habitat‐based model to estimate effective longline fishing effort and relative abundance of Pacific bigeye tuna (Thunnus obesus)

A new habitat‐based model is developed to improve estimates of relative abundance of Pacific bigeye tuna (Thunnus obesus). The model provides estimates of `effective' longline effort and therefore better estimates of catch‐per‐unit‐of‐effort (CPUE) by incorporating information on the variation in longline fishing depth and depth of bigeye tuna preferred habitat. The essential elements in the model are: (1) estimation of the depth distribution of the longline gear, using information on gear configuration and ocean currents; (2) estimation of the depth distribution of bigeye tuna, based on habitat preference and oceanographic data; (3) estimation of effective longline effort, using fine‐scale Japanese longline fishery data; and (4) aggregation of catch and effective effort over appropriate spatial zones to produce revised time series of CPUE. Model results indicate that effective effort has increased in both the western and central Pacific Ocean (WCPO) and eastern Pacific Ocean (EPO). In the WCPO, effective effort increased by 43% from the late 1960s to the late 1980s due primarily to the increased effectiveness of effort (deeper longline sets) rather than to increased nominal effort. Over the same period, effective effort increased 250% in the EPO due primarily to increased nominal effort. Nominal and standardized CPUE indices in the EPO show similar trends – a decline during the 1960s, a period of stability in the 1970s, high values during 1985–1986 and a decline thereafter. In the WCPO, nominal CPUE is stable over the time‐series; however, standardized CPUE has declined by ∼50%. If estimates of standardized CPUE accurately reflect relative abundance, then we have documented substantial reductions of bigeye tuna abundance for some regions in the Pacific Ocean. A decline in standardized CPUE in the subtropical gyres concurrent with stability in equatorial areas may represent a contraction in the range of the population resulting from a decline in population abundance. The sensitivity of the results to the habitat (temperature and oxygen) assumptions was tested using Monte Carlo simulations.

Pelagic shark bycatch in the New Zealand tuna longline fishery

Tuna longline effort declined from 23—26 million hooks per year in 1979mdash;82 to 2mdash;4 million hooks per year in 1995mdash;98. Scientific observer coverage averaged 7.5%since 1988mdash;89, but increased in 1992mdash;93 (mean 23%). Observed catch per unit effort (CPUE) and the numbers of hooks set were used to estimate shark catches. Between 1988mdash;89 and 1997mdash;98,about 450 000 blue sharks (Prionace glauca), 65 000 porbeagles (Lamna nasus ) and 25 000 shortfin makos (Isurus oxyrinchus) were caught. In 1997mdash;98,about 45 000 blue sharks, 4000 porbeagles and 3000 makos were caught. Corresponding weight estimates were 1400 t, 150 t and 200 t. CPUE varied between foreign and domestic vessels, between north and south regions and among years, but there were no consistent temporal trends. Most males and females were immature, and most sharks were alive when recovered. Most sharks were processed, but usually only the fins were retained. The New Zealand tuna longline fishery is probably not seriously affecting pelagic shark stocks, but adequate assessment is not currently feasible. Accurate monitoring of Pacific Ocean catches is an important first step towards ensuring sustainability of their populations.

Turtles on the edge: movement of loggerhead turtles (Caretta caretta) along oceanic fronts, spanning longline fishing grounds in the central North Pacific, 1997–1998

Nine juvenile loggerhead sea turtles tracked during 1997 and 1998 in the central North Pacific by satellite telemetry all travelled westward, against prevailing currents, along two convergent fronts identified by satellite remotely sensed data on sea surface temperature (SST), chlorophyll and geostrophic currents. These fronts are characterized by gradients in sea surface height that produce an eastward geostrophic current, with gradients in surface chlorophyll and SST. Six of the turtles were associated with a front characterized by 17°C SST, surface chlorophyll of about 0.2 mg m–3, and eastward geostrophic current of about 4 cm s–1, while the other three turtles were associated with a front with 20°C SST, surface chlorophyll of about 0.1 mg m–3, and eastward geostrophic flow of about 7 cm s–1. These results appear to explain why incidental catch rates of loggerheads in the Hawaii longline fishery are highest when gear is set at 17°C and 20°C, SST. Further, from the seasonal distribution of longline effort relative to these fronts, it appears that the surface longline fishing ground lies largely between these two fronts during the first quarter and well to the south of the 17°C front, but including the 20°C front, in the second quarter. These findings suggest seasonal or area closures of the longline fishery that could be tested to reduce incidental catches of loggerheads. Finally, these results illustrate the insights which can be achieved by combining data on movement of pelagic animals with concurrent remotely sensed environmental data.