Longline Sets Research Articles

Impacts of fisheries bycatch on loggerhead turtles worldwide inferred from reproductive value analyses

1. Fisheries bycatch has been implicated in declines of many long-lived marine vertebrate populations, but bycatch impacts on these populations vary according to spatio-temporal overlap in fisheries operations and critical ontogenetic habitat, as well as to characteristics of fishing gear. 2. To provide a framework for comparing the relative impacts of different fisheries on populations of loggerhead sea turtles Caretta caretta, we compiled published data on sizes of individual loggerheads taken as bycatch in North Atlantic, North Pacific and Mediterranean fisheries, and used Leslie matrix models to calculate reproductive values (RVs) for bycatch samples of loggerheads within these basins. 3. Sizes and RVs of loggerheads varied significantly based on spatial overlap in fisheries and ontogenetic habitat as well as on fishing gear. Thus, fisheries operating in areas occupied by larger, older turtles (e.g. trawls in neritic areas) tended to interact with more reproductively 'valuable' turtles than fisheries that operated in areas occupied by smaller, younger turtles (e.g. oceanic and pelagic longlines). 4. We also found evidence of size-selectivity among different fishing gears (e.g. wider size variation among loggerheads taken in driftnets and trawls than in longlines) and gear configuration (e.g. smaller loggerheads in shallow longline sets using small hooks). 5. These results suggest that evaluation of fishery impacts on marine megafauna require characterization of fishery activities; understanding of species biology must be considered in order to determine population impacts of fisheries bycatch. Data access and quality can be improved and uncertainty reduced by increasing independent observer coverage on fishing vessels throughout the world's oceans. 6. Syntheses and applications. Our analyses demonstrate that application of reproductive values can allow fisheries managers and biologists alike to identify the most influential bycatch threats to geographically widespread populations of long-lived marine vertebrates, thereby facilitating prioritization of conservation actions and successful management of these animals. For example, our results suggest that effective management of loggerhead catch in trawl gear should be one of the top priorities for conservation of loggerhead populations worldwide.

Field investigation of rare-earth metal as a deterrent to spiny dogfish in the Pacific halibut fishery

Spiny dogfish ( Squalus acanthias) comprise a significant unwanted bycatch on demersal longlines set for halibut, sablefish ( Anoplopoma fimbria), and Pacific cod ( Gadus macrocephalus) in shelf waters of the east and west coasts of North America. Recently, rare-earth magnets and metals have been shown to have deterrent effects on sharks. These effects are likely the result of magnetic or electric fields created by these materials in seawater, which are sensed and avoided by sharks. Our earlier laboratory studies showed that attack rates by spiny dogfish on baits protected with cerium mischmetal (a rare-earth alloy) were reduced and suggested that this metal might reduce unwanted bycatch of spiny dogfish in setline fishing for Pacific halibut ( Hippoglossus stenolepis). We conducted a field study near Homer, Alaska in September 2007 with three hook treatments interspersed on 36 longline sets. These included standard circle hooks used in the halibut fishery, hooks with small pieces of cerium mischmetal attached above the hook, and hooks with a similar (but inert) mild steel piece above the hook. Fewer dogfish were caught on hooks with mischmetal than on either of the two other treatments. Reductions in catch of longnose skate ( Raja rhina) also occurred on hooks protected with mischmetal. However, halibut catch did not increase with protected hooks. Limitations in using mischmetal in commercial operations are expense, hazardous nature, and relatively rapid hydrolysis in seawater.

Pupping areas and mortality rates of young tiger sharks Galeocerdo cuvier in the western North Atlantic Ocean

From 1995 through 2006, 2577 bottom longline sets were conducted in the western North Atlantic Ocean from ~36° N, 75° W to 26° N, 97° W, an area that encompasses both the coastal waters off the southeastern USA (Atlantic) and the northern Gulf of Mexico (Gulf). Over the course of the study, 335 young-of-the-year (YOY) and 219 juvenile tiger sharks Galeocerdo cuvier were cap- tured. In the Gulf, YOY catch per unit effort (CPUE) was highest from ~88 to 83° W. In the Atlantic, the area of highest YOY CPUE occurred from 31 to 33° N. Distribution of juvenile tiger sharks was more uniform than observed for YOY throughout the range of the study. Annual survival rates of YOY and Age 1+ tiger sharks were estimated to be 51 and 62% in the Gulf and 39 and 27% in the Atlantic. Total instantaneous mortality rates of YOY and Age 1+ tiger sharks were, respectively, estimated to be 0.67 and 0.47 in the Gulf and 0.93 and 1.32 in the Atlantic. In the Atlantic, age-specific natural mortality (M) estimates ranged from 0.12 to 0.50 and from 0.08 to 0.36 for YOY and juvenile tigers sharks, respectively. Estimates of age-specific M were more variable in the Gulf, with values ranging from 0.11 to 0.98 for YOY and from 0.08 to 0.57 for juveniles. In the absence of nursery areas, the pro- duction of numerous fast-growing offspring presumably helps the species survive in the face of high total mortality.

Seabird bycatch mitigation and blue-dyed bait: A spectral and experimental assessment

The application of blue-dye to fishing baits is a seabird bycatch mitigation technique used in some pelagic longline fisheries that is thought to make the baits less visible and hence less attractive to seabirds. We tested this assumption in two ways. First, by measuring the spectral profiles of blue-dyed baits (fish and squid) and modelling the spectral profiles of the ocean under set conditions, we assessed how well wedge-tailed shearwaters (Puffinus pacificus) can distinguish dyed baits based on the known visual characteristics of this species. Results showed that no baits were perfectly cryptic against the background ocean, and only blue-dyed squid were relatively cryptic both in terms of chromatic and achromatic contrasts. Second, during at-sea trials blue-dyed and non-dyed baits that were simultaneously presented submerged on a longline or as surface presentations. During 26 longline sets which presented squid only, a 68% reduction in interactions with blue-dyed squid was observed compared to non-dyed squid. During surface presentations only 3–8% of blue-dyed squid baits were struck over the duration of the study compared with 75–98% of non-dyed squid bait. When using fish baits, however, approximately 48% of all blue-dyed baits presented in the first two days of trials received strikes from seabirds but this increased to 90% over the last three days. These results suggest the use of blue-dyed squid bait could decrease seabird bycatch in pelagic longline fisheries whereas blue-dyed fish baits are less likely to have a mitigatory effect.

First characterization of shark nursery habitat in the United States Virgin Islands: evidence of habitat partitioning by two shark species

Little is known of the diversity, demography, and essential fish habitat of sharks within the United States Virgin Islands (USVI) marine ecosystem. To examine species diversity and the relative abundance of elasmobranchs in this region, bottom-longline and hand-gear sampling was conducted in Fish Bay, St. John, USVI, from June 2004 to December 2005. In the 8 sampling trips during this period, 54 standardized longline sets caught 174 elasmobranchs comprising 5 species of sharks and 1 batoid. Overall catch per unit effort [ln(CPUE + 1) ± SE] was 1.83 ± 0.16 elasmobranchs 100 hooks–1 h–1. Lemon sharks Negaprion brevirostris had the highest relative abundance based on log-transformed CPUE data (0.98 ± 0.15), followed by blacktip sharks Carcharhinus limbatus (0.91 ± 0.18), southern stingrays Dasyatis americana (0.28 ± 0.08), nurse sharks Ginglymostoma cirratum (0.08 ± 0.05), blacknose sharks Carcharhinus acronotus (0.06 ± 0.04) and the Caribbean sharpnose Rhizoprionodon porosus (0.03 ± 0.03). The relative abundance of all species was significantly higher in the summer (2.6 ± 0.2) than during the winter (1.1 ± 0.2). For the blacktip (N = 89 captures of 74 individuals) and lemon (N = 66, 48 individuals) sharks, which comprised the bulk of the catch, mean fork length (± SE) was 51.9 ± 0.63 cm and 59.9 ± 1.2 cm, respectively, representing primarily neonatal and young-of-the-year life stages. The recapture rates for blacktip and lemon sharks were 21% and 29%, respectively, and nearly all recaptures occurred within the bay, indicating a high degree of site fidelity. Capture information and limited acoustic tracking provided evidence of spatial and temporal habitat partitioning by these 2 shark species within the bay. Although the CPUE of both species was highest over shallow (<1 m) seagrass substrate, lemon sharks were found and tracked exclusively on shallow, mangrove-fringed seagrass habitat, while blacktip sharks utilized a wider area of the bay. Fish Bay was determined to provide important nursery habitat for young juvenile lemon and blacktip sharks in the USVI.

Spatial distribution ofCubiceps pauciradiatus(Perciformes: Nomeidae) in the tropical Indian Ocean and its importance in the diet of large pelagic fishes

The bigeye cigarfish (Cubiceps pauciradiatus) is a small pelagic nomeid of the tropical world ocean, often recovered in the stomach contents of top predators such as tunas, billfishes and marine mammals. In the Indian Ocean, a few studies have investigated the biology and the ecology of this species that is one of the most abundant fish of the intermediate trophic levels. In this paper, we investigated the spatial distribution of C. pauciradiatus in the Indian Ocean using pelagic trawl catches carried out between 20°N and 45°S, and the importance of bigeye cigarfish in the diet of 9 piscivorous fishes sampled by different fishing gears in the western part of the Indian Ocean. The highest densities were observed along the eastern coast of Africa and in the Arabian Sea (87 000 individuals per square nautical mile) during the South-West Monsoon and in the eastern part of the Seychelles archipelago (62 200 ind. square nmi) during the North-East Monsoon. Small sized bigeye cigarfish (20–80 mm SL) was a regular and abundant prey (20 to 200 ind. per stomach) for schooling predators exploited by purse seine fishery such as large yellowfin and bigeye tunas chasing prey near the sea surface. Large sized bigeye cigarfish (61–150 mm SL) occurred in small numbers (3 to 20 ind. per stomach) in the stomach contents of swordfish and of large yellowfin and bigeye tunas caught by longline sets at great depths. Large concentrations of bigeye cigarfish occurred in zones of high productivity, and that species constituted seasonally a strong link in the transfer of energy from low to high trophic levels in this part of the Indian Ocean.

Measuring the Length of a Pelagic Longline Set: Applications for Management

AbstractRecent U.S. federal fisheries management actions aimed at reducing bycatch have established or proposed limits on the length of a pelagic longline set. However, such management measures are unenforceable if the criteria for measuring the length of a pelagic longline set have not been defined. Differences in the physical gear and deployment strategies suggest high variability among the several methods that can be used to measure set length. This study uses a combination of scientifically monitored sets, fisheries observer data, and federal pelagic logbook data to compare four possible metrics of longline set length: the geographic track of the vessel while deploying the gear, a “folding ruler” approximation based on straight‐line distance between ends of the sections of gear, the straight‐line distance between the beginning and end of the gear, and a dead‐reckoning distance based on vessel speed and the time duration of the gear deployment. Results indicate that while set lengths determined from self‐reported data via logbook submissions are not significantly different from the other proposed methods, the inclusion of an ending location for each set within the logbook form could provide a basis for evaluation of the regulatory measure.

TurtleWatch: a tool to aid in the bycatch reduction of loggerhead turtles Caretta caretta in the Hawaii-based pelagic longline fishery

Operational longline fishery characteristics, bycatch information, and loggerhead turtle satellite tracks were all used in conjunction with remotely sensed sea surface temperature data to identify the environmental area where the majority of loggerhead turtle bycatch occurred in the Hawaii-based longline fishery during 1994 to 2006. In the first quarter of each calendar year from 1994 to 2006, the majority of shallow longline sets and associated loggerhead turtle bycatch were above 28° N, which corresponds to the area near the North Pacific Subtropical Frontal Zone. Based on the thermal ranges of bycatch, sets and the satellite-tagged turtles, it was recommended that shal- low sets should only be deployed in waters south of the 18.5°C (~65.5°F) isotherm to decrease logger- head turtle bycatch. This recommendation formed the basis for the TurtleWatch tool, a map provid- ing up-to-date information about the thermal habitat of loggerhead sea turtles in the Pacific Ocean north of the Hawaiin Islands. TurtleWatch was released to fishers and managers in electronic and paper formats on December 26, 2006, to assist in decision making during the first quarter of 2007. Fishery information from 2007 was later compared with data for the years 2005 to 2006 to assess the response of the fishery to TurtleWatch. The observed fleet movement during the first quarter of 2007 was to the north of the 18.5°C (~65.5°F) isotherm (i.e. in the area recommended for avoidance by the TurtleWatch product) with increased effort and lower bycatch rates. We discuss possible reasons for this decrease in turtle bycatch north of the frontal zone together with future research directions which may lead to refinement of the TurtleWatch product.

Occurrence of deep-water corals on the Mid-Atlantic Ridge based on MAR-ECO data

Abstract Occurrence of deep-water corals on the Mid-Atlantic Ridge between the southern part of the Reykjanes Ridge and the Azores has been examined based on video surveys using remotely operated vehicles (ROV) and bycatch from longline and bottom trawl. Eight sites were surveyed with ROVs, and the bycatch material came from 16 trawl hauls and nine longline sets. Corals were observed at all sites surveyed with ROVs at depths between 800 and 2400 m, but most commonly shallower than 1400 m. The species richness of corals was high, with a total of 40 taxa recorded. Octocorals dominated the coral fauna with 27 taxa. Lophelia pertusa was one of the most frequently observed corals, present at five of the eight surveyed sites. It occurred on basaltic outcrops on the seamounts but always as relatively small colonies (

Species composition and distribution patterns of fishes captured by longlines on the Mid-Atlantic Ridge

Abstract During the 2004 MAR-ECO expedition to the Mid-Atlantic Ridge, the M/S Loran deployed longlines and caught a total of 8518 fish, representing 40 species and 17 families. The 59 longline sets were distributed across the ridge axis at depths ranging from 400 to 4300 m within two sub-areas, i.e. just north of the Azores archipelago and in the Charlie–Gibbs Fracture Zone (CGFZ). Overall, chondrichthyans dominated the catches and contributed nearly 60% in terms of both weight and numbers. This was mainly due to the dominance of Etmopterus princeps in both sub-areas. Multidimensional scaling using species-by-station data indicated an assemblage distribution related primarily to factors varying by depth and latitude. Grouping patterns of stations were not very pronounced, suggesting a gradual spatial change rather than abrupt changes in species composition by depth or latitude. Catch rates peaked at the shallower stations in the CGFZ sub-area, and generally decreased with depth. Relatively large individuals dominated, and the overall mean weight was 2.4 kg. Average fish weight was lower in the CGFZ sub-area than in the southern sub-area. No depth-related pattern was found.

Use of Catenary Geometry to Estimate Hook Depth during Near‐Surface Pelagic Longline Fishing: Theory versus Practice

AbstractManagement and conservation of many highly migratory fish species are based on population assessments that rely heavily on catch and effort data from the pelagic longline fishing industry. In 2003, we monitored hook time at depth for shallow‐set commercial longlines (i.e., four hooks between surface buoys) targeting swordfish Xiphias gladius in the Windward Passage between Haiti and Cuba. We deployed temperature–depth recorders (TDRs) on about every 13th hook and attached them to branchlines just above the hook. Most TDRs were placed on branchlines that were predicted by catenary geometry to be at the deepest hook position between floats. Additional TDRs were also placed at the shallowest predicted hook position. We monitored 10 pelagic longline sets with a length (mean ± SE) of 44.9 ± 2.0 km. Time at depth for each TDR was binned into 5‐m depth intervals. The expected bimodal distributions of hook time at depth were not observed; modes were 40 m for both the shallowest and deepest predicted hook positions. The majority of the hook depth distributions for shallow and deep hook positions achieved only 43% and 31%, respectively, of the depths predicted by catenary equations (i.e., &lt;92 and &lt;127 m). Individual TDRs were poor estimators of hook time at depth for other TDRs in the same catenary hook position during the same set (significant mean depth differences = 76.2–100%) and were even worse predictors of the depths fished during other sets (significant mean depth differences = 100%). Hook depth predictions based on catenary geometry drastically overestimated actual fishing depth in this study. These results indicate that the use of catenary geometry for estimating hook depth and subsequent vertical fishing effort is inadequate and fails to capture both within‐ and among‐set variability, potentially resulting in biased stock assessments.

Oceanographic investigation of the American Samoa albacore (Thunnus alalunga) habitat and longline fishing grounds

AbstractThe American Samoa fishing ground is a dynamic region with strong mesoscale eddy activity and temporal variability on scales of &lt;1 week. Seasonal and interannual variability in eddy activity, induced by baroclinic instability that is fueled by horizontal shear between the eastward‐flowing South Equatorial Counter Current (SECC) and the westward‐flowing South Equatorial Current (SEC), seems to play an important role in the performance of the longline fishery for albacore. Mesoscale eddy variability in the American Samoa Exclusive Economic Zone (EEZ) peaks from March to April, when the kinetic energy of the SECC is at its strongest. Longline albacore catch tends to be highest at the eddy edges, while albacore catch per effort (CPUE) shows intra‐annual variability with high CPUE that lags the periods of peak eddy activity by about 2 months. When CPUE is highest, the values are distributed toward the northern half of the EEZ, the region affected most by the SECC. Further indication of the possible importance of the SECC for longline performance is the significant drop in eddy variability in 2004 when compared with that observed in 2003 – resulting from a weak SECC – which was accompanied by a substantial drop in albacore CPUE rates and a lack of northward intensification of CPUE. From an ecosystem perspective, evidence to support higher micronekton biomass in the upper 200 m at eddy boundaries is inconclusive. Albacore's vertical distribution seems to be governed by the presence of prey. Albacore spend most of their time between 150 and 250 m, away from the deep daytime and shallow nighttime sonic scattering layers, at depths coinciding with those of small local maxima in micronekton biomass whose backscattering properties are consistent with those of albacore's preferred prey. Settling depths of longline sets during periods of decreased eddy activity correspond to those most occupied by albacore, possibly contributing to the lower CPUE by reducing catchability through rendering bait less attractive to albacore in the presence of prey.

Assessing swordfish distribution in the South Atlantic from spatial predictions

Generalized Regression Analysis and Spatial Prediction (GRASP) was used to map the spatial distribution of swordfish ( Xiphias gladius) in the South Atlantic. Generalized additive models (GAMs) were used to relate catch to environmental predictor variables. Catch information from 38,000 Brazilian pelagic longline sets from 1980 to 2000 and size frequency data from 5000 longline sets from 1982 to 2000 were obtained from International Commission for the Conservation of Atlantic Tuna (ICCAT). Results highlight the importance of environmental variables for the fishery and for the spatial distribution of different size classes of swordfish (small, intermediate and large). The distribution of swordfish was closely associated with convergence zones (inter-tropical and sub-tropical), especially in the months of greatest convergence intensity. Spatial distribution patterns differed for the three studied size classes. The smallest size classes were found mainly in coastal zones and in areas with a shallow mixed layer (<20 m). In contrast intermediate-sized swordfish were mostly associated with the inter-tropical convergence and mixed layers of more than 20 m depth and large swordfish were often found in the vicinity of the sub-tropical convergence zone and in areas with mixed layers of less than 25 m and greater than 60 m.

Differential distribution within longline transects of loggerhead turtles and swordfish captured by the Spanish Mediterranean surface longline fishery

Surface longline targeting mainly swordfish is considered a major threat for loggerhead turtle populations in the western Mediterranean Sea. For this reason, it is necessary to analyse the relationship between the characteristics of longline sets and loggerhead turtle by-catch. The objective of this study is to look for differential spatial distribution of loggerhead turtle and swordfish captures along the longline to gain insights that might be useful to diminish unintended, incidental catch of turtles without affecting the captures of the target species.Longlines are divided into transects separated by two consecutives buoys. During the 1999 fishing period of the traditional (without roller) Spanish surface drifting longline fleet (August to November), 59 technically homogeneous fishing operations, composed of 23 sections each, were observed. Chi-square test was used to compare loggerhead turtle and swordfish distributions among transects with those expected at random. Significant differences were obtained only for loggerhead turtles, with 93% of the specimens caught in the second half of the longline transects, which were retrieved from the sea during daytime. Significant differences were not found for the swordfish along the whole longline or for loggerhead turtle captures retrieved during daytime. We conclude that swordfish captures are independent of retrieval time whereas the incidental catch of loggerhead turtles occurs mainly during daytime, probably because loggerhead turtles use vision to locate baits.

Killer whale (Orcinus orca) interactions with the tuna and swordfish longline fishery off southern and south-eastern Brazil: a comparison with shark interactions

Depredation by cetaceans and sharks on longline fisheries is a global issue that can have negative impacts on both animals and fisheries and has concerned researchers, managers and the fishing industry. Nevertheless, detailed information on depredation is only available for a few regions where the problem exists. With the purpose of evaluating killer whale depredation on longline-caught tuna (Thunnus spp.) and swordfish (Xiphias gladius) in the waters off southern and south-eastern Brazil and comparing it to shark depredation, data sheets were distributed to the captains of tuna vessels in Santos, south-eastern Brazil, between 1993 and 1995. Data on the catch per unit effort (CPUE) of tuna and swordfish and some records of interactions were also obtained from fishing vessel logbooks. Dockside interviews with fishermen and with researchers on board tuna vessels provided additional information. Killer whale and shark interactions were analysed per longline set and per trip. Killer whale interactions occurred from June to February, mainly between June and October, while shark interactions occurred year round. The number of sets and trips involving shark interactions was significantly higher than the number of sets and trips involving killer whale interactions. However, when depredation occurred, the proportion of fish damaged by killer whales was significantly higher than by sharks. Furthermore, killer whales removed or damaged significantly more hooked swordfish than hooked tuna, whereas sharks damaged significantly more hooked tuna than swordfish. This study also shows that cetacean by-catch is experienced by the tuna and swordfish longline fishery in Brazilian waters.

El Niño effects in the Palmyra Atoll region: oceanographic changes and bigeye tuna (Thunnus obesus) catch rate variability

AbstractA generalized additive model (GAM) was constructed to separate and quantify the effects of fishery‐based (operational) and oceanographic parameters on the bigeye tuna (Thunnus obesus) catch rates at Palmyra Atoll in the central Tropical Pacific. Bigeye catch, the number of hooks per set, and set location from 4884 longline sets spanning January 1994 to December 2003 were used with a temporally corresponding El Niño‐Southern Oscillation (ENSO) indicator built from sea surface height (SSH) data. Observations of environmental data combined with the results from the GAM indicated that there is an increase in bigeye catch rates corresponding to an increase in eastward advection during the winter months of El Niño events. A seasonal pattern with higher bigeye catch rates from December to April and a spatial pattern with higher rates to the northeast and northwest of the atoll were observed during this study period. It is hypothesized that the combination of the eastward advection of the warm pool coupled with vertical changes in temperature during the winter months of El Niño events increases the availability of bigeye tuna in this region. This increase in availability may be due to a change in exploitable population size, location, or both.

Pelagic longlines: A threat to sea turtles in the Equatorial Eastern Atlantic

Pelagic longlines are widely known to interact with several species of sea turtles, and there is an increasing concern about the by-catch of turtles in commercial fisheries and its impact on their populations. However, information on sea turtle by-catch in the South Atlantic Ocean is scarce, and there are no quantitative by-catch data available on olive ridleys for the Equatorial Eastern Atlantic. In this paper we analyze data collected by observers on board an Uruguayan long-liner targeting swordfish in two areas in the Equatorial Eastern Atlantic: off the Gulf of Guinea and north of Saint Helenaa Island. Specimens of Lepidochelys olivacea and Dermochelys coriacea were hooked or entangled in 26 longline sets. All registered interactions with olive ridleys took place off the The Gulf of Guinea, with captures ranging from 1 to 3 specimens in a single set. The captured specimens, though not measured directly, appeared to be juveniles. In addition, the examination of the stomach contents of one female mako shark showed dermal scutes, vertebrae and the complete head of a sea turtle identified as L. olivacea, allowing us to estimate its curved carapace length. In contrast, adult specimens of D. coriacea were caught in the two fishing areas. The capture of 10 individuals in a single set was recorded. Due to the high rate of sea turtle by-catch observed off the The Gulf of Guinea (1.02 ind/1000 hooks) conservation programs in the area should take into consideration the possible existence of a developmental and feeding area in this zone. Accordingly, longline fisheries in this area should be monitored and mitigation measures put in place to avoid or minimize damage to the pelagic phase of African populations of sea turtles.

Octopus magnificus predation and bycatch in the trap fishery for spiny lobsters Palinurus gilchristi off South Africa

Octopus magnificus inhabits the deep-shelf waters of southern South Africa, where it is a regular bycatch of the trap-fishery for spiny lobster Palinurus gilchristi. We used observer data collected on commercial lobster vessels to assess octopus size distribution, effects of trap location, soak-time and season on octopus catch rates, prey preference upon entering traps (bait or lobster), and octopus-induced lobster mortality rates. Octopuses were absent from traps set at the Agulhas Bank and east of Algoa Bay, and in the remaining area they were present in 54% of longline sets. Soak-times of <24 h recorded the highest mean catch rates, suggesting that many octopuses enter, feed and escape before traps are retrieved. Octopus catch rates peaked in autumn and ebbed in winter, and gut contents showed that 80% of octopus that had a choice between bait and lobsters in traps, ate the bait first. Lobster catch rates were unaffected by octopus bycatch catch rates, apart from <1% of sets which captured ≥8 octopuses per 100 traps. Octopus-induced lobster mortality rates in traps are estimated at ∼2.1%, and octopus is considered to be an additional product of the fishery, rather than a hindrance.

Pelagic longline gear depth and shoaling

Temperature-depth recorders (TDRs) were attached to pelagic longline gear in the Hawaii-based commercial fishery to obtain actual fishing depths and to test the accuracy of catenary algorithms for predicting fishing depths. Swordfish gear was set shallow by typically deploying four hooks between successive floats. The observed depth of the settled deepest hook had a median value of 60 m for 333 swordfish sets. Tuna longline gear deployed more hooks between floats (mean = 26.8), and the observed median depth of the deepest hook was 248 m ( n = 266 sets). Maximum gear depth was predicted from estimates of the longline sag ratio and catenary algorithms; however, depth was not predicted for all TDR-monitored sets because estimating sag ratios proved problematic. Swordfish sets had less slack in the main line and correspondingly smaller catenary angles (median = 54.2°) than tuna sets (median = 63.7°). Median values of the predicted catenary depth were 123 m for swordfish sets ( n = 203) and 307 m for tuna sets ( n = 198). Shallow swordfish sets reached only ∼50% of their predicted depth, while deeper tuna sets reached about 70%. These values indicated that capture depths using traditional catenary equations may be biased without the benefit of TDRs affixed to longlines. Generalized linear models (GLMs) and generalized additive models (GAMs) were developed to explain the percentage of longline shoaling as a function of predicted catenary depth and environmental effects of wind stress, surface current velocity, and current shear. The GAM explained 67.2% of the deviance in shoaling for tuna sets and 41.3% for swordfish sets. Predicted catenary depth was always the initial variable included in the stepwise process, and the inclusion of environmental information in the GAM approach explained an additional 10–17% of the deviance compared to the GLMs. The explanatory ability of the environmental data may have been limited by the scale of the observations (1° in space; weekly or monthly in time) or the geometric (transverse versus in-line) forcing between the environment and longline set. Longline gear models with environmental forcing affecting shoaling may be improved in future studies by incorporating contemporaneous environmental information, although this may restrict analyses to fine-scale experimental longlines.

Analysis of logbook accuracy for blue marlin (Makaira nigricans) in the Hawaii-based longline fishery with a generalized additive model and commercial sales data

Abstract Blue marlin, Makaira nigricans , catch (number per set) in the Hawaii-based longline fishery from March 1994 through June 2002 was analyzed by integrated use of observer reports, commercial logbooks, and auction sales records. The objective was to provide a corrected catch history for this species in this fishery during this period. The study was conducted because official statistics compiled from the logbooks are known to be biased by billfish (Istiophoridae) misidentifications. The initial step entailed fitting a generalized additive model (GAM) of blue marlin catch to environmental and operational data gathered by fishery observers during 8397 longline sets deployed by commercial vessels. The GAM included nine significant predictors and explained 41.1% of the deviance of observed blue marlin catches. The GAM coefficients were then applied to the corresponding predictors in the logbook reports from unobserved sets to estimate catches in evaluations of the accuracy of data from unobserved sets ( N = 87 277 longline sets on 8437 trips; 95.4% of unobserved effort). This was done by regressing the logbook catch data on the predictions, using the residuals to identify trips with systematic misidentifications, and then checking their logbooks against sales records from the public fish auction in Honolulu. The large majority of the misidentifications consisted of striped marlin, Tetrapturus audax , reported as blue marlin, with lesser numbers of shortbill spearfish, T. angustirostris , logged as blue marlin, and blue marlin logged as either striped marlin or black marlin, M. indica . An estimate obtained by use of the GAM and observer data indicated that the nominal catch of blue marlin was inflated by 29.4%. The 95% prediction limits about the GAM-generated estimate (34 201–41 507 blue marlin) did not include the catch total from the logbooks (48 911 blue marlin). The corrections also refined understanding of the distribution of blue marlin by reducing the impression that large numbers of blue marlin are sometimes caught north of Hawaii in the autumn and early winter months. There was no evidence of widespread underreporting of marlins. We conclude that this study significantly improved the accuracy of logbook data for blue marlin and should also contribute to improved understanding of the ecology and distribution of blue marlin. We infer that self-reporting could yield accurate marlins catch data if species identifications were improved because there was no apparent underreporting problem. Finally, we recommend that logbook data accuracy receive serious attention in the context of stock assessments.