Longline Hooks Research Articles

Oscillation and the resulting bait loss observed in pelagic longline hooks

Oscillation and the resulting bait loss observed in pelagic longline hooks

HOOK RATE RAWAI DASAR LAYANGAN (RDL) STUDI PRODUKTIVITAS RAWAI DASAR YANG DIMODIFIKASI DI PERAIRAN SELAT LEMBEH BITUNG SULUT

The research objectives were (1) to examine the difference of productivity level of the catch (hook rate) between the flying bottom longline (RDL) and the conventional bottom longline. (2) record the type and amount of catch. The flying bottom longline (RDL) in this study is a modified construction design of bottom long line, by shortening the branch line and reducing its distance on the main line. The fishing method was changed by spreading the main line on the bottom of the sea. Thus each branch line and hooks will float to the mid surface by taking advantage of the movement of the ocean currents. The research was conducted in August-October 2020 in the waters of the Lembeh Strait, Bitung City, North Sulawesi. The research method used was 6 fishing trips trials. The results of this study indicate that the normality and homogeneity test of the hook rate data distribution is obtained 0.635 for residual value of data normality and a 0.732 of residual value of homogenity, so the data is normally distributed and homogeneous The hook rate on the flying bottom longline (RDL) operated at a depth of 10 m with fresh squid fish bait illustrates that the flying bottom longline hook rate (RDL) is higher than the conventional bottom longline (Rdk). The same thing happened at a depth of 20 m. The hook rate of the flying bottom line (RDL) was higher than that of the conventional bottom line (RDK). The effect of fresh squid bait shows a significant difference in the amount of catch between RDL and RDK. There is a significant difference between RDL and RDK when using decapterus bait. There is no significant difference between the depths of 10 meters and 20 meters on the amount of fish catch. The number of fish species caught using the flying bottom longline (RDL) is eleven species of demersal fish. Total catch using conventional bottom longlines (RDK) is eight species of demersal fish. Keywords: bait; branch line, flying bottom longline; main line, productivity

Oceanic Diel Vertical Movement Patterns of Blue Sharks Vary With Water Temperature and Productivity to Change Vulnerability to Fishing

In the pelagic environment diel vertical movements (DVM) are widespread across taxa, from zooplankton ascending from day-time depths into surface layers at night to avoid visual predators, to apex predators following prey movements to maximise foraging opportunities. The drivers of DVM in large predators such as pelagic sharks have only recently begun to be investigated in detail with the advent of sophisticated archival tags and high-resolution oceanographic datasets. In this study, we satellite tagged adult [>180 cm fork length, (FL)] blue sharks (Prionace glauca) in the North Atlantic Ocean to examine behavioural changes in response to the encountered environment, and therefore, to determine potential risks of capture using pelagic longline fisheries data. Although blue sharks recurrently use surface waters, cyclic diel behaviours were observed, with >95% of night-time spent above 250 m depth and variable day-time depth use. Hence, three different diel behaviours were identified during the tracking period: (i) regular normal DVM (nDVM) (dawn descent – dusk ascent, with over 90% of nighttime spent above 250 m, and between 5 and 50% of the day below this threshold); (ii) surface-oriented behaviour (occupation of surface waters both day and night), and (iii) deep depth-oriented nDVM [dawn descent – dusk ascent, with the majority (>50%) of daytime spent at depth]. Importantly, diel behaviours generally occurred in different ocean regions with nDVM frequently observed in high latitudes, associated with cold, highly productive waters (e.g., North Atlantic Current/Labrador Current convergence zone, West African upwelling area), while depth-oriented nDVM was observed in warm, oligotrophic areas. Thus, day-time occupation of shallow waters significantly increased with lower water temperature at depth (100 m), and with increasing concentration (and decreasing depth) of the chlorophyll a maximum. During nights of full moon blue sharks spent significantly more time in the depth range of longline hooks, while fishing effort and catches were also higher. We demonstrate that increased occupancy of surface layers driven by highly productive, cold waters and greater lunar illumination lead to higher capture risk. Understanding habitat-specific vulnerability to fishing in a commercially important pelagic shark species is essential for improving management and conservation measures.

Retrospective Study of Fishery Interactions in Stranded Cetaceans, Canary Islands.

Estimating cetacean interactions with fishery activities is challenging. Bycatch and chronic entanglements are responsible for thousands of cetacean deaths per year globally. This study represents the first systematic approach to the postmortem investigation of fishery interactions in stranded cetaceans in the Canary Islands. We retrospectively studied 586 cases necropsied between January 2000 and December 2018. Of the cases with a known cause of death, 7.4% (32/453) were due to fishery interactions, and the Atlantic spotted dolphin (Stenella frontalis) was the most affected species [46.9% (15/32)]. Three types of fishery interactions were recognized by gross findings: bycatch [65.6% (21/32)], chronic entanglements [18.8% (6/32)], and fishermen aggression [15.6% (5/32)]. Among the bycaught cases, we differentiated the dolphins that died because of ingestion of longline hooks [23.8% (5/21)] from those that died because of fishing net entrapments [76.2% (16/21)], including dolphins that presumably died at depth due to peracute underwater entrapment (PUE) [37.5% (6/16)], dolphins that were hauled out alive and suffered additional trauma during handling [43.8% (7/16)], and those that were released alive but became stranded and died because of fishery interactions [18.7% (3/16)]. Gross and histologic findings of animals in each group were presented and compared. The histological approach confirmed gross lesions and excluded other possible causes of death. Cetaceans in good-fair body condition and shallow diving species were significantly more affected by fishery interactions, in agreement with the literature. Low rates of fishery interactions have been described, compared with other regions. However, within the last few years, sightings of entangled live whales, especially the minke whale (Balaenoptera acutorostrata) and Bryde's whale (B. edeni), have increased. This study contributes to further improvement of the evaluation of different types of fishery interactions and may facilitate the enforcement of future conservation policies to preserve cetacean populations in the Canary Islands.

Assessing the impact of toothed whale depredation on socio-ecosystems and fishery management in wide-ranging subantarctic fisheries

Marine predators feeding on fisheries catches directly on the fishing gear, a behaviour termed “depredation”, has emerged as a major human-wildlife conflict globally, often resulting in substantial socio-economic and ecological impacts. This study investigated the extent of this conflict in commercial Patagonian toothfish (Dissostichus eleginoides) fisheries across subantarctic waters where both killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus) feed on toothfish caught on longline hooks. Using long-term datasets from six major fishing areas, from southern Chile to the Indian Ocean sector of the Southern Ocean, statistical models were developed to quantify the catch removals due to whale depredation interactions. The results indicated that these removals were large, totalling more than 6600 t of toothfish between 2009 and 2016 with an annual mean of 837 t [95% CI 480–1195 t], comprised of 317 t [232–403 t] and 518 t [247–790 t] removed by killer whales and sperm whales, respectively. Catch removals greatly varied between areas, with the largest estimates found at Crozet, where on average 279 t [179–379 t] of toothfish per year, equivalent to 30% [21–37%] of the total catches. Together, these findings provide metrics to assess the impacts of depredation interactions on the fishing industry, whale populations, fish stocks and associated ecosystems. With an estimated $15 M USD worth of fish depredated every year, this study highlights the large geographic scale and economic significance of the depredation issue and its potential to compromise the viability of some toothfish fisheries which are the primary socio-economic activity in subantarctic regions.

Using autonomous video to estimate the bottom-contact area of longline trap gear and presence–absence of sensitive benthic habitat

Bottom longline hook and trap fishing gear can potentially damage sensitive benthic areas (SBAs) in the ocean; however, the large-scale risks to these habitats are poorly understood because of the difficulties in mapping SBAs and in measuring the bottom-contact area of longline gear. In this paper, we describe a collaborative academic–industry–government approach to obtaining direct presence–absence data for SBAs and to measuring gear interactions with seafloor habitats via a novel deepwater trap camera and motion-sensing systems on commercial longline traps for sablefish (Anoplopoma fimbria) within SGaan Kinghlas – Bowie Seamount Marine Protected Area. We obtained direct presence–absence observations of cold-water corals (Alcyonacea, Antipatharia, Pennatulacea, Stylasteridae) and sponges (Hexactinellida, Demospongiae) at 92 locations over three commercial fishing trips. Video, accelerometer, and depth sensor data were used to estimate a mean bottom footprint of 53 m2 for a standard sablefish trap, which translates to 3200 m2 (95% CI = 2400–3900 m2) for a 60-trap commercial sablefish longline set. Our successful collaboration demonstrates how research partnerships with commercial fisheries have potential for massive improvements in the quantity and quality of data needed for conducting SBA risk assessments over large spatial and temporal scales.

Size-selectivity for British Columbia Sablefish (Anoplopoma fimbria) estimated from a long-term tagging study

The underlying size-distribution of commercial fish stocks is usually unknown, so fishery size-selectivity must be estimated as a latent process embedded within age-structured stock assessments. However, dome-shaped fishery size-selectivity, in particular, is often inestimable because decreasing selectivity is confounded with mortality at older ages. In this paper, we test for dome-shaped selectivity in British Columbia's Sablefish fishery using a long-term tagging data set. We incorporate alternative fishery size-selectivity assumptions within a mark-recapture framework based on an asymptotic logistic model and dome-shaped models using gamma and normal probability density functions. We also fit each model using both time-invariant and time-varying parameterizations. Our results strongly suggest dome-shaped size-selectivity for tagged-Sablefish in longline trap, longline hook, and bottom trawl fisheries. Time-varying models were generally favored over time-invariant models, although alternative time-varying models often produced similar statistical fits. Dome-shaped selectivity in longline fisheries could be a function of fishery targeting, fish movement, or by lower reporting rates for large size-classes.

At‐sea trialling of the Hookpod: a ‘one‐stop’ mitigation solution for seabird bycatch in pelagic longline fisheries

AbstractBycatch of pelagic seabird species in longline fisheries is recognized as one of the most important and pervasive sources of mortality, contributing to an increased risk of their extinction. Uptake of mitigation measures to reduce seabird bycatch has not been widespread by the industry. Here, we present the results of 18 at‐sea trials conducted between 2011–2015 onboard pelagic longliners targeting tuna (Thunnus spp) and swordfish (Xiphias gladius) in South African, Brazilian and Australian waters, using a recently designed seabird bycatch mitigation device. The ‘Hookpod’ is a polycarbonate capsule that encases the point and barb of baited pelagic longline hooks to prevent seabirds from becoming hooked and drowning during line‐setting operations. The assessment was based on efficacy (i.e. reducing rates of seabird bycatch without impacting target catch rate) and practicality (i.e. how the Hookpod fitted into fishing operations). We observed 59 130 experimental branchlines over 129 sets and recorded a single seabird mortality on the Hookpod branchlines compared to 24 on the control branchlines, a bycatch rate of 0.04 birds/1000 hooks and 0.8 birds/1000 hooks, respectively. No difference in catch rate of target fish species between Hookpod and control treatments was detected. These findings demonstrate that Hookpods do not negatively affect catch rate of target species and could make an important contribution to halting the decline of many seabird populations if adopted as a mitigation measure by the pelagic longline fishing industry.

Effects of pelagic longline hook size on species- and size-selectivity and survival

Pelagic fisheries can have profound effects on ecosystem structure and functioning, affecting ecosystem services, including fisheries production, and threaten vulnerable bycatch species. Controlling hook size could manage the species- and size-selectivity and survival of target and incidental catch. To test this hypothesis, we conducted experimental pelagic longline fishing in the western tropical Pacific testing a control hook and two hooks with wider minimum widths. Data such as catch, length and condition were fit to response-specific Bayesian geo-additive generalized additive and linear mixed regression models. Model fits were assessed using posterior predictive check tests. Catch rates of both retained and discarded species were significantly higher on medium hooks. Target tuna species were significantly larger and had significantly higher at-vessel survival rates on wider hooks. Significantly larger billfishes, also market species, were caught on narrowest hooks. These effects of hook width on length and survival, however, are a much smaller determinant of economic value of the catch than effects on catch rates. If input controls are limiting, then, relative to medium hooks, continued use of narrowest hooks would maintain current economic viability without causing a significant increase in discard catch levels, including of vulnerable sharks. If market species output controls are limiting, because the ratio of retained to discarded catch on medium hooks was greater than on narrowest hooks, medium hooks would generate lower discard levels. Further research assessing single-factor effects of longline hook width is needed to support robust meta-analyses that account for fishery-specific effects.

Estimation of tuna longline hook depth for improved performance in Fiji

In pelagic longline, deploying the gear such that the depth of the hook is the same as that of the target fish is important to improve the fishing performance and selectivity. In this study, the depth of the tuna longline hook was estimated using the mass-spring model, catenary curve method, and secretariat of the pacific commission Pythagorean method in order to improve the performance of the longline gear in Fiji. The former two methods were estimated to be relatively accurate, and the latter showed a large error. Further, the mass-spring model accounted for the influence of tidal current in the ocean, which was found to be appropriate for use in field trials.

Fatty acid profiles of Trachinus radiatus Cuvier, 1829 (Perciformes-Trachinoidei, Trachinidae)

The goal of this study was to analyse the fatty acid (FA) profiles of the streaked (starry)weever (Trachinus radiatus), a prized food fish in the countries of its distribution. Fish(N = 20) were sampled in July 2011. Location: 42.761019°N, 17.765090°W ; Adriatic Sea, Elaphite Islands near Dubrovnik, Croatia, at 5–10 m depth using longline hooks ; body length ranges: 24.1–47.2 cm, weight ranges: 120–960 g. Morphological species determination was genetically confirmed (Folmer region of COI gene). Biochemical analysis of T. radiatus muscular tissue (filet) revealed average ± SD dry matter of 252.3 ± 14.8 g/kg w.w. ; moisture of 747.7 ± 14.8 g/kg w.w., and ash of 28.0 ± 6.9 g/kg w.w. Intramuscular crude protein content exceeded the total lipid (TL = 11.9 ± 4.0 g/kg w.w.) content approximately 17.7 fold. Unsaturated FA (UFA) was higher than saturated FA (SFA), with a predominance of polyunsaturated FA (PUFA). The ω3:ω6 ratio was 4.9:1, respectively. Among individually determined fatty acids, the PUFA 22:6n3(DHA) was highly present (29.99 ± 2.75% TL) followed by a relatively high 20:5n3(EPA) content. There was 25 fold higher EPA content than of substrate αLNA, and a 15.5 fold higher DHA content to DPAn3. Such ratios indicate that besides trophic ingestion, FA bioconversion elongase/desaturase synthesis pathways toward ω3 PUFA in Weevers could be highly efficient.

Review of effects of pelagic longline hook and bait type on sea turtle catch rate, anatomical hooking position and at-vessel mortality rate

Bycatch in pelagic longline fisheries is one of the most serious threats to some sea turtle populations. Hook shape, hook minimum width and bait type have been the focus of research and management measures to mitigate problematic bycatch of vulnerable taxa. To assess the current state of knowledge and progress over the past decade, we reviewed findings on the effects of hook and bait type on pelagic longline sea turtle catch rates, anatomical hooking position and at-vessel mortality. Fish versus squid for bait lowered catch rates of leatherback sea turtles (Dermochelys coriacea) and individual species of hard shelled turtles. Fish bait also reduced hard-shelled turtle deep hooking. Wider circle hooks reduced both leatherback and hard-shelled turtle catch rates relative to narrower J and tuna hooks, and reduced the proportion of caught hard-shelled turtles that were deeply hooked. Wider circle hooks with fish bait reduced leatherback and hard-shelled turtle catch rates relative to narrower J and tuna hooks with squid bait. Wider versus narrower circle hooks reduced hard-shelled sea turtle catch rates and deep hooking. The mechanisms for hook and bait type effects on turtle interactions are reviewed. Research designed to assess single factor effects is needed, in particular for hook shape and minimum width, and for hook and bait effects on anatomical hooking position and survival rates. Fishery-specific and holistic assessments are needed to account for variability between fisheries in a bycatch mitigation method’s commercial viability, relative risks to affected populations and possible conflicting effects on vulnerable taxa.

Longline hook testing in the mouths of pelagic odontocetes

Abstract Several species of odontocete cetaceans depredate bait and catch and, as a result, become hooked and entangled in pelagic longline fisheries. The present study measured how selected commercial longline hooks, including “weak hooks”, behaved within odontocete mouths. Five hooks (Mustad-16/0, Mustad-18/0, Mustad J-9/0, Korean 16, and Korean 18) were tested on three species of odontocetes known to interact with longline fisheries—short-finned pilot whales (Globicephala macrorhynchus), Risso's dolphins (Grampus griseus), and false killer whales (Pseudorca crassidens). Specimens were secured to a stanchion, hooks were placed in the mouth at multiple positions along the dorsal lip, and the force required to pull each hook free was measured. The soft tissue lips of these odontocetes were capable of resisting forces up to 250 kg before failing. The polished steel M-16, M-18, and J-9 hooks straightened at forces between 50 and 225 kg, depending on hook gauge. When straightened, these hooks exposed the sharpened barb, which sliced through the lip tissue, usually releasing the hook intact. The K-16 and K-18 hooks behaved very differently, breaking at higher forces (110–250 kg) and consistently just at the barb; usually, there was measurable soft-tissue loss and often shards of the hook were retained within those soft tissues. The different behaviours of these two hook types—the M and J type polished steel vs. the K type carbon steel—were consistent across all species tested. Mechanical tests were also conducted to determine if hooks could fracture the mandible of these same odontocetes. Only the M-18 and K-18 hooks had sufficiently large gapes to hook around the mandible, and both hook types fractured bone in short-finned pilot whales and Risso's dolphins. These results support other lines of evidence indicating that longline hooks can cause serious injury to these species, and suggest possible steps to mitigate these impacts.

An assessment of oceanic seabird abundance and distribution off the southern Brazilian coast using observations obtained during deep-water fishing operations.

The use of discarded fish over baited hooks used in longline fishery, and fish caught in gillnets, as a food source for gulls, albatrosses and petrels has been intensively studied in northern and southern oceans. This study describes the occurrence and abundance of seabirds observed from 20 foreign vessels which operated during the period between July 2001 and May 2005, off the southeastern and southern Brazilian coast. A total of 353,557 seabirds were observed; comprising eight families and 28 species. The most abundant species was Procellaria conspicillata followed by Daption capense, Puffinus gravis, Thalassarche melanophrys and Oceanites oceanicus. Ten species of seabirds (392 individual birds) were incidentally captured in gillnets; and 122 birds (9 species) by longline hooks, with P. gravis, D. capense and Procellaria aequinoctialis having the largest capture rates.

Calibration of pelagic stereo‐BRUVs and scientific longline surveys for sampling sharks

SummaryOur understanding of the ecology of sharks and other highly mobile marine species often relies on fishery‐dependent data or extractive fishery‐independent techniques that can result in catchability and size‐selectivity biases. Pelagic Baited Remote Underwater stereo‐Video Systems (pelagic stereo‐BRUVs) provide a standardized, non‐destructive and fishery‐independent approach to estimate biodiversity measures of fish assemblages in the water column. However, the performance of this novel method has not yet been assessed relative to other standard sampling techniques.We compared the catch composition, relative abundance and length distribution of fish assemblages sampled using pelagic stereo‐BRUVs and conventional scientific longline surveys. In particular, we focused on sharks of the family Carcharhinidae (requiem sharks) to assess the sampling effectiveness of this novel technique along a latitudinal gradient off the coast of Western Australia. We calibrated the sampling effort required for each technique to obtain equivalent samples of the target species and discuss the advantages, limitations and potential use of these methods to study highly mobile species.The proportion of sharks sampled by pelagic stereo‐BRUVs and scientific longline surveys was comparable across the latitudinal gradient.Carcharhinus plumbeuswas the most abundant species sampled by both the techniques. Longline surveys selected larger individuals of the family Carcharhinidae in comparison with the length distribution data obtained from pelagic stereo‐BRUVs. However, the relative abundance estimates (catch per unit of effort) from the pelagic stereo‐BRUVs were comparable to those from 5 to 30 longline hooks.Pelagic stereo‐BRUVs can be calibrated to standard techniques in order to study the species composition, behaviour, relative abundance and size distribution of highly mobile fish assemblages at broad spatial and temporal scales. This technique offers a non‐destructive fishery‐independent approach that can be implemented in areas that may be closed to fishing and is suitable for studies on rare or threatened species.

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.

The blood chemical status of Atlantic cod Gadus morhua following capture by jig and demersal longline with differential hook removal methods

Common haematological [haematocrit (Hct)], primary (serum cortisol) and secondary (serum glucose and plasma lactate) analytes were utilized to compare blood biochemical status of Gadus morhua captured rapidly by jig with that of G. morhua captured by commercial demersal longline. In general, the physiological status of G. morhua, despite blind hook times, was significantly more disrupted (pronounced haemo-concentration and significantly elevated concentrations of cortisol, glucose and lactate) following longline capture relative to capture by jig, while no differences were detected among longline-caught fish as a function of dehooking method (or concomitant extent of overt physical trauma). Blood profiles from the more stressed G. morhua, a possible function of more extended longline hook times, were similar to the most stressed values reported for this species. The results also demonstrate that, although acute blood biochemical status is an effective gauge of relative stress, it does not reflect physical injury status, which has been shown to exert a strong influence on delayed mortality in previous studies in this species. Thus, acute blood chemical status alone may not be the most complete predictor of mortality. Future studies should evaluate physiological repercussions from capture-handling against physical trauma during more extended post-release periods for this species.

Performance of methods used to estimate indices of abundance for highly migratory species

Estimating indices of abundance from fishery-dependent data requires that catch-per-unit-effort (CPUE) be standardized to account for factors that may have affected CPUE but are not related to changes in abundance. Such standardization is particularly important for highly migratory species (e.g., tunas, pelagic sharks, and billfishes), because of time-varying mismatches between distributions of abundance and the distribution of fishing effort. Two commonly applied methods for standardizing CPUE are generalized linear models (GLMs), which can account for changes in fishing practices in a straightforward linear fashion, and habitat-based standardizations (e.g., statHBS), which use nonlinear analysis to relate the distribution of fishing effort to the species distribution. We evaluated the accuracy of these methods over three patterns in vertical catchability as related to ocean temperature profiles, and 50 possible biomass trajectories using a simulation framework that followed the general effort dynamics of the Japanese longline fishery in the Atlantic Ocean from 1956 to 2009. Additionally, we propose a method for directly incorporating vertical habitat information into the linear models. Overall, we found the most accurate approach to be a delta-lognormal GLM with our unique habitat factor. The statHBS approach was the most accurate when catchability was simulated to peak in surface waters. However, statHBS was much more sensitive to errors in estimates of longline hook depths (i.e., habitats exploited). Based on these results, we recommend that relative abundance be estimated for highly migratory species following a delta-GLM approach that considers vertical habitats fished.

Monitoring the behaviour of longline gears and the depth and time of fish capture in the Australian Eastern Tuna and Billfish Fishery

We describe the results of a detailed monitoring program undertaken to understand the fishing practices and operational characteristics of longline gears deployed in the Australian Eastern Tuna and Billfish Fishery. Gear monitors were used to ascertain both the time-at-depth-profiles of the deployed longline hooks and the time- and depth-profiles of fish capture. Operational details and environmental conditions recorded by observers were also used to gain an understanding of factors influencing the depths attained by hooks and the shape of the longline. The results indicate that while hooks fish a range of depths down to around 400m the most common depths fished are relatively shallow, with 34 percent of all hooks deployed during the project period being between 40 and 60m. The depths to which hooks were observed to sink were found to be highly variable for a given longline configuration and hook-number, though the mean depths attained across sets were found to be reasonably well predicted by the longline catenary equation. Current-speed, hook-position and wind-speed were found to be the main factors influencing variation from the predicted depth. While the depth profiles fished by sets deploying less than 10 hooks-per-float were found to be similar, for sets deploying 10 or more hooks-per-float a near linear relationship was found in the mean depth fished by all hooks and the number of hooks-per-float. Finally, a procedure is outlined to estimate the availability-by-depth of the species caught and the resulting profiles were found to be similar to profiles obtained from studies using archival tags. The results of this study support the need to improve the information used to account for factors influencing the fishing effectiveness of longline gears and the use of set-based analyses to standardise catch rates.

Use of management strategy comparison scatterplots to identify clusters of high‐performing strategies

Abstract A management simulation model was developed to test quota, trip limits and reduced longline hook numbers as alternatives to yearly seasonal (November) closure in South Australian snapper fisheries. Population dynamics equations and maximum‐likelihood parameter estimates were supplied by an age‐ and length‐based stock assessment model. The relative performance of tested management strategies was quantified by comparing change in egg production vs change in average catch, to optimise the trade‐off between reproductive sustainability and catch foregone. Clusters of better‐performing strategies were visually identifiable in management strategy scatterplots. These scatterplots display percentage change from the status quo in egg‐production‐per‐female‐recruit (y‐axis) vs percentage change in catch‐per‐recruit (x‐axis). Evaluated strategies fell into three distinct performance clusters. The best performing strategy extended November closure by 2 weeks. Strategies that retained the November closure performed uniformly better than those that removed it. Further simulation showed this resulted from non‐closure strategies regulating only the commercial sector.