Demersal Longline Fisheries Research Articles

Post-release survival of Whiteleg skate (Amblyraja taaf) caught as bycatch on demersal longlines in deep waters of the Southern Ocean (Crozet Archipelago)

Abstract Whiteleg skate (Amblyraja taaf) is a bycatch species commonly discarded in demersal longline fisheries targeting Patagonian toothfish around the Crozet archipelago (Southern Ocean). While there are global conservation issues on elasmobranchs, the post-release survival of this deep-sea species remains unknown. This study is the first tagging programme conducted on Whiteleg skates to investigate their fate after being released at sea. Almost 3000 skates were captured, double-tagged with conventional tags and released between 2020 and 2022 to assess survival using capture–mark–recapture (CMR) models. Our primary result was an estimated annual apparent survival rate higher than 92% for skates released in good condition (with 185 skates recaptured after up to 944 days at liberty). Low detection rate highlighted methodological challenges for CMR models. However, skate size and depth of capture had an effect on apparent survival probability with smaller individuals having lower values (<65 cm; 0.84 ± 0.04) as well as skates caught at shallower depths (<1200 m; 0.83 ± 0.04). Overall, this study suggests that the mandated release of skates in good condition is effective to limit fishing impact on skates in this fishery. To further limit this impact, we recommended changes in fishing practices (hauling speed and soak time).

Vulnerability of elasmobranchs caught as bycatch in the grouper longline fishery in the Gulf of Gabès, Tunisia

The present study aims to quantify the effect of the grouper demersal longline fishery to elasmobranchs in the Gulf of Gabès to support future conservation actions for the most vulnerable species. Data from 162 demersal longline sets carried out during the grouper fishing seasons in 2016 and 2017 were analyzed. At least 17 elasmobranch species were caught, representing about 50% of the total catch in number which exceeded the target catch of groupers Epinephelus spp. (44.15%). Elasmobranchs were present in 139 (85.80%) out of the 162 examined sets, while the frequency of occurrence per set varied greatly depending on the species. The nominal Catches Per Unit Effort of all species combined was 3.07 specimens per 1,000 hooks/hour of fishing. Sharks and batoids contributed equally to the total elasmobranch catch, comprising 51.84% and 48.16% in terms of number, respectively. Mustelus mustelus, Carcharhinus plumbeus and Squalus spp. were the most abundant species, representing 45.65% of the total elasmobranch catch, whereas the remaining shark species were rarely caught (6.19%). Batoids were dominated by Glaucostegus cemiculus, which represented 19.07% of the total elasmobranch catch in number, followed by Dasyatis spp. (8.91%), Raja clavata (7.1%) and Taeniura grabata (4.59%). The elasmobranchs discarded due to their low commercial value and/or small size represented 11.34% of the total catch in number. According to the results of the Productivity and Susceptibility Analysis, the coastal viviparous elasmobranch species in the study area are highly vulnerable to the grouper longline fishery activity and should be carefully managed to reduce the likelihood of overfishing.

From sealing to the MPA - A history of exploitation, conservation and management of marine living resources at the South Sandwich Islands

The exploitation of marine resources of the South Sandwich Islands (SSI) began with the hunting of fur seals for their pelts in the early decades of the 19th Century. Pelagic whaling in the region started a century later with catches recorded until the mid-1970s. Blue and fin whales dominated the catches accounting for 80% of the total. Trawl fisheries for demersal finfish and krill (Euphausia superba) were established around many sub-Antarctic islands in the late 1960s and through the 1970s, but they did not become established at the South Sandwich Islands despite fisheries research expeditions from several nations visiting the region during this period. The first licensed demersal longline fishery for toothfish (Dissostichus spp.) was initiated by a UK flagged vessel in 2005 following earlier expeditions by Chilean and Bulgarian fishing vessels. The fishery for toothfish is now conducted by a maximum of two vessels and represents the only fishing carried out in the SSI region with total annual catches of around 40 t per annum, with a fishing footprint restricted to less than 4% of the SSI Maritime Zone (MZ). This MZ extends 200 nm from the island chain and forms the eastern half of the 1.24 million km2 MZ of the UK Overseas Territory of South Georgia & the South Sandwich Islands (SGSSI) which was established in 1993. The MZ around the SSI lies within the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) management Subarea 48.4. Fishing within the MZ is licensed by the Government of South Georgia & the South Sandwich Islands (GSGSSI) who, under domestic legislation, are required to adopt all fisheries management regulations that have been agreed for the region each year by CCAMLR. In addition, a suite of additional management measures are enforced. In 2012, GSGSSI established a sustainable use Marine Protected Area (MPA) within the SGSSI MZ to conserve the marine biodiversity of the region. Enhancements were introduced in 2013 and 2019 extending No-Take Zones (NTZs), where all fishing is prohibited, across 261,000 km2 of the MZ around the SSI including the deepest regions of the Southern Ocean, the South Sandwich Trench. The SSI marine ecosystem has been relatively poorly studied but has recently been a focus of two dedicated UK research cruises providing a considerable new amount of information to assist with the management of this remote marine region.

Quantifying killer whale depredation in the blue-eye trevalla commercial fisheries of south-east Australia

Large marine predators feeding on fish caught on fishing gear, a behaviour termed as depredation, can incur socio-economic costs for fisheries and have serious implications on the depredating species, fish stocks and associated ecosystems. The quantity of depredated fish as well as the quantity of depredated fish relative to the retained catch dictates the severity of these impacts, yet these depredated quantities are often unknown due to the complexity of assessing depredation. This is the case for many demersal longline fisheries experiencing depredation by killer whales (Orcinus orca), which remove entire fish from hooks when interacting with the fishing gear. In the present study, we used Generalized Linear Models fitted to the Catch per Unit Effort (CPUE) to quantify killer whale depredation in the blue-eye trevalla (Hyperoglyphe antarctica) commercial longline fishery of south-east Australia. Our results showed that during days when killer whales were present around fishing vessels (7.3% of total fishing days), blue-eye trevalla CPUE decreased by 45.6%. Killer whales were estimated to have removed a total of 63.9 t (95% CI: 45.3–82.7 t) of blue-eye trevalla from the hooks between 2006 and 2017, with a mean removal of 5.3 t (3.8–6.9 t) per year. The mean rate of killer whale depredation in south-east Australia was estimated to be 5.2% (4.0–6.4%) for the entire study period. We estimated that the current depredation rate equates to an additional 27 fishing days per year across the fishery. This figure grows to 354 additional fishing days if depredation was assumed to occur during every fishing operation. Considering that killer whale observations are voluntarily reported in logbooks, this likely underestimates the depredation in our study. Together, these findings suggest that the impacts of killer whale depredation in the blue-eye trevalla fishery of south-east Australia are not negligible given the small scale of the fishery and the historical decline of its catches in this region. We recommend estimates provided in this study to be used as baselines for the future assessment and management of the blue-eye trevalla stocks and for assessing the socio-economic and ecological implications of this issue. In addition, regardless of the limitations associated with assessing depredation, our study also highlights the importance of cross-sectoral and interdisciplinary collaborations between scientists, fishers and fishery managers for effectively improving the management of commercially important fisheries globally.

Ege Denizi dip paragat balıkçılığında iğne ve yem tipinin birim çabada av miktarına etkisi

This study aimed to determine the impact of hook and bait type on the catch composition and catch per unit effort. Effects of hook and bait types on catch composition, catch per unit effort (CPUE), length and weight distributions in demersal longline fishery were determined by experimental surveys on demersal longline sets in the Aegean Sea. A total of 12 samplings corresponding to 4800 hook fishing effort were performed between April 2014 and September 2014. Two bait types; sardine (Sardina pilchardus) and grooved razor shell (Solen marginatus) and two hook types; J-hook and C-hook were tested. CPUE values were calculated for each species and assessed between different hook-bait combinations. A total of 623 individuals were captured belonging to 3 families and 9 species. It was found that more than 60% of total catch was captured by grooved razor shell and more than 50% of the total catch was caught with J type hook. J hook was found to be close to significant (p=0.06) and grooved razor shell was found significant (p=0.02) for CPUE. The effect of bait type was found to be more significant than that of hook type for CPUE and length distribution. Hook-bait combination differed according to species and C hook baited with sardine was determined to be the best combination for Sparus aurata as the most targeted fish in the study area. Discard ratio was calculated to be 34% in terms of weight and 42.5% in terms of total number of individuals for pooled data. The condition value (K) of the species ranged from 1.05 to 1.68 and differed according to bait type. Most of the high commercial value species caught with any hook-bait combination experimented within this study are larger than minimum fishing length according to minimum landing size regulations of Turkish fishery and maturity studies.

Acoustics and photo-identification provide new insights on killer whale presence and movements when interacting with longline fisheries in South East Australia

Removal of target catch from longlines by marine mammals, known as depredation, is a global issue creating animal welfare, socioeconomic and management concerns. The killer whale (Orcinus orca) is a key species of concern for longline depredation due to their global presence and ability to remove large quantities of caught fish. Currently, data on whale behaviours around fishing vessels, including timing of depredation and ability to follow vessels, is not completely understood. This lack of knowledge prevents a complete assessment of depredation both spatially and temporally, which is important to reduce the chance of underestimating depredation events. Our study utilised both photo-identification and acoustics as complementary approaches to investigate depredation on the blue-eye trevalla (Hyperoglyphe antarctica) demersal longline fishery in Australia. Of the 14 d when depredations were recorded acoustic detection of killer whales prior to visual confirmations occurred in 13 (93 %). Photo-identification revealed individuals repeatedly interacting with the vessel, sometimes over long distances (>1000 km), with increasing inter-depredation times as the vessel travelled further. These findings suggest that killer whales move to known fishing areas well before detection from surface monitoring occurs. While this study has revealed aspects of killer whale behaviours when encountering and interacting with longline fisheries, it has highlighted how traditional monitoring methods underestimate depredation and that this aspect needs to be further investigated.

A comprehensive large‐scale assessment of fisheries bycatch risk to threatened seabird populations

Abstract Incidental mortality (bycatch) in fisheries remains the greatest threat to many large marine vertebrates and is a major barrier to fisheries sustainability. Robust assessments of bycatch risk are crucial for informing effective mitigation strategies, but are hampered by missing information on the distributions of key life‐history stages (adult breeders and non‐breeders, immatures and juveniles). Using a comprehensive biologging dataset (1,692 tracks, 788 individuals) spanning all major life‐history stages, we assessed spatial overlap of four threatened seabird populations from South Georgia, with longline and trawl fisheries in the Southern Ocean. We generated monthly population‐level distributions, weighting each life‐history stage according to population age structure based on demographic models. Specifically, we determined where and when birds were at greatest potential bycatch risk, and from which fleets. Overlap with both pelagic and demersal longline fisheries was highest for black‐browed albatrosses, then white‐chinned petrels, wandering and grey‐headed albatrosses, whereas overlap with trawl fisheries was highest for white‐chinned petrels. Hotspots of fisheries overlap occurred in all major ocean basins, but particularly the south‐east and south‐west Atlantic Ocean (longline and trawl) and south‐west Indian Ocean (pelagic longline). Overlap was greatest with pelagic longline fleets in May–September, when fishing effort south of 25°S is highest, and with demersal and trawl fisheries in January–June. Overlap scores were dominated by particular fleets: pelagic longline—Japan, Taiwan; demersal longline and trawl—Argentina, Namibia, Falklands, South Africa; demersal longline—Convention for Conservation of Antarctic Marine Living Resources (CCAMLR) waters, Chile, New Zealand. Synthesis and applications. We provide a framework for calculating appropriately weighted population‐level distributions from biologging data, which we recommend for future fisheries bycatch risk assessments. Many regions of high spatial overlap corresponded with high seabird bycatch rates recorded by on‐board observers, indicating that our approach reliably mapped relative bycatch risk at large spatial scales. Implementation of effective bycatch mitigation in these high‐risk regions varies considerably. Although potential bycatch risk appears to have decreased since the early 2000s, albatross and petrel populations from South Georgia and elsewhere are still declining, emphasizing the need for much improved observer coverage and monitoring of compliance with bycatch regulations.

Evidence of deep-sea interactions between toothed whales and longlines.

Toothed whales (odontocetes) feeding on fish caught on hooks in longline fisheries is a growing issue worldwide. The substantial impacts that this behaviour, called depredation, can have on the fishing economy, fish stocks and odontocetes populations, raise a critical need for mitigation solutions to be developed. However, information on when, where and how odontocete depredation occurs underwater is still limited, especially in demersal longline fisheries (fishing gear set on the seafloor). In the present study, we investigated depredation by killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus) on demersal longlines in the French Patagonian toothfish fishery (Southern Ocean). Using a combination of animal-borne behavioural and longline-attached data loggers, we demonstrated that both species are able to depredate longlines on the seafloor. This study, therefore, suggests that odontocetes whales-longline interaction events at depth may be unrecorded when assessing depredation rates from surface observations during hauling phases only. This result has implications for the management of fisheries facing similar depredation issues as underestimated depredation rates may result in unaccounted fish mortality in fish-stock assessments. Therefore, while further research should be conducted to assess the extent of deep-sea whale-longline interaction events during soaking, the evidence that depredation can occur at any time during the whole fishing process as brought out by this study should be considered in future developments of mitigation solutions to the issue.

Circle hook versus J-hook: A case study of the Sultanate of Oman

This paper analyses the operational efficiency and economic performance of the circle hook and the J-hook, commonly used by traditional fishers in the demersal longline fishery of the Sultanate of Oman. A longline experiment was conducted at three fishing locations at Masirah Island of Al-Sharqiyah Governorate. A total of 6,120 baited J-hooks and circle hooks were deployed over a 17-day period. The findings from this experimental research suggest that the overall performance (measured under various operational yardsticks such as hooking status and location, catch composition and quality, catching efficiency, and time and cost efficiency) of the circle hook is better than its counterpart. The results also indicate that the use of circle hooks has the potential to yield better financial returns. It is acknowledged that the reliance on three fishing locations may restrict the scientific generalizations. However, it is hoped that the results from this study will provide insight into the design of future experiments to ensure the validity of the present results and make effective regulatory decisions.

Circle hook versus J-hook: A case study of the Sultanate of Oman

This paper analyses the operational efficiency and economic performance of the circle hook and the J-hook, commonly used by traditional fishers in the demersal longline fishery of the Sultanate of Oman. A longline experiment was conducted at three fishing locations at Masirah Island of Al-Sharqiyah Governorate. A total of 6,120 baited J-hooks and circle hooks were deployed over a 17-day period. The findings from this experimental research suggest that the overall performance (measured under various operational yardsticks such as hooking status and location, catch composition and quality, catching efficiency, and time and cost efficiency) of the circle hook is better than its counterpart. The results also indicate that the use of circle hooks has the potential to yield better financial returns. It is acknowledged that the reliance on three fishing locations may restrict the scientific generalizations. However, it is hoped that the results from this study will provide insight into the design of future experiments to ensure the validity of the present results and make effective regulatory decisions.

Vessel monitoring systems (VMS) reveal an increase in fishing efficiency following regulatory changes in a demersal longline fishery

A global expansion of satellite-based monitoring is making fisher behavioral responses to management actions increasingly observable. However, such data have been underutilized in evaluating the impacts of fishing on target and non-target fish stocks or the ramifications of management strategies on fishers. We demonstrate how vessel monitoring system (VMS) data can provide a suite of metrics (such as effort) for improving inputs to stock assessments, dynamic delineation of fishing grounds, and evaluation of regulatory or other (e.g., climatic) impacts on fisher performance. Using >1 million VMS records from the Gulf of Mexico grouper-tilefish demersal longline fishery, we first develop a generalized additive modeling approach that predicts fishing duration with ∼85% accuracy. We combine model predictions with logbook data to compare the fishery before and after implementation of a suite of regulatory changes (e.g., a shift to catch share management). We find a large-scale reduction in fleet size, accompanied by reduced fishing effort (duration * number of hooks), shorter trips, lower operational expenses, higher catch rates, and more earnings for those vessels that remained in the fishery. We discuss how the combination of VMS and associated metrics can be expanded for use in management strategy evaluation, parameterizing economic models of fisher behavior, improving fishery-dependent stock assessment indices, and deriving socioeconomic indicators in fisheries worldwide.

Effect of Bait Type and Bait Size on Catch Efficiency in the European Hake Merluccius merluccius Longline Fishery

AbstractWe investigated the effect of bait type and bait size on the catch efficiency of a demersal longline fishery targeting European Hake Merluccius merluccius in the North Sea. Automation of the labor‐intensive processes onboard fishing vessels requires finding alternatives to the traditional bait used in the fishery (i.e., whole European Pilchard Sardina pilchardus). Of the six alternative baits investigated, four resulted in significant reductions in catch efficiency ranging from 32% to 90%. Only chopped Atlantic Herring Clupea harengus was a reasonable alternative bait, with an estimated non‐significant loss of only 2.12% in European Hake catch efficiency. Our results demonstrated that choice of bait type and size can affect the catch efficiency of different sizes of European Hake. Thus, the choice of bait may also affect the size distribution of the catch. The latter highlights the importance of considering fish size when inferring the effect of bait choice on the catch efficiency of longline fisheries.

Reconstructing Western Australian white shark (Carcharodon carcharias) catches based on interviews with fishers

The assessment of fisheries-related effects on protected species, such as white sharks (Carcharodon carcharias), is often hampered by a lack of historical catch information. In the present study, historical catch estimates derived from interviews with fishers operating in the Western Australian Temperate Demersal Gill-net and Demersal Longline Fisheries were matched with fishing effort data reported in statutory fishing returns. Catch estimates obtained from interviewed fishers were extrapolated to account for total catch in two different ways, resulting in mean (95% confidence intervals) estimates of 1232 (476–2245) and 1039 (505–2096) white sharks caught between 1988 and 2012. These estimates were then used to reconstruct catches over a 59-year period, from the start of commercial gillnetting in the mid-1950s. The reconstructed catch trend reflected the history of gill-net fishing effort, peaking in the late 1980s at a level approximately fourfold greater than the estimated catch of ~30 sharks year–1 in 2014. More than one-third of fishers interviewed expressed doubts about the accuracy of self-reported white shark catch data, a requirement of current legislation. Given the benefits of reporting data from protected species bycatch, efforts to accurately record this information should be continued and improved.

Do commercial fisheries display optimal foraging? The case of longline fishers in competition with odontocetes

Depredation in longline fisheries by odontocete whales is a worldwide growing issue, having substantial socioeconomic consequences for fishers as well as conservation implications for both fish resources and the depredating odontocete populations. An example of this is the demersal longline fishery operating around the Crozet Archipelago and Kerguelen Island, southern Indian Ocean, where killer whales (Orcinus orca) and sperm whales (Physeter macrocephalus) depredate hooked Patagonian toothfish (Dissostichus eleginoides). It is of great interest to better understand relationships of this modern fishery with its environment. Thus, we examined the factors influencing the decision-making process of fishers facing such competition while operating on a patch. Using optimal foraging theory as the underlying hypothesis, we determined that the probability captains left an area decreases with increasing fishing success, whereas in presence of competition from odontocete whales, it increases. Our study provides strong support that fishers behave as optimal foragers in this specific fishery. Considering that captains are optimal foragers and thus aim at maximizing the exploitation of the resources, we highlight possible risks for the long-term sustainability of the local ecosystems.

Inter-specific niche partitioning and overlap in albatrosses and petrels: dietary divergence and the role of fishing discards

Although fisheries discards are recognized as a key food source for many seabirds, there have been few thorough assessments of their importance relative to natural prey, and of their influence on the trophic structure of pelagic seabird communities during the non-breeding period. Competition for resources in Procellariiformes appears to be reduced mainly by avoiding spatial overlap, which is supposed to influence diet composition. However, artificial food sources provided by fisheries might relax niche partitioning, increasing trophic niche overlap. Using bycaught birds from pelagic longline fisheries, we combined the conventional diet and stable isotope analyses to assess the importance of fishing discards in the diet of eight species of Procellariiformes. Both methods revealed the high contribution of trawl discards to the non-breeding diet of three neritic species and a moderate contribution in several other species; discards from pelagic and demersal longline fisheries were considerably less important. There was a clear contrast in diets of neritic vs. oceanic species, which are closely related taxonomically, but segregate at sea. Niche partitioning was less clear among neritic species. They showed an unexpectedly high level of diet overlap, presumably related to the large volume of trawl discards available. This is the first study combining the conventional diet and stable isotope analyses to quantify the importance of fishery discards for a community of non-breeding seabirds, and demonstrates how the super-abundance of supplementary food generates high levels of overlap in diets and allows the coexistence of species.

Seabird mortality in the Namibian demersal longline fishery and recommendations for best practice mitigation measures

AbstractSeabird bycatch is widely regarded as the greatest threat globally to procellariiform seabirds. Although measures to reduce seabird–fishery interactions have been in existence for many years, uptake in fleets with high risk profiles remains variable. We recorded seabird bycatch and other interactions in the Namibian demersal longline fishery. Interaction rates were estimated for seasonal and spatial strata and scaled up to fishing effort data. Bycatch rates were 0.77 (95% CI 0.24–1.39) and 0.37 (95% CI 0.11–0.72) birds per 1,000 hooks in winter and summer, respectively. Scaling up to 2010, the most recent year for which complete data are available, suggests 20,567 (95% CI 6,328–37,935) birds were killed in this fishery that year. We compared bycatch rates to those from experimental fishing sets using mitigation measures (one or two bird-scaring lines and the replacement of standard concrete weights with 5 kg steel weights). All mitigation measures significantly reduced the bycatch rate. This study confirms the Namibian longline fishery has some of the highest known impacts on seabirds globally, but implementing simple measures could rapidly reduce those impacts. In November 2015 the Ministry of Fisheries and Marine Resources introduced regulations requiring the use of bird-scaring lines, line weighting and night setting in this fishery. A collaborative approach between NGOs, industry and government was important in achieving wide understanding and acceptance of the proposed mitigation measures in the lead up to the introduction of new fishery regulations.

Incidental bycatch of northern fulmars in the small-vessel demersal longline fishery for Greenland halibut in coastal Norway 2012–2014

With seabird populations in rapid decline, understanding and reducing anthropogenic mortality factors is essential. One such factor is incidental bycatch in fisheries. Here we analyze bycatch in the small-vessel demersal longline fishery for Greenland halibut outside the coast of Northern Norway in 2012–2014, by means of self-reporting from fishers and independent observers. A sample of killed birds were analysed for sex, age, reproductive status and condition. Nearly all were northern fulmars. Estimated total bycatch for this fishery for the 3-year period was about 312 birds (SE ≈ 133) using a stratified estimator. Bycatch rate per 1000 hooks was estimated to approximately 0.031 (SE ≈ 0.012). Exploring per trip bycatch rates utilizing generalized linear mixed models, we found no convincing trends of environmental, spatial and temporal variables in explaining bycatch. However, trips using longlines with non-swivel hooks had a more than 100-fold larger bycatch rate (mean ≈ 0.760, SE ≈ 0.160) than those using swivel hooks (mean ≈ 0.008, SE ≈ 0.002). Further, trips with external observers had higher bycatch estimates (mean ≈ 0.75, SE ≈ 0.16) compared with trips where bycatch was registered by the fishers (mean ≈ 0.02, SE < 0.01). Of the analysed birds, about two-thirds were adult birds and males dominated (71.1%). A majority were in good or moderate condition. The findings suggest that the incidental bycatch in the Greenland halibut fishery along the Norwegian coast is more limited than previous studies indicated, and that the use of swivel hooks can significantly reduce such bycatch. However, the impacts on the red-listed, diminishing population of fulmars breeding in mainland Norway should be assessed further and requires a method to assign killed birds to regions/colonies. Also, gaining a better understanding of what triggers events with extreme bycatch numbers is important to reduce the problem further and to improve bycatch modelling.

A preliminary study on the potential use of an alternative bait for demersal longline fishery; Sepietta sp.

Bu çalışma Eylül 2007-Nisan 2008 tarihleri arasında deneysel amaçlı hazırlanmış bir paragat takımıyla Ege Denizi’nde gerçekleştirilmiştir. Geleneksel yemler olan Sardina pilchardus ve Solen vagina, Ege Denizi demersal trol avcılığında ıskarta edilen bir yem olan Sepietta sp. ile birlikte kullanılmıştır. Altı familyaya ait toplam 438 birey yakalanmış ve toplam ağırlıkları 43,1 kg olarak bulunmuştur. Yakalanan bireylerin yarısına yakın kısmı Sepietta sp. ile yakalanmıştır. Bununla beraber bu alternatif yem ile yakalanan bireyler toplam avın ağırlık olarak %45’ini oluşturmuşlardır. Sonuçlar Sepietta sp.’nin tüm yemler içinde en etkili yem olduğunu göstermiştir. Bu türün paragat balıkçılığında yem olarak kullanımının, trol avcılığından ıskarta edilen bir türün değerlendirilmesi açısından ayrı bir önemi bulunmaktadır

Rapid fish stock depletion in previously unexploited seamounts: the case of Beryx splendens from the Sierra Leone Rise (Gulf of Guinea)

Fish stocks associated with seamounts may be particularly susceptible to overexploitation. From January to July 2001, the Spanish Oceanographic Institute (IEO) conducted an experimental fishing survey entitled ‘Palguinea-2001’ on the seamounts of the Sierra Leone Rise. Beryx splendens (commonly called alfonsino) is the main commercial demersal fish associated with this area. The aim of this study was to investigate the effects of a demersal longline fishery targeting B. splendens on the previously unexploited small Machucambo Seamount over a short time-scale, and also to consider trends in pooled catch rate at another four seamounts. During 110 fishing days at Machucambo, a total catch of 207 tonnes of B. splendens was taken with a fishing effort of 1 309 070 hooks. A spectral analysis and red-noise spectra procedure (REDFIT) algorithm was used to identify the red-noise spectrum from the gaps in the observed time-series of catch per unit effort by weight. Our results show the potential impact of longline fishing pressure on an unexploited ecosystem – after approximately 50 fishing days, the stock appeared to decline substantially, as reflected by a marked drop in catch per unit effort. The apparent rapid decline of the stock might be related to the small size and the virgin state of the Sierra Leone seamounts. The results could be extrapolated to similar small seamounts elsewhere.

Aspects of the reproductive biology of dusky, spinner and sandbar sharks (Family Carcharhinidae) from the Tasman Sea

Increased harvest pressure exerted on sharks worldwide has created a necessity for more information concerning the basic biology of targeted species. This study marks the first dedicated assessment of the reproductive biology of Carcharhinus obscurus, C. brevipinna and C. plumbeus in eastern Australian waters, where these species support a demersal longline fishery. We found all three to be late maturing species of low reproductive output, suggestive of an overarching susceptibility to stock depletion in the region. Length- (L50, cm LT) and age-at-maturity (A50, years), for females and males were respectively 281.1 and 15.5, and 271.9 and 14.6 for C. obscurus; 224.9 and 10.1, and 208.9 and 8.5 for C. brevipinna; and, 174.8 and 9.5, and 164.5 and 7.0 for C. plumbeus. Uterine fecundity (i.e. litter size) was observed at 5–12 (=9.6), 5–14 (=10.6) and 3–12 (=7.8) for the same three species respectively, and increased significantly with maternal length in C. brevipinna. Length-at-birth (L0, cm LT) ranged from 92 to 96 for C. obscurus, 79–82 for C. brevipinna and 66–76 for C. plumbeus, and all three species exhibited lengthy gestation periods, overall embryonic sex ratios of 1:1 and synchronous parturition in autumn. However, given limitations with respect to sample size and temporal distribution inherent in this study, the reproductive parameters defined herein are necessarily preliminary. Nevertheless, this research challenges a range of findings emanating from other parts of the world and, in doing so, raises pertinent questions relating to the resilience to targeted fishing activities of these species in New South Wales waters compared to other oceanic regions. Moreover, it reinforces the importance of locally derived demographic parameters for population modelling and stock assessment.