Fly-shoot seining for demersal species is a commercially important fishery for Irish vessels operating in the Celtic Sea. We conducted a pilot-scale assessment of cod (Gadus morhua) survival in the ...fly-shoot seine fishery using survivorship pop-up satellite archival tags (sPATs). Supporting information on fish condition including vitality on a modified four-point scale - excellent, good, poor and moribund - was collected for all cod caught during the trial. Ten cod of suitable size for tagging (≥ 55 cm) in excellent and good vitality were tagged and released. Cod in poor and moribund vitality were assumed to have died. A Kaplan-Meier estimator was used to assess cod survival. Of 96 cod caught during the trial, 53 (55%) were ≥ 55 cm with 18 (34%) of these found to be in excellent or good vitality. Tagged fish were representative of the latter component of the catch. All 10 tags reported data with tag deployment period ranging from 2 to 21 days and a mean survival of 10 days. Assuming a 34% survival probability at time zero, survivability gradually decreased to zero at day 20 when all fish were assumed to have died. Barotrauma was likely the main cause of poor survival in this study. Alterations to fishing operations to reduce barotrauma are technically feasible but unlikely to be commercially viable. The sPATs and tagging procedures used in this study generally worked well in estimating cod survival rates, elucidating mortality causes and have potential applications for other species.
•Study on post-capture cod survival in the Irish fly-shoot seine fishery successfully completed.•Cod survival was poor mainly due to barotrauma.•Survivorship pop-up satellite archival tags were effective in assessing cod survival and elucidating mortality causes.•sPATs have major potential to assess survival in other species.
•Significant reductions in unwanted fish catches were achieved in the Nephrops mixed fishery.•Effective separation of Nephrops and fish species in the trawl facilitates application of alternative ...selectivity measures.•The combination of catch based management and quota based incentives drives uptake of more selective gears.
Trawlers that target Nephrops norvegicus and a range of demersal fish species need gear options to reduce bycatch and address European Union landing obligation requirements. We demonstrated how this can be achieved by using inclined panels to separate fish species into an upper codend with 90 mm T90-mesh, and Nephrops norvegicus into a lower codend with 80 mm diamond-mesh. A nested mixed effects model was developed to compare proportional catch at length of key species retained in test or control trawls, and based on this, the conditional probability of retention in the upper and lower test trawl codends. Haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) < minimum conservation reference size (MCRS) were significantly reduced, while haddock ≥ MCRS, Nephrops, cod (Gadus morhua), monkfish (Lophius piscatorius), and commercial flatfish catches were generally maintained in the test trawl. Model outputs demonstrated effective separation of Nephrops into the lower codend, and size dependent separation of haddock and whiting into the upper codend. Strong performance of this catch separation device facilitates alternative selectivity measures depending on landing obligation requirements. Additional benefits such as improved catch quality and reduced catch sorting times, but also the need for further incentives to encourage industry uptake are discussed.
•Large Atlantic bluefin tuna tagged off the coast of Ireland utilize the warm North Atlantic Current to access foraging areas in the North Atlantic Ocean.•Five hotspots of Atlantic bluefin tuna occur ...in the central and eastern North Atlantic Ocean. Each are in regions with long-lived, quasi-stationary anticyclonic features (i.e., eddies or recirculation).•Daily maximum depth and time at mesopelagic depths (i.e., greater than 200 m) are positively correlated with absolute dynamic topography in the open ocean.•In the Winter, a majority of Atlantic bluefin tuna tagged in Irish waters travel to the Newfoundland Basin, a region with intense mesoscale eddy activity and high mesopelagic fish biomass.•Some Atlantic bluefin tuna migrated in the Spring to the Mediterranean Sea, an important spawning ground for this species.
Electronic tagging of Atlantic bluefin tuna (ABT; Thunnus thynnus) has shaped our understanding of their movements and migrations throughout the Atlantic basin. In this study, we used pop-up satellite archival tagging data to examine the movements of 51 large (CFL µ ± σ: 215 ± 15 cm) ABT tagged off the coast of Ireland. When combined with satellite oceanographic data, we found that ABT take advantage of the warm North Atlantic Current to access foraging areas in the North Atlantic Ocean. We identified four potential foraging regions: (1) off the coast of Ireland, (2) the Bay of Biscay, (3) the Newfoundland Basin, and (4) the West European Basin. In addition, 14 ABT migrated to their spawning grounds in the Mediterranean Sea, entering by May 16 and exiting by July 7, on average. In all five regions, anticyclonic ocean features (i.e., eddies or recirculation) were present. In the open ocean, these features often co-occurred with areas where the daily maximum depth of tuna exceeded 400 m and tuna spent extended time at mesopelagic depths (i.e., greater than 200 m). We hypothesize that ABT exploit anticyclonic structures to forage on the abundant mesopelagic fish communities. Additionally, our results suggest that ABT are travelling across the North Atlantic Ocean in a directed migration to the Newfoundland Basin to reach what may be one of the best mesopelagic feeding grounds in the world.
•Quantitative estimates of seal bycatch in Irish set-net fisheries are provided.•Key factors affecting bycatch include mesh size, depth and landings of target species.•Potential seal bycatch ...mitigation measures at an operational level are discussed.
A dedicated observer programme was carried out in gillnet and entangling net fisheries off the west and southwest coasts of Ireland to monitor interactions with seals. No seals were observed as bycatch in gillnet fisheries suggesting the risk of bycatch in observed gillnet fisheries is low. Grey seals (Halichoerus grypus) and to a lesser extent harbour seals (Phoca vitulina) were observed as bycatch principally in large mesh tangle nets targeting crawfish (Palinurus elephas). Observed bycatch levels, proximity of grey seal colonies to crawfish fisheries and similar habitat preferences suggest that the risk of seal bycatch in tangle net fisheries for crawfish on the west and southwest coasts of Ireland is high. Factors affecting bycatch in tangle nets were modeled to investigate potential bycatch mitigation measures. Crawfish and monkfish (Lophius spp.) catches, depth of gear deployment and larger mesh size were significantly positively correlated with seal bycatch. Development of mitigation measures such as improved net visibility, use of smaller mesh size and reintroduction of pots has major potential to reduce seal bycatch in the observed tangle net fishery. Growing seal populations in regions where tangle netting for crawfish is most prevalent could be related to factors such as benefits accrued from depredation and possible immigration from adjacent populations in the UK. More explicit conservation objectives will likely be needed to provide an impetus for development of proposed mitigation measures and bycatch reductions in Ireland. Results of this study also have broader ramifications for management of pinniped bycatch in large mesh gillnet and entangling net fisheries, which are widespread but poorly studied in European Community waters.
Fisheries bycatch represents a major anthropogenic threat to marine megafauna worldwide. To identify populations at risk, it is essential to estimate the total number of individuals removed from a ...population as bycatch. However, estimating total bycatch remains challenging due to the often-limited scope of monitoring programmes. In this study, we aimed to maximise the value of limited bycatch data collected by scientific observers and self-reported by fishers to provide estimates of total seal bycatch for static net fisheries operating in Irish waters. We constructed a model of bycatch rate as a function of known predictors of seal bycatch, and used this to predict bycatch rates throughout the Irish Exclusive Economic Zone. Annual estimates of seal bycatch, from 2011 to 2016, ranged between 202 (90% CI: 2-433) and 349 (90% CI: 6-833) seals per annum. Estimated bycatch exceeded the precautionary threshold of Potential Biological Removal (PBR = 165-218; Fr=0.5) for the national grey seal population but was below less conservative threshold values (PBR = 330-437; Fr=1.0), with confidence intervals spanning both. Further research on the population structure of grey seals in the Northeast Atlantic is needed to set appropriate bycatch thresholds. Nonetheless, this study shows that by utilising predictive models to maximise the value of limited bycatch observer effort, we can produce informative estimates of protected species bycatch and highlight areas of high bycatch risk. We present this as a case study for maritime nations with comparatively limited bycatch data to fill key data gaps in protected species bycatch worldwide.
•Issues regarding the accuracy and precision of CPUE standardizations prevail.•Standardization using normal and finite mixture models is compared.•Residual diagnostics in the finite mixture model are ...markedly improved.•Precision of estimates is increased.•Fish abundance and fleet behavior are more accurately reflected.
Catch-per-unit-effort (CPUE) data are routinely standardized to account for factors that influence catch rates that are not related to resource abundance. Despite improvement in the methods applied to CPUE standardization, for many datasets model diagnostics can still indicate poor conformity to modeling assumptions, imprecision and unexplained fishing behaviors. In this study we examine catch rate data of an Irish mid-water pair trawl fleet targeting albacore tuna (Thunnus alalunga) in the North East Atlantic. A fleet strategy of searching and congregating on fish aggregations combined with negative skew in model residuals suggest that multiple components exist within the dataset. Assuming up to five components, finite mixture models are applied and compared using the Bayesian information criterion. The two component model most consistently explained observed distributions in fishing behaviors and catch rates. Finite mixture modeling markedly improved conformity to modeling assumptions, resulting in substantial improvement in the precision of specific components used in CPUE standardization and reduced inter-annual variability of the catch rate trend. These methods may facilitate investigations of technological creep but also raise questions on how best to use the results in assessment.
Bycatch of protected species in static net fisheries is a global conservation concern and is currently considered the dominant anthropogenic threat facing many marine mammal species. Effective ...bycatch mitigation remains challenging, contingent on an understanding of the underlying mechanisms that cause individuals to become entangled.
We combined data collected by scientific observers and fishers to identify predictors of seal bycatch in static net fisheries along the west, southwest and south coasts of Ireland. We first analysed the broad regional and seasonal trends in seal bycatch before identifying environmental variables that could explain these patterns.
Based on negative binomial generalized linear mixed effects models, seal bycatch significantly varied with season and decreased with greater distance to major seal colonies and in clearer, less turbid water.
Synthesis and applications. Our results suggest that distance to major seal colonies was a significant driver of spatial variation in seal bycatch, and water turbidity a major driver of seasonal trends. These findings will enable us to identify future bycatch risk and target mitigation measures effectively. This is the first study to identify the effect of water turbidity on bycatch of a protected marine species. Increasing net visibility in turbid waters may provide a novel approach to mitigating against protected species bycatch in static net fisheries.
Our results suggest that distance to major seal colonies was a significant driver of spatial variation in seal bycatch, and water turbidity a major driver of seasonal trends. These findings will enable us to identify future bycatch risk and target mitigation measures effectively. This is the first study to identify the effect of water turbidity on bycatch of a protected marine species. Increasing net visibility in turbid waters may provide a novel approach to mitigating against protected species bycatch in static net fisheries.
•Pop-up archival satellite tags were successfully deployed in albacore tuna.•Shallow nocturnal and variable diurnal depth preferences were exhibited.•Deep diving behaviour was adapted to deal with ...cold water.•Environmental factors significantly affected vertical depth preferences.
Pop-up archival satellite tags were deployed on North Atlantic albacore tuna (Thunnus alalunga) in the first experiment using this technique on this species. New information on movement, habitat use and fish behaviour is presented. Data from five successful tag deployments for periods of up to 106 days revealed consistently shallow nocturnal depth preferences and progressively deeper diurnal depth preferences as seasons unfolded. In situ environmental variables such as chlorophyll-a and mixed layer depth significantly affected vertical depth preferences. Adapted deep diving behaviour to colder water below the mixed layer was also exhibited in the first record of repetitive bounce diving in albacore tuna.
Reports from the fishing industry suggest that seal depredation in Irish bottom-set gillnets and entangling net fisheries has increased substantially in recent years. A dedicated observer program was ...conducted in a range of such fisheries off the southwest and west coasts of Ireland to provide the first quantitative estimates of seal depredation. Zero inflated negative binomial and Poisson regression models found positive correlations between depredation and factors such as latitude, depth, timing of a haul within a trip and quantities of gear hauled. Soak time was significant in the inshore gillnet fishery for pollack species (Pollachius spp.) but not significant in the deeper more offshore gillnet fishery for hake (Merluccius merluccius). Results suggest that soak times should be kept short in shallow areas while faster hauling speeds, and systems which actively deter seals from the vicinity of Vessels operating in deep water should be explored.
•Pop-up satellite tagged albacore were predated.•Shortfin mako and porbeagle sharks are identified as the likely predator.•The observed predation rate is thought to be elevated due to irregular ...post-release behavior.
Three out of eight albacore (Thunnus alalunga) tagged with pop-up satellite archival tags (PSATs) in the greater Bay of Biscay area of the North East Atlantic were predated as indicated by missing light level data, sudden increases in temperature, and substantial changes in depth profiles. Recorded stomach temperatures and differences between stomach and ambient water temperatures were consistent with shortfin mako (Isurus oxyrinchus) and porbeagle (Lamna nasus) sharks. Vertical behavior in the form of diel migrations, deep dives and extensive periods spent close to the surface during day and night time was again consistent with Lamnid sharks. The observed predation rate was considered to be elevated due to irregular post-release behavior as opposed to symptomatic of natural mortality.
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