Mitigating slipping-related mortality from purse seine fisheries for small pelagic fish: case studies from European Atlantic waters

The release of unwanted catches (UWC) from purse seines, while the catch is still in the water, is known as “slipping”. Once thought to be a benign process, compared to discarding UWC overboard from the fishing vessel, it is now recognised that “slipping” can lead to significant mortality in the released fish if done inappropriately. In this chapter, we examine purse seining and slipping operations, and discuss what drives slipping and potential mitigation measures to reduce slipping mortality. We use three examples of purse seine fisheries for small pelagic species in the North-east Atlantic; from Norway, Portugal and Spain. The ideal solution (identifying and avoiding UWC before the net is set) requires the development of tools to enable fishers to better characterise target schools in terms of key selection criteria, e.g., with respect to species, individual size and catch biomass. Such tools are being developed, based primarily on hydro-acoustic technology. However, some UWC in purse seine catches are inevitable, and operational improvements in slipping practices have been shown to significantly reduce stress and mortality in the released UWC. We conclude with a discussion on the challenges currently facing the implementation of the European Union (EU) Landing Obligation with regards to minimising slipping related mortality.info:eu-repo/semantics/publishedVersio

In Search of New Beginnings: On Poetry and Hope

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Pre-catch and discard mortality in Northeast Atlantic herring and mackerel fisheries: consequences for stock estimates and advice

Unaccounted mortality caused by discarding or pre-catch losses is a major challenge for fisheries management. In pelagic fisheries, a considerable proportion of catches may be lost due to intentional release of unwanted catch (slipping) or net bursts (fishing net tears due to the weight of the catch). Here we review and estimate ranges of discard and pre-catch mortality for two important pelagic fisheries, the Northeast Atlantic (NEA) mackerel and Norwegian spring spawning (NSS) herring, and explore the effects on stock estimates and catch advice. We show that mortality caused by discarding, slipping, and net bursts is unknown but probably corresponds to a considerable percentage of total registered catches. Including estimated unaccounted mortality into assessment models leads to underestimation of the stock levels by 3.7–19.5% and 2.8–6.8% for NEA mackerel and NSS herring, respectively, corresponding to up to several million tonnes of fish that die annually due to fishing without being landed. If discard and pre-catch mortality were eliminated, allowed catches could increase by 10–20%. We demonstrate that unaccounted mortality in pelagic fisheries may be substantial, affecting stock estimates and catch advice. This may undermine the sustainable management and efficient use of pelagic resources.publishedVersio

Effects of fish behaviour on abundance and length frequency estimates from in-trawl stereo cameras

In-trawl stereo cameras can provide fine-scale spatial and temporal information on species along the trawl path and record small-sized and fragile organisms typically absent from catches. Reliable estimates of abundance and length frequency from in-trawl cameras will improve ecosystem understanding and lessen the need for physical catches on scientific surveys. However, determining these estimates from camera footage is challenging since the same individual can appear in multiple frames and swim repeatedly in and out of the camera’s field of view. The manual image analysis performed in this study provides important information on how the swimming behaviour of three abundant pelagic taxa in the Norwegian Sea, along with a camera’s field of view and frame rate, affect the number of repeated appearances. Moreover, these manual annotations serve as a valuable dataset for validating automatic image analyses. Our results show that, depending on the taxa, swimming orientation, length, density of individuals, and distance to the camera affect the extent of time an individual is observed. If the repeated appearance of individuals is not accounted for, taxa or length classes with fewer appearances are under-represented in relative abundance and lead to skewed length frequency distributions. Compared to herring and blue whiting, a large fraction of mesopelagic fishes remains undetected during automatic analysis (RetinaNet). Assessing the factors driving repeated appearances improves our understanding of in-trawl camera data and highlights the importance of integrating tracking with automatic image analysis.publishedVersio

First Test Results from the New Norwegian Lidar for Monitoring of Marine Resources

Following promising lidar test flights in 2002, Institute of Marine Research (IMR), Bergen in cooperation with NTNU University, Trondheim started the process of designing and building a new lidar to be ready by summer 2004. It is based on the National Oceanographic and Atmospheric Administration (NOAA) lidar used during the 2002 test flights. The paper presents the first results obtained during the mackerel (Scomber scombrus) survey in July 2004. This is an annual survey using two commercial vessels to map the distribution and density of mackerel in the Norwegian Sea during summer feeding using sonar and trawl. The lidar flights cover the same tracks as the vessels and the results are compared. An intercalibration routine between lidar and sonar was tested. The idea was to cover a school of fish observed on the vessel sonar by numerous lidar flights. Finally, the plans for mounting the lidar onboard R/V “G.O. Sars” is presented

Mortality of North Sea herring that is crowded and subsequently slipped from a purse seine

Catch regulation by slipping whole or parts of the catch has traditionally been used in NE-Atlantic purse seine fisheries for pelagic species if the catches are considered too big or the quality or size of the fish is considered unsatisfactory. This is particularly the case when the prize differs between sizes or quality groups of fish (high grading) as is often found with herring. No information is, however, available on the survival rate of herring that is slipped from the purse seine or how significant this mortality is in relation to total fishing mortality. The aim of this study is to quantify mortality of herring crowded to different degrees in the purse seine and subsequently slipped. Large-scale open-sea survival experiments were carried out in the North Sea in 2008 and in 2009. Herring caught by purse seine were allowed to swim from the seine to large circular net pens in an early phase of hauling. Commercial crowding conditions were simulated by lifting the bottom of the net pen. The mortality rate four to five days after crowding ranged from 1.8% in the least crowded to 50.7% and 52.0% in the hardest crowded groups. Control group mortality was low, between 0.9% and 2.0%. These results provide important information on what crowding densities can be tolerated in the purse seine fisheries for herring and suggest a need to revise the legislation on slipping in these fisheries. Keywords: unaccounted mortality, slipping, purse seine, herring, crowdin

Norsk litteraturkritikk i mellomkrigstiden : med hovedvekt på Eugenia Kielland og Sigurd Hoel

Masteroppgave i nordisk språk og litteratur- Universitetet i Agder, 201

Survey report: Developing methods for abundance estimation of bluefin tuna in Norwegian waters

The major aim of this survey was to map and quantify distribution, school size and behaviour of Atlantic bluefin tuna (Thunnus thynnus) migrating to and feeding in Norwegian waters.The survey covered the region between Bergen and Molde, between 4th to 15th October, onboard hired vessel M. Ytterstad (75 m long) with promising preliminary results for acoustic methods for monitoring the distribution, abundance and behaviour of bluefin tuna in Norwegian waters.publishedVersio

Testing of trawl-acoustic stock estimation of spawning capelin 2022

This report describes the fourth in a series of trawl-acoustic monitoring surveys of the spawning stock of capelin during the migration to the coast. The survey is a response to a proposal from the industry to evaluate the possibility of using winter monitoring of maturing capelin as an input to the capelin assessment and advice. The timing and geographic coverage of the survey are such that the results would be relevant to use for advice given that the output is reliable. Pre-defined areas off the Troms and Finnmark coast were covered using two vessels, Vendla surveying the eastern part and Eros the western part. A stratified random transect design was originally adopted with two complementary zig-zag grids, the first going in a west-east and the second in an east-west direction over the same strata. However, the high capelin abundances north of Varangerhalvøya lead to changes in the design, and a new stratum with increased effort covering the area with high abundance. The biomass estimate we present is based on the second coverage, but evaluation of the mobility of the fish can be done by comparing the coverages. Echo sounders with frequencies from 18-333 kHz were run together with sonars, and target trawls were carried out on significant pelagic aggregations. Capelin abundance was estimated using 38 kHz data. The median biomass of maturing capelin in the coverage area based on 500 bootstrap runs was estimated at 426 618 tons, with a CV of 42%. The 5% lower and 95% upper confidence limits were 167 555 and 757 229 tons, respectively. The confidence bands overlap with the prediction from the autumn 2021, but the wide confidence interval shows that the survey result is uncertain. The high uncertainty despite the good survey coverage is likely due to the very patchy distribution. Capelin aggregations recorded north of Varanger in the eastern coverage area totally dominated in the estimate. Capelin concentrations were also observed in the west, in areas associated with Fugløybanken and Malangsgrunnen, but the capelin abundance in these areas seemed to decrease over the survey period. Capelin at age 3 dominated in the samples and were estimated to constitute more than 6 times the biomass of the other age groups combined. The 3-year-olds had an average length of 15.5 cm and weight 16.3 g. Maturation had progressed further in the western than the eastern area similar as last year, and quite a high proportion of the females sampled towards the end of the survey in the western area were spent. We undertook 7 stations with a submersible probe measuring acoustic target strength of capelin. Preliminary results indicate that the target strength and acoustic frequency response of capelin this year were more in accordance with expectations for capelin than last year. A thorough evaluation of this survey series and its usefulness as input to the capelin advice will be prepared for the capelin benchmark in June this year.publishedVersio

How to obtain clear images from in-trawl cameras near the seabed? A case study from the Barents Sea demersal fishing grounds

Underwater camera systems are commonly used for monitoring fish and fishing gear behaviours. More recently, camera systems have been applied to scientific trawl surveys for improved spatial resolution and less invasive sampling and to commercial fisheries for better catch control and reduced by-catch. A challenge when using cameras in demersal trawls is poor image clarity due to the door and ground gear generated sediment plume. In this study we have measured the height of the sediment plume produced by a large commercial trawl in the Barents Sea using acoustic methods and investigated its effect on in-trawl camera image clarity. The trawl extension was lengthened, and additional buoyancy added to lift the camera system in the aft end of the trawl. The camera system was tested at increasing heights above seabed until no sediment plume was visible in the images. Based on the acoustic data the sediment plume was measured to be on average 4–5 m (SD 1.7 m) above sea floor. Image clarity improved significantly as the camera system clearance from seabed increased from 4 to 11 m. No effect of sediment type on image clarity was identified. The trawl modifications did not affect the trawl’s opening geometry or bottom contact. However, the increased length and angle of the under panel aft in the trawl and in the extension appears to have resulted in reduced water flow and may influence the passage and retention of fish. The feasibility of using camera systems in demersal trawls and this and other solutions for obtaining clear images are discussed.How to obtain clear images from in-trawl cameras near the seabed? A case study from the Barents Sea demersal fishing groundspublishedVersio