Codend selectivity of the Danish seine investigated by the trouser trawl method

During June 1986 codend selectivity of the Danish seine was measured using both diamond mesh codends and square mesh codends. Comparisons between different codends were made by the trouser trawl method, having two parallel 12 m codends with either different meshshape or different meshsize. A special construction panel in front were ("trousers") with an 8 m lang vertical used for dividing the extension piece/rear belly into two equal parts

Height and spread of the Danish seine

Height and spread of trawls is successfully measured by the wireless "Scanmar" system. Several experiments made by fishermen have shown that this system will not work on the Danish seine. A modified version of the "Scanmar" system have been tested for geometrical measurements on the Danish seine. During May 1987 a total of 11 valid hauls were made by two different types of Danish seines on fishing grounds in Northern Norway. Hauling and fishing procedures were carried out as in the normal, commercial fishery. The measurements clearly demonstrated that it is possible to control the geometry of the Danish seine throughout the haul by this modified version of the "Scanmar" system. In addition to read the height and spread of the net, it was possible to detect whenever a rope got fastened around rocks, and to see when it loosened again. Signals from the height and spread sensors were lost for shorter periods during the haul. Using this (modified version of the) system, the skipper will get continuous information about the geometry of the Danish seine, and thereby know exactly when to start hauling the ropes and when to change the hauling speed, in order to keep a steady/continous closing of the wings

Trawl Selectivity in the Barents Sea Demersal Fishery

This chapter provides a general overview of the Barents Sea demersal trawl fishery. First, it reviews historical catch levels and current biomass status of four commercially important demersal species (cod, haddock, Greenland halibut, and redfish) and includes an overview of their management plan that has been carried out by the Joint Norwegian–Russian commission. Then, it presents the evolution of the technical regulations for improving size selectivity in this fishery and describes current challenges in gear selectivity. Later, this chapter describes the concept of size selectivity, introduces the selective parameters that define a selection curve, and progressively introduces different parametric models that describe the selection process. The most common experimental methods and gear used to collect selectivity data are described, and their advantages and disadvantages are discussed. Finally, this chapter describes an alternative, or a complementary method, to the conventional estimation of trawl selectivity—the FISHSELECT method. This method is based on morphology measurements and fish penetration models to estimate the selective properties of different mesh shapes and sizes at different mesh openings, which are later used to provide simulation-based prediction of size selectivity. FISHSELECT has already been applied to four important species of the Barents Sea Demersal Fishery, and the results have in all cases showed to be coherent with the results obtained from sea trial results

Effect of grid section design on trawl size selectivity

Sorting grids are introduced in trawl fisheries to improve size selectivity and reduce the variability of results obtained with sorting devices constructed of netting e.g., codends. Grids are rigid or semi-rigid structures where the bar spacing defines the sizes of fish that can pass through. However, for size selection to occur, fish need to be oriented towards the grid in specific ways, which may depend on the netting construction where the grid is installed. In the Barents Sea gadoid trawl fishery, fishermen are allowed to use different netting configurations within the same type of grid, but it is unclear to which degree the performance of sorting grids depends on the construction characteristics of the netting section where they are installed. This study compares the size selective properties of a steel grid mounted in three different netting section configurations: 2-panel section, 4-panel section, and 4-panel section with a modified lifting panel. Overall, the results of the study demonstrate that the 2-panel configuration performs better than the two 4-panel configurations tested. The differences were smaller when the 2-panel and the 4-panel section with a modified lifting panel were compared. Specifically, the grid contact probability for haddock with the 2-panel configuration was 0.92 (0.90 – 0.95) and significantly higher than for the 4-panel configuration, which was estimated to be 0.82 (0.77 – 0.89). For cod, the grid contact probability between the different configurations did not vary more than 6% and the differences were not significant. The 4-panel configuration led to significantly higher retention of cod between 40 to 60 cm and haddock between 20 and 54 cm than the 2-panel configuration. The results also show that despite having been assumed to provide constant size selectivity, the performance of sorting grids is sensitive to relatively small design changes in the section where they are mounted

The effect of semi-pelagic trawling on american plaice (hippoglossoides platessoides) by-catch reduction in the northeast arctic shrimp fishery

The present study reports results of a small-scale preliminary experiment to evaluate whether lifting trawl doors and sweeps from the seabed can lead to a reduction of by-catch in the Northeast Arctic shrimp trawl fishery. We carried out a catch comparison and catch ratio analysis between two gear configurations: one with trawl doors and sweeps on the seabed (traditional rigging) and the other with doors and sweeps clear of the seabed (semi-pelagic rigging). The study focused on the by-catch of American plaice (Hippoglossoides platessoides) and showed that the gear was significantly less efficient at catching this species when rigged in the semi-pelagic mode. When rigged this way, the gear captured 52%–66% fewer American plaice between 10 and 40 cm compared to traditional rigging. Moreover, this difference was significant for sizes between 12 and 31 cm, and it increased with fish size. The herding efficiency of doors and sweeps for American plaice was estimated to be 100% and significantly higher than 0 for these sizes. Finally, the analysis carried out did not detect a significant reduction in shrimp catch (in kg) with the experimental gear. This study demonstrates that in addition to having recognized environmental advantages, lifting the doors and sweeps from the seabed could help mitigate American plaice by-catch in the Northeast Arctic shrimp fishery. Although these preliminary results are promising, more extensive data collection is required before definitive conclusions can be reached.acceptedVersionpublishedVersio

Field demonstration of a novel towed, area bubble-plume zooplankton (Calanus sp.) harvesting approach

SINTEF post https://www.sintef.no/en/publications/publication/?pubid=CRIStin+1097811.This paper presents field experiment results of using a new technology to harvest copepods (Calanus sp.) in the ocean by bubble upwelling. Two largo-scale bubble rafts, a 21 m2 and a flexible 75 m2 bubble raft with tow parallel and perpendicular sparger elements, respectively, were field tested in areas with high, upper 25-m water column Calanus densities. Measured bubble-driven upwelling velocities (Vup) at different air flows (Q) and depths (z0), found Vup-(f21 under stratified marine conditions, in agreement with other results for stratified conditions. Bubble trawls significantly enhanced Calanus concentrations in the upper water column, as much as 1416% integrated over the upper 2·m, with the best results for the tow-transverse raft, which also was tested under unstratified seas. Bubble trawl perfonnance aJso was related to stratification with the highest enhancement for the lowest stratification. Catch species analysis showed reduced bycatch. Thus, this new harvesting technology showed a potential to develop an economically robust, environmentally benign, and sustainable fishery on a renewable resource at lower trophic levels in the food web, within the context of ecosystem-based managemen

MultiSEPT - Full scale tests of the semicircular spreading gear (SCSG)

Source at http://hdl.handle.net/11250/2463877This reportdescribes full scale tests at sea with the semi-circular spreading gear (SCSG). This activity is part of Work Package 3 (WP3: Development of new ground gear) of the MultiSEPT project. The spreading of the SCSG and rockhopper gear was measured by useof MARPORT distance sensors. Bottom contact was monitored by use of SCANMARtrawl sensors and by underwater cameras mounted in the headline and bellyof the trawl. Fishing efficiency of these two gears was assessed by comparing the size distributions of fish caught by each gear. The results showed that the spreading (distance between wing-ends)was approximately 7 % higher with the SCSG than with the rockhopper gear for the same door spreading. The SCSGhad good bottom contact and passed bottom obstacles (e.g. stones) easily. The size distribution of fish caught with the SCSG was very similar to that caught with the rockhopper, but apparently more cod (over 65 cm) and more haddock (of all sizes) was caught by this gear. However, the number of hauls performedwith the SCSP and with the rockhopper was too small to draw a clear conclusionon catch efficiency. The SCSGis a gear that is easy to rig and handle on deck, it does not require accurate adjustments, it has few control points, it has low weightand the results indicate that its performance at sea is comparable or better than the rockhopper gear for the given bottom conditions. Further development of the SCSG should emphasise on the choice of material that can reduces wear ofthe gear and the extent its life

Managing size selectivity: the relevance of compulsory and alternative selection devices in the Northeast Atlantic bottom trawl fishery

Two decades of use of the compulsory selectivity gear configuration in the Northeast Atlantic bottom trawl fishery, which consists of a rigid sorting grid followed by a diamond mesh codend, have revealed problems with performance and efficiency. Size selectivity gear alternatives to this configuration are being pursued, and recent studies of codends with shortened lastridges have reported promising results. In this study, we compared the size selectivity and catch efficiency for cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) caught using a sorting grid with 55 mm bar spacing followed by a 130 mm knotted diamond mesh codend (i.e. the compulsory gear) that same configuration but considering only the catch in the 130 mm diamond mesh codend (regular codend), and an alternative knotless codend with shortened lastridge ropes and a thinner twine. We also addressed the question of whether size-selective gear is even needed to avoid exceeding the bycatch limits of the fishery. Our results demonstrated that the alternative codend improved size selectivity by reducing the loss of fish above minimum legal size (MLS), with a minor increase in the retention of fish below MLS, compared to the compulsory combined grid and codend configuration. The regular codend also reduced the loss of fish above MLS, but the retention of haddock below MLS approached the legal limit of 15% by number. Our results also showed that the abundance of cod below MLS in the fishing area during the trials was low. They also suggested that spatial and seasonal closures in combination with a flexible choice of selection device and modern monitoring tools could be an alternative to rigid enforcement of the use of selective fishing gear.publishedVersio

Effect of Codend Design and Postponed Bleeding on Hemoglobin in Cod Fillets Caught by Bottom Trawl in the Barents Sea Demersal Fishery

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