Shorter trawls improve size selection of northern shrimp

Discards of small northern shrimp (Pandalus borealis) are a problem in the Skagerrak northern shrimp trawl fishery. To reduce catches of small shrimp, we studied the effect of trawl belly length on size selectivity in November 2017 and June 2018 onboard 15 and 27 m double-rigged shrimp trawlers. The selectivity of the vessels’ standard trawl was compared with a trawl differing only in the belly length, being 37% shorter. The trawls had 40 mm bottom panels and cod ends of 35 mm mesh sizes. Eleven and 14 hauls were made, respectively, in 2017 aboard the 15 m vessel and in 2018 aboard the 27 m vessel. The trawls fished shrimp above 19 mm carapace length equally, while catch rates of shrimp below 15.5–16 mm carapace length in the shorter trawl were more than halved. The results were consistent between the two vessels. In short, modifying trawl length is a simple design modification that can reduce catches of small shrimp. Bycatch of Norway pout (Trisopterus esmarkii) was slightly reduced in the shorter trawl, unrelated to fish length.acceptedVersio

Surface selection of haddock and cod in the Norwegian demersal seine fisheries

Size selection in active fishing gears is a continuous process and undersized fish may escape during the whole fishing operation. Fish that escape during the surface hauling operation are likely to experience higher mortality due to barotrauma-related stress than fish escaping at the fishing depth during the towing process. A well-functioning selectivity device should therefore select mostly at depth for enhancing survival probabilities of escaping fish. The current gear regulation in the Norwegian demersal seine fishery is likely to cause large proportion of undersized fish to escape at the surface. In this study, we estimated surface selection of haddock and cod in demersal seine by using an automatic release system and a small meshed codend that collected fish escaping during surface hauling. The collecting bag contained 19% undersized haddock compared to 10% in the conventional square-mesh codend indicating that about 50% of undersized haddock brought to the surface were released. The proportions of undersized cod were 8% for the collecting bag and 1% for the conventional square-mesh codend. These results demonstrate that surface selection is significant for both haddock and cod. Based on this finding, we discuss methods to improve size selectivity at the fishing depth.publishedVersio

Selective flatfish seine: A knee-high demersal seine barely catches cod

Norwegian coastal cod (Gadus morhua) protection restricts the use of active fishing gears. Demersal seines, acknowledged as being efficient for targeting flatfish, are therefore largely excluded from the fjords. To exploit plaice (Pleuronectes platessa), a species-selective gear that avoids catching cod is needed. We therefore designed a low-rise demersal seine with a 0.6 m vertical opening and tested it on fishing grounds in Lofoten (Northern Norway), comparing it with a conventional seine that had a vertical opening of ∼3.5 m, and fished both during the day and at night. Six to nine hauls were taken with each of the four gear/time-of-day categories (32 hauls in total). The low-rise seine caught no fewer plaice during day-time fishing, but less at night. Cod and haddock (Melanogrammus aeglefinus) catches were reduced by 94% and 98%, respectively, while catches of sole (Solea solea) increased with the low-rise seine. No catch differences were found for halibut (Hippoglossus hippoglossus), common dab (Limanda limanda), or monkfish (Lophius piscatorius). The low-rise seine therefore enables targeting flatfish while avoiding gadoid catches, although loss of plaice during night-time fishing is to be expected.publishedVersio

Development and evaluation of dynamic catch control devices in the demersal seine fisheries for cod (Gadus morhua)

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Artificial light improves size selection for northern shrimp (Pandalus borealis) in trawls

Size selection in the northern shrimp (Pandalus borealis) trawl fisheries is a widely studied topic. While the focus has largely been on codend and grid selectivity, studies have shown the importance of other design changes and the application of artificial light to evoke behavioural responses. LED lights of three different colours; green (~470–580 nm), white (~425–750 nm) and red (~580–670 nm), were mounted in the belly section of a shrimp trawl to investigate their influence on the overall selectivity of the trawl. The study was conducted using a twin-trawl setup, one with light and the other without light. For catch-comparison analysis, a polynomial regression with random effects was applied. The number of valid hauls with green, white and red lights were eleven, eight, and nine, respectively. All lights tested significantly affected the length-dependent retention of shrimp. Green light had the greatest effect, red the least. Significant loss was observed for shrimp below 17.5 mm carapace length (CL) for green light, 19.5 mm CL for white and 20.8 mm CL for red light.acceptedVersio

Fangstkontroll i kolmulefisket — M/S Morten Einar, mars 2023

Prosjektet fokuserer på å utarbeide tekniske innretninger for å redusere faren for sprenging av trålposer i fisket etter kolmule. Fisket etter kolmule med norske fartøy foregår i all hovedsak med pelagisk trål og i størst grad i havområdene vest av Irland og videre opp i færøysk sone. Fiskeposen utsettes ofte for stor belastning som følge av store fangster. Den største belastningen inntreffer når posen er full når den kommer opp til havoverflaten. Årsaken til dette er de store trykkendringene som gjør at fisken får luft i svømmeblæren og volumet av fisk i fiskeposen øker og krever mere plass. Er det da for lite plass til å kunne kompensere for volumøkningen så kan fiskeposen sprekke (sprenges). Konsekvensen av dette er tap av fangst og ødelagt fiskepose. Prosjektet startet opp i 2018 med møter med næringen om problemstillingene i dette fiskeriet. Havforskningsinstituttet har det faglige ansvaret for prosjektet. I 2019 ble det fulgt opp med praktiske fiskeforsøk og logging av trålposer i fiske ved hjelp av sensorer som ble montert på trålen til seks kolmuletrålere. På grunn av koronasituasjonen ble det ikke gjennomført forsøk som planlagt i 2020, slik at det første organiserte fiskeforsøket i prosjektet ble gjennomført i mars 2021. Det som ble testet da gav oss et godt bilde av hva en burde utvikle videre for å imøtekomme ovennevnte problemstillinger. For videre forsøk i 2022 tok vi med oss erfaringene fra 2021, og kom fram til et konsept som ut fra forsøkene kan være en god løsning for videre praktisk uttesting i kommersielt fiske. Konseptet er et fangstbegrensningssystem som fungerer slik at overskuddsfangst slippes ut av trålen på fiskedypet gjennom stor åpning i trålens underpanel. Fisken som er kommet inn i fiskeposen holdes tilbake ved hjelp av et fiskelås som hindrer utslipp av fisk fra fiskeposen i havoverflaten. Størrelsen på fangstene reguleres ved å binde over fiskeposen slik at den tar ønsket mengde fisk.Fangstkontroll i kolmulefisket — M/S Morten Einar, mars 2023publishedVersio

Catches in abandoned snow crab (Chionoecetes opilio) pots in the Barents Sea

During a 2018 retrieval cruise for abandoned snow crab (Chionoecetes opilio) pots in the Barents Sea, approximately 8600 pots abandoned 1.5 years earlier were recovered. Forty-three percent of a subsample of 1000 pots contained snow crabs, with an average of three crabs per pot. Most of the crabs were alive (~98%) and dominated by large males. Pinch injuries and limb loss were common and tended to decline with increasing crab size. Reflex testing showed that the crabs were vital (i.e. the crabs moved their legs, chelipeds and maxillipeds when stimulated), which was supported by a relatively high meat content. However, energy reserves in the digestive glands (hepatopancreas reserves) were low, indicating overall energy deficiencies. Our results indicate considerable unaccounted mortality due to self-baiting, continued catch and cannibalism. The findings demonstrate that snow crab pots which are lost or abandoned in the Barents Sea fishery maintain huge potential for ghost-fishing impacts.publishedVersio

Pelagisk trål/Semipelagisk trål – friere redskapsvalg til levendefiske – kunnskap om redskapsbruk i sammenheng med kvalitet og effektivitet

Semipelagisk tråling erfares som lite effektivt redskap for å fange torsk. Dette kan skyldes at med dørene og sveipene hevet noe opp fra havbunnen vil en ikke oppnå samme sveipeeffekt for å samle fisk like godt som ved fiske med konvensjonell bunntrål. En av grunnene til det redskapsvalget var at trålen hadde skjørt istedenfor rockhoppergear, og kunne derfor tas inn på nettrommelen. Redskapen er dessuten av utforming tilsvarende moderne bunntrål som har vært brukt med gode resultater av torsketrålere. Vårt inntrykk er at fangstratene har vært betydelig lavere enn det som tilsvarer forventet 30 % fangsttap. På det siste toktet ble det, ved hjelp av instrumenter og undervannsopptak, bekreftet god bunnkontakt av selve trålen. Vurdert fra resultatene på restitusjon av levende fisk vil det være gunstig med korte hal og begrenset fangstmengde for å gi gode forutsetninger for overlevelse og restitusjon. Ut fra våre undersøkelser tyder det på at dødsårsaker for den leverte torsken er en kombinasjon av skadene som oppstå under fangst, på skinn, finner og indre blødninger som fører til sekundære komplikasjoner når fisken overføres til merd for lagring. Fisken med slike skader vil dø i løpet av de to første ukene etter den er satt i merd.Pelagisk trål/Semipelagisk trål – friere redskapsvalg til levendefiske – kunnskap om redskapsbruk i sammenheng med kvalitet og effektivitetpublishedVersio

Catch Control in the Blue Whiting Pelagic Trawl Fishery - Research Cruise Report [2022825]: FV “Vikingbank”

This cruise has demonstrated that the catch limitation system can successfully limit catches in the blue whiting pelagic trawl fishery, with negligible loss during the fishing operation while effectively releasing excessive fish when the codend is full (sections 3 & 6),. This solution can significantly reduce the risk of burst codends and discarding of excess fish, thereby making the blue whiting fishery more sustainable and potentially more profitable, through appropriate certification. However, further work is needed on an effective fish lock design (section 4) and a bycatch release system that does not release excessive target catch (section 6). The depth triggered releasers for reliable release of the codend choker also require further development and testing, which should include defining the optimal release depth (sections 5 & 9). In addition to catch limitation, this cruise also investigated the potential risk to the safety of the vessel and crew, during haul-back and pumping, by estimating the weight in water of the catch with total loss of swimbladder gas buoyancy (section 10). Also tested was a prototype method to mitigate this risk during pumping, by automating the release of the pump by a depth triggered releaser (section 7).Catch Control in the Blue Whiting Pelagic Trawl Fishery - Research Cruise Report [2022825]: FV “Vikingbank”publishedVersio

A catch limitation device to avoid excessive catches in the blue whiting (Micromesistius poutassou) Northeast Atlantic pelagic trawl fishery

Blue whiting (Micromesistius poutassou) is harvested in the Northeast Atlantic by a multinational fleet of pelagic trawlers. Occasionally, vessels take catches which exceed their remaining holding capacity and in extreme cases large catches cause codends to burst, resulting in spill of catch. To control catch quantity, a catch limitation system was developed and tested. The system consists of three components: 1) escape opening(s) in front of the codend to release excess fish, 2) a fish lock to prevent loss of fish through the escape opening(s) during haulback and at the surface, and 3) a choking unit to match codend capacity to the desired size of catch. Blue whiting escaped through both longitudinal slots and large (≥ 2 m) meshes in front of the codend. However, video observations showed that with large meshes in the upper panel, large amounts of blue whiting escaped long before the codend was full. Therefore, a design with large openings in the bottom was combined with longitudinal slots in the side and top panels. Two fish locks were tested: an oblique netting panel designed to seal off the codend when vessel speed reduce during haulback, and a cylinder of netting with a choking rope that closed it after the codend was filled. Both fish lock designs inhibited release of fish during haulback and at the surface, but the attachment of the constricting rope in the cylinder frequently broke. The choking unit consisted of a depth triggered releaser connected to a strap of rope wrapped around the codend. Mechanical releasers with factory-set depth trigger and electronic versions were tested. Both freed the choke point as intended, but occasionally at unpredictable depth. A reliable relationship (r2 = 0.94) was attained between codend choking position and catch amount, demonstrating that codend capacity could be adjusted to achieve target catch quantities. Our work shows that controlling catch quantity in the blue whiting pelagic trawl fishery can be achieved effectively through relatively simple modifications to the codend section Future work is needed to optimize the fish lock design and ensure the codend choking rope releasers trigger reliably and at the proper depth.A catch limitation device to avoid excessive catches in the blue whiting (Micromesistius poutassou) Northeast Atlantic pelagic trawl fisherypublishedVersio