Tråling over steindekte rørledninger i Nordsjøen

Between 17 June and 1 July 1997 an experiment was conducted to determine to what extent gravel dumped on pipelines crossing the sea floor obstructs bottom trawling in the North Sea. The investigation was carried out by the Institute of Marine Research, Bergen, and was financed by Statoil. Representatives of two fishermen's organisations and the Directorate of Fisheries joined the study as observers. The experiment was conducted on board the research vessel «Michael Sars». The following trawl types, representative for gears normally used in the North Sea fisheries, were used: shrimp trawl («Ballong») with sabb; two industrial fish trawls ("Expon"), one rigged with a small bobbin gear, the other with sabb. During a total of 23 hauls the stone covered pipelines were crossed 41 times. Eighteen hauls were made with the industrial fish trawl and five with the shrimp trawl. During hauling the trawl performance was documented by an underwater camera carried by a towed vehicle (FOCUS) and by trawl instrumentation (Scanmar) that measured the trawl geometry and tension behind the trawl doors. The experiments were performed at several locations along the Zeepipe IIB gas pipeline, around the Draupner platform, and across pipelines between underwater installations and the Statfjord C platform. The gravel dumps turned out to obstruct fishing with industrial fish trawl and shrimp trawl in the investigation area. The trawl was severely damaged in 10 of the 23 hauls. In eight cases the damage could be ascribed to the gravel dumps or the bare pipelines. This conclusion was drawn on the basis of the following observations: trawl inspections with UTV just prior to passing the gravel dumps showed the gear looking normal. Just after passage it was observed that the trawl was severely damaged. In the majority of the hauls in which the trawl was torn, the door spread increased, the trawl opening height decreased and the wire tension decreased just after the trawl had passed the gravel dumps. The damage was fairly similar in all hauls in which tears occurred. The lower belly had been torn from the lower wing or just behind the fishing line and back towards the cod-end. In the majority of hauls it was observed that gravel or stones passed over the gear or sabb and entered the trawl while it was crossing the stone dumps. Gravel was also found in the cod-end after hauling, but never in such large amounts that the weight of the stones in the cod-end itself could explain why the trawl was torn. Stones lying on the rather thin netting in the wings or the belly may increase the friction between the netting and the sea bottom and thus cause small tears, which may easily hook on an uneven bottom and increase the damage. The shrimp trawl and the industrial fish trawl rigged with a sabb were damaged in almost every haul tested. The industrial fish trawl with a bobbins gear was damaged significantly less often. This may be explained by the difference in bottom contact caused by sabb and bobbins gear. In a trawl rigged with a sabb, the trawl opening from wing to wing closely follows the bottom contour, and is likely to dig into the bottom and catch any stone in the trawl path. The bobbins gear, however, has the ability to roll across small objects or uneven seabed and is thus less likely to catch stones, get hooked and break. It may be concluded that light trawl gears rigged with a sabb are unsuitable for trawling over gravel covered pipelines. NORSK SAMMENDRAG: I perioden 17. juni til 1. juli 1997 gjennomførte Havforskningsinstituttet etter oppdrag fra Statoil en undersøkelse som skulle belyse i hvilken grad steinfyllinger på rørledninger i Nordsjøen kan være til hinder for utøvelse av fiske med bunntrål. Undersøkelsen ble utført i samarbeid med representanter fra Fiskeridirektoratet og fiskeriorganisasjoner. Trålforsøkene ble utført ombord på forskningsfartøyet «Michael Sars». Følgende tråltyper, som er ordinære trålredskaper som brukes i kommersielt fiske i Nordsjøen, ble benyttet: Reketrål (type «Ballong») med sabb og to industrifisktråler (type «Expo»), den ene med et lite bobbins-gear, den andre med sabb. Det ble gjennomført 23 hal i forsøksperioden med til sammen 41 kryssinger over steinfyllinger/rørledninger. 18 av halene ble gjort med industrifisktrål og 5 med reketrål. Under halene ble trålen video-overvåket med undervannsfarkosten FOCUS og med trålinstrumentering (Scanmar) som målte trålens geometri og strekk bak tråldørene. Forsøkene ble gjennomført følgende områder: - over steinfylte partier langs Zeepipe IIB (40") gassrørledning - i kryssingspunktene mellom Zeepipe IIB (40") gassledning og Sleipner kondensat (20") rørledning, og mellom Statpipe S35 (36") og Sleipner kondensat rørledning - over steindumpede rørledninger rundt Draupner-plattformen - langs produksjons- og vanninjeksjonsrør mellom Statfjord C og undervanns produksjonsbrønner - langs Statpipe S34 (28") gassledning i Norskerenna Steinfyllingene viste seg å være til hinder for fiske med industrifisktrål og reketrål i undersøkelsesområdet. I 10 av de 23 halene som ble gjennomført i forsøksperioden ble trålen sterkt skadet. I åtte av disse er det sannsynlig at tråling over steinfyllinger/rørledninger var årsaken til skadene. Følgende observasjoner støtter opp om dette. I flere hal kontrollerte man med FOCUS at trålen var hel like før steinhellingen ble passert. Når trålen ble inspisert igjen like etter passering, hadde den store synlige skader. I de fleste hal der trålen ble ødelagt ble det registrert at dørspredningen økte, åpningshøyden gikk ned og wirestrekket ble redusert like etter at steinfyllingen ble passert, noe som indikerer at trålen er skadd. I så godt som alle hal der trålen ble revet, var hele underbelgen ødelagt fra undervinge eller garneringen bak gear/sabb bakover til forlengelsen eller trålposen. I de fleste hal gjorde man video-observasjoner av at stein gikk inn i trålen under passering av steinhellingene. Det ble også funnet stein i trålposen etter hiving, men aldri så mye at tyngden av steinen i seg selv kunne forklare at trålen ble ødelagt. Reke- og industrifisktrålene er laget av relativt tynt nett, og stein som har lagt seg oppå nettet i vinger eller underbelg har trolig forårsaket økt friksjon mot sjøbunnen og forårsaket rifter. Slike små rifter hekter seg lett fast i ujevnheter på bunnen slik at skadene forstørres. Industritrålen som var utstyrt med sabb ble skadet i alle hal. Det samme gjaldt reketrålen, med unntak av to hal som ble gjort på bløt bunn i Norskerenna. Frekvensen av skader på industrifisktrålen med bobbinsgear var mindre. Det var trolig gearet og ikke selve trålen som var viktigste årsak til forskjell i skadefrekvens. Sabb går tettere i bunnen enn bobbinsgear, hekter lettere i ujevnheter og tar inn mer stein, og er derfor mer utsatt for skader. En kan konkludere med at trålredskaper utstyrt med sabb ikke er egnet til å krysse steinfylte rørledninger

Mortality and injuries of haddock (Melanogrammus aeglefinus) that are caught by pelagic longline

By-catches of haddock (Melanogrammus aeglefinus) below legal size (44 cm total length) in the seasonal pelagic longline fisheries for haddock off the coast of Finnmark, northern Norway, are often high. The small fish are torn off the hook at the vessel side by means of a crucifier or a gaff and returned to the sea. It is generally thought that most of the discarded haddock die. An investigation to quantify this mortality was done in the season for this fishery in 1997. The undersized haddock that were torn off the longline hook were recaptured by gently catching them in a dip net as they reached the sea surface. Survival of haddock torn off by means of crucifier alone was compared to haddock torn of the hook by means of a gaff. The fish were transferred in tanks onboard a vessel to holding pens made of small meshed knotless netting floating at the surface. They were visually monitored for 7-11 days. The control group consisted of haddock fished with clean hooks without barbs and gently released by hand. At the end of the observation period the live and dead fish were examined for external damage. The experiment showed a total mortality of 39% for fish that had been torn off by means of a crucifier, and 53% mortality of fish released by means of a gaff. The mortality of the control group was 9%. The injuries of the fish were also analysed

Overleving av torskefisk som unnslipper fra reketrål med Nordmørsrist

The survival of one year old cod (Gadus morhua), haddock (Melanogrammus aeglefrzus) and whiting (Merlangius merlangius) escaping from a shrimp trawl with a metal grid sorting device (the Nordmøre grid) was studied during two subsequent seasons. The fish escapees were collected in cylindrical net cages connected to a cover net covering the exit window above the sorting grid. The cages were released from the trawl and anchored for 5 to 12 days. The fish in the cages were observed daily by underwater television and in the second season also by divers. The escapees from separate trawl hauls were analysed for fish body damage. No mortality was found among the young gadoids during the observation period, except for one haddock in a control group. There was almost no visible skin injuries or scale loss of cod, while that of whiting and haddock was significantly larger. No correlation was observed between the amount of scale loss and fish size.NORSK SAMMENDRAG:Hensikten med å montere inn skillerist (Nordmørsrist) i en reketrål er i hovedsak å sortere ut bifangst av fisk fra rekefangstene. For å studere om ett år gammel torsk (Gadus morhua), hyse (Melanogrammus aeglefinus) og hvitting (Merlangius merlangius) som unnslipper fra en slik reketrål overlever, ble det gjennornført feltforsøk i fjordene i Troms gjennom to feltsesonger. For å fange opp fisk som ble sortert ut fra trålen under tauing, ble utslippsvinduet over skillerista dekket med et finmasket dekknett. Til dekknettet var det festet sylinderformede netting-bur som kunne 1øses fra trålen nede i fiskedypet ved hjelp av en akustisk utløsningsmekanisme. Til kontrollgruppe ble det samlet opp fisk fra trålhal der oppsamlingsburet var festet direkte til trålens forlengelse uten skillerist. Etter en tauetid på 30 minutter ble burene frigjort og ankret opp like ved trålbanen i 5 til 12 dager for å studere overlevelsen til den oppsarnlede fisken. Burene ble overvåket ved hjelp av undervannskamera, og i den andre feltsesongen også ved hjelp av dykker. Bur med fisk fra tre trålhal ble brakt direkte til overflaten for analyse av skjelltap og ytre skader. Det ble ikke observert dødelighet av ett år gammel torsk, hyse eller hvitting under forsøkene, med unntak av en hyse fra kontrollgruppen. På torsk ble det bare funnet ubetydelige hudskader og skjelltap, mens hyse og hvitting hadde noe større skader. Det ble ikke funnet noen sammenheng mellom mengde av skjelltap og fiskens størrelse. Ett år gammel torskefisk ser dermed ut til å tåle utsorteringen fra en reketrål med Nordmørsrist godt

Mortality and injuries of haddock (Melanogrammus aeglefinus) that are caught by pelagic longline

By-catches of haddock (Melanogrammus aeglefinus) below legal size (44 cm total length) in the seasonal pelagic longline fisheries for haddock off the coast of Finnmark, northern Norway, are often high. The small fish are torn off the hook at the vessel side by means of a crucifier or a gaff and returned to the sea. It is generally thought that most of the discarded haddock die. An investigation to quantify this mortality was done in the season for this fishery in 1997. The undersized haddock that were torn off the longline hook were recaptured by gently catching them in a dip net as they reached the sea surface. Survival of haddock torn off by means of crucifier alone was compared to haddock torn of the hook by means of a gaff. The fish were transferred in tanks onboard a vessel to holding pens made of small meshed knotless netting floating at the surface. They were visually monitored for 7-11 days. The control group consisted of haddock fished with clean hooks without barbs and gently released by hand. At the end of the observation period the live and dead fish were examined for external damage. The experiment showed a total mortality of 39% for fish that had been torn off by means of a crucifier, and 53% mortality of fish released by means of a gaff. The mortality of the control group was 9%. The injuries of the fish were also analysed

Mortality and injuries of haddock, cod and saithe escaping through codend meshes and sorting grids

Mortalities and injuries of the gadoids haddock (Melanogrammus aeglefinus L.), cod (Gadus morhua L.) and saithe (Pollachius virens L.) were studied after codend and grid escapement in two full scale trials in 2000 and 2001 in the Barents Sea. The escaped fish were sampled using small meshed cages. Trawl caught controls were sampled by removing the cod end and attaching the cage directly to the cod end extension. In the 2001 trial, control fish were sampled in fish traps in addition. Acoustic closing and releasing devices were used to time the sampling. Survival rates of cod and saithe escaping through codend and sorting grid were 100%. Mortality of haddock were 26.2 to 50.4% (codend escapees), 1.6 to 20% (grid escapees), 4.1 to 26.5% (trawl caught controls) and 0% (trap caught controls). The haddock mortality and injuries decreased with increasing fish length in all groups, with a mortality peek of the mesh escapees with girth approximately the mesh size circumference. Cod and saithe had significantly less skin and fin injuries than haddock, and in general, frequency of skin injuries increased towards the tail. Grid escaped gadoids had significantly less skin and fin damages than the mesh and control groups

Catch comparison between rockhoppers and bobbins ground gear on the Norwegian bottom sampling trawl

In a study aimed at improving the reliability of the Norwegian bottom sampling trawl, comparative fishing between standard bobbins gear and rockhopper ground gear were carried out in the Barents Sea. The catch ratios bobbins/ rockhopper (B/R) for cod and haddock show a higher efficiency for rockhopper than for bobbins, especially for small fish. The catch ratio B/R for cod varied by time of day. Rockhopper seemed to have a higher efficiency compared to bobbins during daytime. The same trend was not found for haddock. These experiments confirm earlier experiments that rockhoppper is more effective than bobbins gear in catching fish close to the bottom. This is particulary true for small cod

Avoidance reactions of ultrasonic tagged cod during bottom trawling in shallow water

Introduction: Observations made by echo sounder (Ona 1988) have shown that cod (Gadus morhua L.) may avoid an approaching survey vessel horizontally and vertically. However, there has been a lack of information on the escapement behaviour of the individual fish. This short note discribes an experiment carried out in a shallow fjord in Northern Norway, where the reactions of single cod towards a small survey vessel during bottom trawling were studied, using ultrasonic tagging technique