Abstract:To determine whether seismic exploration affected abundance or catch rates of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), acoustic mapping and fishing trials with trawls and longlines were conducted in the central Barents Sea 7 days before, 5 days during, and 5 days after seismic shooting with air guns. Seismic shooting severely affected fish distribution, local abundance, and catch rates in the entire investigation area of 40 × 40 nautical miles.Trawl catches of cod and haddock and longline catches of haddock declined on average by about 50% (by mass) after shooting started, which agreed with the acoustic abundance estimates; longline catches of cod were reduced by 21%. Reductions in catch rates were observed 18 nautical miles from the seismic shooting area (3 × 10 nautical miles), but the most pronounced reduction occurred within the shooting area, where trawl catches of both species and longline catches of haddock were reduced by about 70% and the longline catches of cod by 45%; a relatively greater reduction was found (in catches and acoustic estimates) for large (>60 cm) than for small fish. Abundance and catch rates did not return to preshooting levels during the 5-day period after seismic shooting ended.Résumé : Pour déterminer si l'exploration sismique nuit à l'abondance de la morue (Gadus morhua) et de l'églefin (Melanogrammus aeglefinus) ainsi qu'au taux de capture de ces espèces, on a procédé à des travaux de cartographie acoustique et à des essais de pêche au chalut et à la palangre dans la partie centrale de la mer de Barents 7 jours avant, 5 jours durant et 5 jours après une série de tirs sismiques au canon pneumatique. Les tirs ont considérablement modifié la répartition du poisson, abaissé sa densité locale et gravement réduit le taux de capture dans l'ensemble de la région d'étude (soit un secteur mesurant 40 milles marins de côté). En moyenne, les prises de morue et d'églefin au chalut et les prises d'églefin à la palangre ont diminué d'environ 50% (en masse) après que la série de tirs ait commencé. Ces résultats concordent avec ceux des relevés acoustiques d'abondance. Les prises de morues à la palangre ont été réduites de 21%. Des baisses des taux de prise ont été observées jusqu'à 18 milles marins du secteur des tirs sismiques (un secteur de 3 × 10 milles marins), mais l'effet le plus prononcé a été observé à l'intérieur de ce secteur : les prises au chalut des deux espèces et les prises à la palangre de l'églefin ont diminué d'environ 70%, les prises de morue à la palangre d'environ 45%. La réduction est proportionnellement plus marquée dans le cas des poissons de grande taille (>60 cm) que celle des poissons de petite taille, tant dans les prises que dans les estimations par relevé acoustique. Cinq jours après la fin des tirs sismiques, l'abondance et les taux de capture ne s'étaient pas encore rétablis. [Traduit par la Rédaction]

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1999. How vertical fish distribution may affect survey results. -ICES Journal of Marine Science, 56: 345-360.At a selected location in the Barents Sea, acoustic observations were made and bottom-and pelagic-trawl data were collected over a 10-day period. A large proportion of fish were found in the acoustic bottom dead zone. Only during a few hours in the daytime were high acoustic values obtained, mainly consisting of large haddock ascending from the bottom. Small haddock and redfish dominated the acoustic recordings at night. The bottom-trawl catches showed greater variability and higher average catch rates during the day than at night, but the diel variations were relatively less pronounced than those of the acoustic recordings. The largest reductions in catch rates from day to night were observed in small haddock and redfish. This was consistent with the observation that these species were pelagic at night. The acoustic observations and the bottom-trawl catch rates were found to be correlated with diel cycles in observed light level and semi-diel cycles in current speed. The results are interpreted in terms of the variable availability and efficiency of the bottom trawl and of the variable availability of the echosounder. 1999 International Council for the Exploration of the Sea

Trawlers involved in the Antarctic krill (Euphausia superba) fishery use different trawl designs, and very little is known about the size selectivity of the various gears. Size selectivity quantifies a given trawl's ability to catch different sizes of a harvested entity, and this information is crucial for the management of a sustainable fishery. We established a morphological description of krill and used it in a mathematical model (FISHSELECT) to predict the selective potential of diamond meshes measuring 5–40 mm with mesh opening angles (oa) ranging from 10 to 90°. We expected the majority of krill to encounter the trawl netting in random orientations due to high towing speeds and the assumed swimming capabilities of krill. However, our results indicated that size selectivity of krill is a well-defined process in which individuals encounter meshes at an optimal orientation for escapement. The simulation-based results were supported by data from experimental trawl hauls and underwater video images of the mesh geometry during fishing. Herein we present predictions for the size selectivity of a range of netting configurations relevant to the krill fishery. The methods developed and results described are important tools for selecting optimal trawl designs for krill fishing.

Abundance estimates of North-east Arctic cod and haddock are given annually on the basis of combined bottom trawl and acoustic surveys.Both survey methods use data from bottom trawl catches in the calculations.The surveys are conducted in the Barents Sea and the Svalbard area with the same bottom trawl, but with different sweep wire length.The bottom trawl indices are calculated for both areas assuming an effective path width of 25 m.In this paper results from parallel trawling experiments with the standard sampling trawl equipped with different sweep lengths are analyzed.These show that the total catch increases with increasing sweep length.Small fish are relatively underestimated by the trawl with the longer sweeps, but no species selection was observed.It is concluded that combining survey results without compensating for the effect of using different sweep length will bias the estimates.The variability in the· presented results demonstrates the complexity of combining data from trawl surveys conducted with different sweep lengths on the bottom trawl.