Journal Article

Seasonal variation in trawl codend selection of northern North Sea haddock

H. Özbilgin, R.S.T. Ferro, J.H.B. Robertson, G. Holtrop and R.J. Kynoch

in ICES Journal of Marine Science

Published on behalf of ICES/CIEM

Volume 63, issue 4, pages 737-748
Published in print January 2006 | ISSN: 1054-3139
Published online January 2006 | e-ISSN: 1095-9289 | DOI: http://dx.doi.org/10.1016/j.icesjms.2005.01.025
Seasonal variation in trawl codend selection of northern North Sea haddock

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We investigate the variation in three separate months of trawl codend size selection for haddock, paying attention to changes in the length/girth relationship, and size of the total catch in the codend. Three cruises were conducted on the same fishing grounds using the same fishing gear on board the same commercial trawler in April 1995, September 1995, and February 1996. The periods coincided with postspawning in April, post-summer feeding in September, and prespawning in February. The seawater temperature at the seabed on the fishing grounds was near its annual maximum in September (12.2°C) and annual minimum in February (7.2°C) and in April (7°C). There was significant variation in selectivity of haddock with month and total catch in the codend. Fish length and, for two of the three cruises, fish maximum girth were used as variables in selectivity analysis. The probability of being retained was lowest in September, when the fish were in good condition and water temperature was highest, but girth was also largest (50% retention length L50 = 33.5 cm; girth at L50 = 172 mm). The retention probability was intermediate in February when the water was cold and the fish were prespawning, again with relatively large girths (L50 = 31.3 cm; girth at L50 = 158 mm). The retention probability was highest in April when the fish were postspawning, the water was cold, and the fish had the smallest girth (L50 = 27.3 cm; girth at L50 = 129 mm). Although care was taken to maintain similar conditions during all trips, there were some differences between April and the other months. Overall, retention was not related simply to girth and length. In September and February, at constant girth, codend retention increased with length, suggesting that within a given month, fish condition has an effect on selectivity. However, at constant length an increase in girth in February led to an increase in retention, but in September to a decrease in retention. A possible explanation is that, in February, the girth of a fish increases owing to its sexual maturity, whereas in September, increased girth is due to maximum volume of somatic tissue. Water temperature and volume of fish muscle affect the maximum rate and power of swimming. Swimming ability may be a significant factor in determining selectivity.

Keywords: haddock; Melanogrammus aeglefinus; North Sea; seasonal variation; selectivity

Journal Article.  6801 words.  Illustrated.

Subjects: Environmental Science ; Marine and Estuarine Biology

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