Relative selectivity in trawls, longline and gillnets on Greenland halibut
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Selectivity parameters for Greenland halibut (Reinhardtius hippoglossoides, Walbaum) are compared to catches reported fiom trawl, gillnets and longline in the Norwegian scientific fisheries for Greenland halibut. A trouser trawl selectivity experiment reported here gives an L50 at 43 cm in 135 mm codend. A selectivity analysis of the gillnets using loglinear models is done, and show maximum retention probability for lengths at 40.6 - 63.8 cm for the five mesh-sizes used. The effect of the fishiig strategy is analysed in respect to the selectivity of the gear used and the diskibution of length and age in the catches. To avoid possible bias from strong dominating yearclasses and selection in these comparisons, length-at-age data are used. The sex-ratio in gillnet catches is shown to be a linear fiinction of meshsize. Our data show no trend in length distribution with depth. We show that calculated growth of female Greenland halibut is affected by the selectivity of the gears. It is shown that growth parameters calculated fiom gillnet catches may be biased due to the selection properties in the gillnets. These analyses will provide a better understanding of possible sarnpling bias when sampling a stock with only one gear.
Gear selection and sampling gears. Proceedings of the seventh IMR-PINRO Symposium. Murmansk, 23-24 June 1997.