Size selection of large catches:: using sorting grid in pelagic mackerel trawl

Two sorting grid systems with three different bar spacings for size selecting mackerel in pelagic trawls have been tested during four surveys. in the mackerel fishery, the value of the catch depends on fish size; the largest mackerel are priced substantially higher than the smaller fish. Using a grid-selection system to improve size selection in this fishery would therefore increase fishermen's income. Between 8 and 51% of each catch was sorted during 12 hauls, with catch weights ranging from less than 300 kg to more than 170 000 kg. There were significant differences in individual mean weight between mackerel that passed through the grid and mackerel retained in the codend in all hauls as a result of the reduction in the proportion of mackerel below 400 g, and an increase in the mackerel above 600 g. Using 1999 prices as the standard, grid selection increased the value of these hauls from 8 to 18%. Selectivity parameters are presented for all hauls and show that the sorting grid delivered a reasonably sharp size selection in all hauls. Four different models are presented to explore the selection data, and possible influence of catch size on selectivity parameters is tested. Data on grid angle and water flow through the grid are presented from four hauls. (C) 2002 Elsevier Science B.V. All rights reserved.