Abstract
Strengthening bags are used in demersal trawl fisheries to prevent the cod-end from bursting when it is being lifted aboard the vessel. Very little is known, however, concerning their effect on the selective properties of the cod-end. This paper reports on covered cod-end experiments to measure the selective performance of a 110 mm cod-end and a 120 mm cod-end, both with and without strengthening bags attached. The design and position of the strengthening bag satisfy current EU regulations. Results were obtained only for haddock. The l 50 of the 110 mm cod-end with a strengthening bag was ∼29.5 cm and was ∼31.4 cm without a strengthening bag. The selection range in each case was ∼4.5 cm. For the 120 mm cod-end, the l 50 was ∼32.4 cm with a strengthening bag and ∼34.3 cm without a strengthening bag and the selection range in each case was ∼5.2 cm. The standard error estimates of each of these parameters were small. These results indicate that the combination of recent Scottish legislation to ban strengthening bags and technical conservation measures of the EU Commission to increase the minimum mesh size from 110 to 120 mm may have increased the 50% retention length of the cod-ends used by the Scottish whitefish fleet by up to 4.8 cm. This may be an overestimate however, since it does not take into account the 90 mm square mesh panel that had to be inserted in the 110 mm mesh cod-end.
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