Biological and economic aspects of mesh size regulation in the multispecies demersal fishery of the Adriatic Sea

Abstract

       Taking into account present high level of exploitation of the Adriatic demersal resources regulative measure as application 40 mm cod-end mesh size by GFCM was recommended (1978) and accepted by member countries.

       As far as this management measure is concerned, a series of selectivity experiments of the various cod-ends have been undertaken along the eastern Adriatic coast by covered cod-end technique, in order to assess and forecast short, mostly economic, and long-term effects especially of the 40 mm streched cod-end mesh size. Five populations: hake (Merluccius merluccius L.), striped mullet (Mullus barbatus L.), pandora (Pagellus erythrinus L.), horse mackerel (Trachurus trachurus L.) and Norway lobster (Nephrops norvegicus L.) were studied.

       Considerations of the short-term effects of the 40 mm cod-end, i.e. number (biomass) of individuals that might be lost throughout of cod-end meshes (retained in the cover) per unit time of one hour pointed out, that in the Adriatic trawl multispecies fisheries with present economic difficulties, losses of U$ 8 to U$ 16 might constrain, further activities.

       Calculations of the changes in yield per recruit (Y/R), assuming present level of exploitation of the studied stocks, either by changing size at first capture (C) or fishing mortality rate (F), gave such results that in all cases higher efficiency (eumetric catch) in management strategy would be realized by increasing 50% retention point (l_c) of the 40 mm cod-end. Increaments of (l_c) values to the optimum (C), except for horse mackerel population, would endoubtedly lead to economic loss. At the same time, results of cod-end mesh size selectivity experiments over the multispecies demersal stocks in the Adriatic Sea noted that Beverton and Holt (1966) vield-isopleth diagram model is not so suitable approach. Regulation of total fishing effort should be an esential objective of the Adriatic trawl fishery policy.