THE DERIVATION OF AND CHANGES IN NEPHROPS PER UNIT EFFORT VALUES FOR THE NORTHERN-IRELAND FISHING FLEET

Nephrops is commercially the most valuable species landed from the Iri sh Sea (ICES Division VIIa). The largest proportion (51%) of this catc h is taken by the Northern Ireland fishing fleet, for which catch per unit effort (cpue) values in kilograms per rated horsepower hours over the period 1981-1990 are calculated. Results for some 200 vessels and 13 000-15 000 landings per year are given by three alternative method s. In the analyses, data are grouped by months and proportions of spec ies caught. Results produced by different methods are compared and the difficulties of making comparisons between values for different perio ds are discussed. Monthly results for a subset of individual vessels w hich remained consistent throughout the period are isolated and used t o provide paired comparisons between periods, in which statistical tes ts show significant changes between years. There was a gradual and sta tistically significant reduction in cpue following an increase in mini mum allowable mesh size in 1986, with an upsurge in cpue in 1990. Incr eased total effort in recent years shown in the results has been assoc iated with a gradual decline in cpue, despite changes in practice towa rd both longer voyages and night fishing.