The exploitation of megrim Lepidorhombus whiffiagonis by the Irish demersal fleet

Abstract

Two species of megrim occur in Irish waters but Lepidorhombus whiffiagonis constitutes the bulk of the catch by the Irish demersal fleet. Megrim is exploited as a by-catch of gadoid fisheries and is particularly associated with fisheries for Nephrops, angler and ray. Over the past 25 years megrim landings from the ICES divisions adjoining Ireland have fluctuated between 6,000 and 21,000 tonnes but there has been a gradual upward trend of which Ireland has taken an increasing share, rising to 1,800 tonnes in 1980. Spain exerts most influence in this fishery, being the location of the main market for megrim as well as having the greatest catching power. The most significant developments in the exploitation of megrim over the past twenty years are identified as the establishment of the European Exclusive Fishery Zone in 1977 and the entry of Spain to the Community in 1986. The importance of division Vlla as a producer of megrim has declined and the main fisheries for the species are in divisions VlIg-k. This assessment of the species is based on material gathered in 1989 and 1990 when length frequency data from the landings were collected from the west and south coasts. Lengths of approximately 8,000 megrim from landings from all divisions were collected in 1989, 28,000 the following year, and 1,400 were aged in the two years. Samples of the landings are comparable with graded exports from the fleet. The most intensive work was undertaken on a Nephrops and mixed whitefish fishery in Vllj. Megrim discards (round) are estimated to weigh 13.2% of landings (gutted). Few megrim of less than 20 cm were captured and no 0 group megrim was encountered. A growth curve is calculated for females and for females and males combined. For males the to value is outside the range of that parameter recorded elsewhere. Males are however a small proportion of the total, their higher mortality and slower growth giving them a skewed frequency distribution and reducing their contribution to the landings. Catch curves provided values for Z of 0.45 and 0.49 for females and combined sexes. Yield per recruit curves indicate exploitation is close to F max for females and on the negative slope for the sexes combined.