Fishery Associated Changes in the Whelk Buccinum undatum Stock in the Southwest Irish Sea, 1995-2003

Abstract

The whelk fishery of the southwest Irish Sea had a turnover of approximately €18 million and employed 250 people directly and indirectly in catching and processing in 2003. In the nine years, 1995-2003 inclusive, whelk landings to Ireland from the southwest Irish Sea fluctuated between 3,800 and almost 10,000 tonnes(t) per year; from an estimated 5,000 – 15,000 boat-days annually. A collapse in landings was recorded in 1997 and again in the spring of 2004. The fishery is divided for assessment purposes into four sectors, the central two consisting largely of nursery area and the north and south extremities of the fishery populated by more older, larger and mature whelk. Logbooks were not completed by fishermen participating in this fishery which was, in theory, managed by size limit, but the regulations were not enforced. Boats fishing whelk made daily fishing trips and daily weights recorded by processors were used to estimate biomass in each of the sectors by depletion regressions. Total biomass fluctuated between 12,720 t in 1999 and 37,319 t in 2002. The estimates based on a full season’s landings are used to compare the fishery from one year to the next and to supply weighting factors for other parameters of the population. Exploitation rates were lowest in one of the central sectors of the fishery (where they averaged 27% annually). At the southern extremity they averaged 39%. Depletion estimates based on landings records from approximately April to 15 June provided higher exploitation rates and lower biomass. The mortality coefficient Z, calculated from a catch curve, peaked in two years, 1998 and 2002, as did an index of recruitment. The age at full recruitment declined after 2000. Throughout nine years of documented history, one of the central sectors of the fishery assumed progressively greater dominance over the others, providing 77% of the landings from the entire fishery to its ports in 2003. Some sectoral changes to the whelk population may be irreversible: the oldest animals have been removed from the northern extremity of the fishery and while the whelks which are exploited at the southern end between 2000 and 2003 were similar in size to those exploited in the mid-1990s, their tonnage between 1999 and 2003 decreased from 47 to 4% of the landings in 1995. A substantial recruitment in 2001 or 2002 was followed by an increase in fishing effort of 42% between 2002 and 2003 and this is identified as the reason for the collapse in 2004.