Population dynamics and predictions in the Isle of Man fishery for the great scallop, Pecten maximus L.

B. D. Beukers-Stewart 0 1 M. W. J. Mosley 0 1 A. R. Brand 0 1 0 Department of Agriculture , Fisheries and Forestry statis- 1 B. D. Beukers-Stewart, M. W. J. Mosley, and A. R. Brand: Port Erin Marine Laboratory , Beukers-Stewart: tel.: There has been a fishery for the great scallop, Pecten maximus, around the Isle of Man, since 1937. Research up to the end of the 1980s suggested that these scallop stocks were in continuous decline. The fishery is now largely dependent on the strength of each recruiting year-class, placing it at considerable risk from recruitment failure. This study utilised data on the spat settlement, age structure, abundance and commercial catch rates of scallops, collected between 1987 and 2001, to examine recent population dynamics and the potential for predicting future catch rates. Spat settlement was generally low, but there were two exceptionally strong year-classes. Surveys of the stock revealed high exploitation rates during each fishing season (November to May inclusive) with variable recovery due to recruitment by the following October. In 1997/1998, scallop catch rates reached a 20-year high on several grounds and have generally remained high since. The strong spat settlements in 1989 and particularly 1994 were largely responsible for recent rises in catch rates, although the maintenance of high catch rates between 1999 and 2001 has occurred despite poor spat settlement between 1995 and 2000. Within stock surveys, the density of 2year-old scallops was generally an accurate predictor of the density of 3- and 4-year-old scallops, 1-2 years later. The nature and strength of these relationships varied considerably between fishing grounds due to spatial variation in both scallop biology and patterns of exploitation. Results from fishery independent surveys did not always correlate well with commercial catch rates, however, suggesting the need for an expansion of the survey on some grounds. Overall, our study indicated that current levels of exploitation appear to be sustainable in the Isle of Man scallop fishery. Our results also demonstrated that monitoring of both spat settlement and the abundance of juveniles has considerable potential for predicting future catch rates of commercial sized scallops. Introduction One of the central problems in fisheries science is variability in recruitment of young fish and shellfish into populations and our inability to predict this variation (Sissenwine, 1984). Recruitment into scallop stocks is notoriously variable with many scallop fisheries exhibiting boom and bust cycles as a result (Fairbridge, 1953; Young and Martin, 1989; Orensanz et al., 1991). Fluctuations of this nature are very difficult to accommodate into the strategies of fishermen and managers and may lead to the complete collapse of fisheries (Frank and Brickman, 2001). There has been a commercial fishery for the great scallop, Pecten maximus (L.), around the Isle of Man, since 1937. This species presently constitutes over 65% of the value of all fisheries production on the Island (Isle of Man tics). The fishery for P. maximus has been regulated in a variety of ways since its inception, including a minimum legal landing size of 110 mm shell length (SL) and an annual closed season (June to October inclusive; Brand et al., 1991a). Initial catch rates in the fishery were very high but had fallen to a low level by the end of the 1980s despite regulation (Brand et al., 1991a). The age structure of the P. maximus population has also shifted during this period of exploitation from being dominated by scallops 10 years or older to 5 years or less (Brand et al., 1991a; Brand, 2000). As a consequence, the fishery is now largely dependent, each season, on the strength of the recruiting year-class. Although recruitment of P. maximus around the Isle of Man does not appear to be as variable as in some other scallop fisheries (Brand et al., 1991a,b), occasional strong and weak year classes have occurred. Staff from the Port Erin Marine Laboratory have collected detailed information on spat settlement, size, age, abundance and commercial catch rates of P. maximus around the Isle of Man between 1975 and 2001. The aims of this paper are twofold. The first is to use this dataset to examine recent trends in the population dynamics of P. maximus in the waters around the Isle of Man. The second aim is to examine the potential of this dataset for predicting future trends in the abundance of great scallops at a relatively fine spatial scale. Such predictions would be extremely useful to both fishermen and managers when planning future utilisation of stocks (Pace, 2001). For example, the prospect of good catches in the future may encourage investment by fishermen and processors in terms of new vessels, premises or staff (Phillips, 1986). Alternately, early warning of poor recruitment could allow management authorities to instigate catch (i.e. quotas) or effort restrictions that aim to maintain spawning stock above a critical level and therefore reduce the likelihood of recruitment overfishing (Caputi and Brown, 1986). The Isle of Man government is in an ideal position to implement such legislation quickly and effectively due to its control of territorial waters up to 12 miles offshore and unique parliamentary system (Brand et al., 1991a,b). A number of workers have previously attempted to use the abundance of newly settled or juvenile fish and shellfish to predict the future size of adult populations (e.g. Caputi and Brown, 1986; Sause et al., 1987; Helle et al., 2000; Sakuramoto et al., 2001). Results from these studies are commonly incorporated into fisheries stock assessment models (Hilborn and Walters, 1992). The reliability of such predictions depends upon accurate, long term datasets with demonstrated and consistent relationships between the abundance of individuals at different life-history stages (Phillips, 1986). The practice of monitoring the settlement of pueruli has enabled accurate predictions of commercial catch rates up to 4 or 5 years in advance in several lobster fisheries (Phillips, 1986; Gardener et al., 2001). However, attempts to predict commercial catch rates of scallops have produced mixed results. The settlement of spat was a reliable predictor of commercial catch rates 1218 months in advance in the fishery for Pecten fumatus in Port Phillip Bay, Australia (Sause et al., 1987; Coleman, 1988). In the nearby fishery for the same species at Lakes Entrance, however, there was no such relationship (Coleman, 1988; Young et al., 1988). Likewise, only weak or intermittent relationships have been found between spat settlement and adult abundance for the bay scallop, Argopecten irradians, in Florida (Arnold et al., 1998; Marelli et al., 1999) and North Carolina (Peterson and Summerson, 1992; Peterson et al., 1996) and P. maximus off the west coast of Scotland (Fraser, 1991). In contrast, spatfall of the queen scallop, Aequipecten opercularis off w (...truncated)