Fluctuations and forecasts in the fishery for queen scallops (Aequipecten opercularis) around the Isle of Man

B. J. Vause 0 B. D. Beukers-Stewart 0 A. R. Brand 0 0 B. J. Vause, B. D. Beukers-Stewart, and A. R. Brand: Port Erin Marine Laboratory, University of Liverpool , Port Erin, Isle of Man IM9 6JA, British Isles. B. D. Beukers-Stewart: tel: The annual success of the queen scallop fishery around the Isle of Man in the northern Irish Sea is dependent on the strength of recruitment. We examined data from surveys and commercial logbooks on the annual density of spat, juvenile, and adult queen scallops in the fishery between 1982 and 2002. These were used to examine past population and fishery trends and the potential for formulating a predictive model for the fishery. The results were highly variable on both temporal and spatial scales, but there were some general trends. Density appeared to have been relatively stable during the 1980s, declined sharply from the early to mid-1990s, then recovered to produce relatively good catch rates thereafter. There was no relationship between spat settlement and the subsequent density of juveniles or adults in stock surveys or with commercial catch rates. However, within the stock surveys, there were three different significant relationships between cohort densities over time. Additionally, there was a significant relationship between the density of 1-year-olds caught on the surveys and commercial catch rates the following year. Monitoring juvenile queen scallop density would therefore allow prediction of recruitment and fisheries variations at least 1 year in advance, allowing perhaps for more effective management, including reducing the fluctuations in the fishery and helping to ensure long-term sustainability. Introduction Fluctuations in the abundance of fish and shellfish populations are often attributed to variations in recruitment (Sissenwine, 1984). This is generally caused by several interacting factors (Rothschild, 2000), most commonly the size of the spawning stock (Ricker, 1954; Beverton and Holt, 1957; Shepherd, 1982) and environmental conditions (Caputi, 1993; Neill et al., 1994; Hofmann and Powell, 1998; Le Pennec et al., 2003). It is important that such variation in recruitment be accounted for in managing exploited stocks, even when the cause of the variation cannot be identified. An ability to predict the level of recruitment, defined here as the abundance of individuals entering the fishery, contributes to more effective fisheries management (Smith, 1993). Such prediction would be particularly valuable for fisheries that are heavily dependent on the strength of the recruiting year class, such as many shellfish stocks. The queen scallop (Aequipecten opercularis) is a commonly exploited species in the Northeast Atlantic (Brand, 1991), and is common around the British Isles (Mason, 1983; Ansell et al., 1991). It is rare to find scallops older than 6 years (Brand et al., 1991a), possibly because of the combined effect of cumulative fishing mortality and the onset of senescence (Allison and Brand, 1995), but in this time, they can grow to 90 mm shell height (Brand et al., 1991a). The most valuable fishery for the species is in the Irish Sea; 9286 t (45% of the total landings in the Northeast Atlantic) were taken from the region in 2001 (FAO, 2003). A significant part of the fishery operates out of the Isle of Man, with an average of .2300 t of queen scallops landed there each year between 1982 and 2002 (Isle of Man Department of Agriculture, Fisheries and Forestry statistics). The Manx queen scallop fishery operates mainly during summer (June October inclusive), and is prosecuted with two different types of fishing gear, dredges or trawls. Both are size-selective to target animals .55 mm shell height, because of the economics of processing (Brand et al., 1991a). During the fishing season, there is a change in the size structure of the population, caused by rapid growth of scallops in summer. This can produce a change in the age structure of the catch as the recruiting cohort, generally 2-year-olds, often attain 55 mm in the later part of the season and so become vulnerable to fishing (Allison, 1993). The fishery is dominated by scallops 2 4 years old (Brand et al., 1991a; Allison and Brand, 1995) and the few age classes in the exploited population dictates that the success of the fishery each year is very dependent on the strength of the incoming year class. For fisheries such as this, recruitment variation is a major contributor to temporal variations in the commercial catch. Scallop stocks have a well established reputation for being temporally and spatially variable, and the main causes have been summarized into three groups; recruitment variability, catastrophic mortality, and the longevity of the species (short-lived species have no buffer zone if there is a period of poor recruitment, making them more vulnerable to recruitment failure) (Orensanz et al., 1991). The rapid early growth, short lifespan and high motility of A. opercularis led Mason (1983) to state that queen scallop stocks needed no protection and that the best fishing strategy was to fish them hard before some other predator gets them. Similarly, Hancock (1979) considered recruitment for some scallop populations to be so unreliable that for management purposes, they should be regarded as a non-renewable resource. Many previous studies of fish (Helle et al., 2000; Nash and Geffen, 2000; Arnott and Ruxton, 2002) and shellfish (Chcharo and Chcharo, 2001; Beukers-Stewart et al., 2003; Caputi et al., 2003) have documented good relationships between the abundance of cohorts at different life history stages. In the northern Irish Sea, Beukers-Stewart et al. (2003) recorded a strong link between settlement of great scallop (Pecten maximus) spat and commercial catch per unit effort (cpue) 4 5 years later, and a previous study on queen scallops in Scottish waters suggested a positive relationship between spat settlement and the subsequent abundance of juveniles and adults (Fraser, 1991). Our study brings together 20 years of data from three monitoring programmes that were designed to assess the queen scallop populations and their fishery around the Isle of Man. These programmes measured the density of queen scallops at different life history stages. Year classes were sampled for the first time as spat settled out of the plankton onto spat collectors, and stock surveys caught juveniles and adults which supported length frequency analysis as a basis for determining relative cohort densities. Additionally, commercial fishers contributed catch and effort data in the form of logbooks. All these data were combined to investigate the potential for developing a predictive model for the fishery. Material and methods Data on spat settlement, on stock structure from the surveys, and on commercial catch and effort from logbooks were analysed for four fishing grounds around the south and east of the island (Figure 1), for various periods between 1982 and 2002. Spat set (...truncated)