© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Population dynamics and predictions in the Isle of Man fishery for the great scallop, Pecten maximus L.
Port Erin Marine Laboratory, University of Liverpool Port Erin, Isle of Man IM9 6JA, UK
*Correspondence to B. D. Beukers-Stewart; tel: +44 1624 831038; fax: +44 1624 831001. e-mail: brycebs{at}liv.ac.uk.
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 1975 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 2-year-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.
Keywords: stock assessment, predictions, exploitation rates, recruitment, scallops, Pecten maximus, Irish Sea
Received 8 July 2002; revision received 4 December 2002; accepted 6 December 2002.
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Introduction |
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 Top Introduction Materials and methodsResultsDiscussionReferences |
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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 Department of Agriculture, Fisheries and Forestry statistics). 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 1981 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 12–18 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 western Scotland appears to be a good indicator of future stock size (Fraser, 1991).
It would seem likely that relationships between spat settlement and adult abundance will be strongest for short lived, fast growing, scallop species such as P. fumatus and A. opercularis in relatively enclosed bodies of water (e.g. Port Phillip Bay and Scottish sea lochs). Whether such a relationship would exist in the open waters around the Isle of Man, where P. maximus takes 3–5 years to reach commercial size (Brand et al., 1991a) was not clear. We therefore also examined the potential of measuring the abundance of juvenile (2-year-old) scallops as a means of either confirming or refining predictions made from spat settlement.
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Materials and methods |
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Study area
This study was based on the coastal waters around the Isle of Man, in the north Irish Sea. Three main data collection programmes related to the Isle of Man's fishery for the great scallop, P. maximus, have been undertaken between 1975 and 2001. This analysis will concentrate on eight study sites around the west, south and east of the Isle of Man (see Figure 1), for the period between 1987 and 2001, for which we have the most comprehensive datasets.
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Spat settlement
Spat settlement was monitored every year from 1987 to 2000 at between two and five locations (generally three) around the south and west of the Island (Figure 2). At each site three single lines of spat collectors were placed approximately 100 m apart in 20 m water depth. On each line a single onion mesh spat bag was placed at 2 m intervals between 2 and 12 m above the seabed to ensure good coverage of the water column. Each onion mesh bag (500 mmx600 mm, 10 mmx3 mm mesh size) was loosely stuffed with 1.5 m2 of 6 mmx6 mm plastic mesh, giving a total surface area available for settlement of approximately 1.90 m2. Spat collectors were generally deployed in July and retrieved in October or November each year to coincide with the period of peak settlement of P. maximus (Brand et al., 1980). Our measurement of settlement therefore only represents the number of spat present at the end of the settlement season and not necessarily the total number that may have settled. The abundance of the P. maximus in the spat bags was counted and expressed as a mean per site for each year, together with an overall mean for each year (using sites as replicates). The authors obtained these data between 1995 and 2000. Prior to this period, data were obtained from previous studies (Brand et al., 1991b; Whittington, 1993; Wilson, 1994; Zhang, 1996). Annual variation in spat settlement was analysed by one-way ANOVA using the two most commonly sampled sites, Bradda and Bay Fine (see Figure 2), as replicates. Tukey's test (honestly significant difference method—HSD) was then used to identify which years were significantly different from one another.
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Scallop surveys
The eight fishing grounds examined in this study (see Figure 1) were surveyed for P. maximus size, age and abundance, generally twice every year, from 1991 to 2001. Surveys were conducted just before the start of each scallop fishing season (October) and just after the end (June), using the RV "Roagan", a 20 m converted beam trawler. Each ground was surveyed using three or four replicate tows, approximately 2 nautical miles (3.70 km) in length, of spring loaded Newhaven type dredges (see Chapman et al., 1977) to simulate local commercial practice. Average tow speed was approximately 2.5 knots (4.63 km h–1) resulting in tow durations of approximately 45 min. Exact tow lengths and locations were recorded using an onboard differential global positioning system (DGPS) linked with Microplot software (Sea Information Systems, Aberdeen). As far as possible, tows covered the same area of seabed on each survey. On one side of the research vessel, a gang of four standard scallop dredges was fished while on the other side four queen scallop (A. opercularis) dredges were used. All dredges were 0.76 m in width. The queen dredges differ from scallop dredges in having 10 teeth of 60 mm length (compared with nine teeth of 110 mm) and belly rings of 55 mm internal diameter (compared with 80 mm). These queen dredges were used to increase the sample size of small scallops below the minimum legal landing size (110 mm shell length—SL) (Beukers-Stewart et al., 2001; unpublished data).
All scallops taken were counted, aged (using validated annual growth rings; Allison et al., 1994) and measured to the nearest mm (SL). Tow length data (from Microplot) were combined with the dimensions of the scallop gear to calculate the area of seabed swept by the dredges on each tow. Data from any non-functional dredges were removed from the analysis. It was therefore possible to calculate relative total and age-specific densities of scallops caught by the two types of dredge for each survey date and fishing ground. Relative densities were expressed as the mean number of scallops per 100 m2 swept area using each tow as a replicate. In this study, no attempt was made to use the estimates of dredge efficiency and selectivity to convert relative densities into actual densities.
After testing for normality and homogeneity of variances (Cochran's C test), and transformation where necessary, these data were analysed in several different ways. Initially, the relationship between the relative density of scallops captured by queen and scallop dredges was determined by regression analysis. Variation in the relative density of scallops captured by queen dredges in October of each year was then analysed for each fishing ground by one-way ANOVA. Analysis was restricted to October as this is generally when the upcoming year-class (2 year olds) have first grown to a size susceptible to the dredges (see Results). If the ANOVA was significant, a Tukey's test (HSD method) was conducted to identify which years were significantly different from one another. Analysis of the scallop dredge catch rates was restricted to scallops above the minimum legal size (110 mm SL). To examine the effect of each fishing season on scallop density, the relative density of scallops (
110 mm) captured in October each year (before the season) was compared with the density captured the following June (at the end of the season) for each fishing ground. This was done by two-way ANOVA with the two fixed factors being month of survey (October or June) and fishing season (where a season runs from October to June). Only seasons sampled in both October and June could be included in this analysis. Tukey's test was again used to identify significant differences. Finally, variation in the relative density of scallops (110 mm) captured by scallop dredges in October of each year was again analysed for each ground by one-way ANOVA followed by Tukey's test.
Logbooks
A voluntary logbook scheme for local scallop fishermen has been running since 1981 (Brand and Allison, 1994). In these logbooks, fishermen have filled out details of their daily activity, including hours fished, numbers of scallops caught and their location within pre-designated 5x5 nautical mile boxes (see Figure 1). These boxes generally correspond to individual fishing grounds for the smaller grounds, while the larger grounds occupy two or more boxes. The width and number of scallop dredges fished by each boat is also recorded. These data could therefore be converted into measures of catch per unit effort (CPUE) at a relatively fine spatial scale. CPUE was expressed as the number of scallops caught per metre of dredge towed per hour. Scallop gear is consistently towed at a speed of approximately 2.5 knots (4.63 km h–1), making records from different boats comparable. There has been little or no change in fishing gear design or the average size of fishing boats and engines over the course of the study (unpublished data), therefore catch rates were also considered to be comparable over time. Approximately 30% (10–25 boats) of the local fleet participated in the scheme on average.
Linking the data and predicting catches
The other purpose of this investigation was to examine the strength of relationships between the abundance of P. maximus at different life history stages with an aim of developing a model capable of predicting catch rates up to 5 years in advance. We therefore performed regression analysis on relationships between spat settlement each year (mean number of spat per bag across sites) and the density of 2-year-old scallops (on each ground) taken by queen dredges in the October scallop surveys 2 years later. We also analysed relationships between the abundance of 2-year-old scallops (as measured above) and 3-, 4- and 5-year-old scallops taken with scallop dredges on each ground 1, 2 and 3 years later, using data from our scallop surveys. We then determined the relationship between CPUE from the scallop surveys (using scallop dredge data only) and logbooks to examine the applicability of these results to the commercial fishery. To ensure CPUE values were comparable we took the average of scallop survey catch rates from October (at the start of the season) and the following June (at the end of the season) to represent average scallop abundance for each fishing season. Finally, we performed regression analysis on relationships between spat settlement each year (mean number of spat per bag across sites) and the mean CPUE (from the logbooks) on each fishing ground 3–5 years later.
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Results |
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Spat settlement
Spat settlement of P. maximus around the Isle of Man was generally very low (mean no. spat/bag <25), between 1987 and 2000 (Table 1; Figure 3). However, 2 years (1989 and especially 1994) had significantly higher settlement than many other years, while 2 other years (1999 and 2000) had significantly lower settlement (Tables 2 and 3). Mean settlement in 1989 was approximately 5 times higher than the average of all other years and in 1994 it was approximately 15 times higher. Within years there was often considerable variation in settlement at different sites. For example, during 1994 there was very high settlement at Bradda and Bay Fine, medium to high settlement at Bay Stacka and low settlement at Niarbyl. During 1989, the pattern was reversed somewhat, with high settlement at Niarbyl but only medium to low settlement at the other sites.
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Scallop surveys
In general, the relative density of all P. maximus collected by scallop dredges was lower, but reflective, of the density collected with queen dredges (regression analysis; df=133, F=413.63, p<0.001, R2=0.757), (Figure 4). Scallop density tended to increase between June and October each year as the upcoming year class grew to a size susceptible to the dredges and then decline substantially between October and the following June (i.e. during the fishing season). There was significant variation in the density of scallops captured by queen dredges each October on all grounds except Targets and Peel (Tables 4 and 5; Figure 4). The Chickens ground generally had the highest density of scallops, with significant peaks in abundance in October of 1996, 1997, 1998, 2000 and 2001. From October 1999 to 2001 there has also been a dramatic and significant increase in the density of scallops on the Laxey ground. The Bradda Inshore and Bradda Offshore grounds tended to have intermediate densities of scallops, although there have also been significant increases on these grounds from October 1999 to 2001. The other grounds, Targets, Peel, 15 Miles South and East Douglas have had lower, but relatively stable scallop densities over the study period. However, several surveys still stand out, such as October 1995 at 15 Miles South and October 1997 at East Douglas.
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The relative density of legal sized scallops (
110 mm SL) collected with scallop dredges during the October surveys has also fluctuated significantly on the different grounds over the study period, from as low as 0.25 scallops per 100 m2 at Laxey in 1995 to as high as 1.50 scallops per 100 m2 at Chickens in 1998 (Tables 6–9; Figure 5). In contrast, the end of season (June) densities have generally remained fairly stable at between 0.2 and 0.4 scallops per 100 m2, regardless of pre-season (October) abundance. The exception has been on the Targets ground, which has seen a rise in both June and October catch rates since 1996/1997. Correspondingly, the timing of surveys (October or June) had a significant effect on scallop densities on all grounds except 15 Miles South (Tables 8 and 9). Differences between the two surveys appeared to be related to the amount of recruitment into the legal sized stock each year. For example, after several instances of good October recruitment (e.g. Bradda Inshore and Bradda Offshore in 1999, Chickens in 1998 and Laxey in 2000) there was a significant (60–70%) decline in density over the course of the following fishing season (Tables 8 and 9; Figure 5). Low recruitment levels on the same grounds (e.g. Bradda Inshore, Bradda Offshore and Chickens in 1996 and Laxey in 1995) resulted in little difference between the October and June surveys. On average, Chickens again supported the highest densities of legal sized scallops, with a number of strong peaks in abundance, particularly in October 1998 (as mentioned earlier). Densities at Laxey were particularly high during the October surveys from 1999 to 2001, and there was also significant recruitment of legal sized scallops at the Bradda Inshore and Bradda Offshore grounds in October 2001 (Tables 8 and 9; Figure 5). The density of legal sized scallops has gradually increased during October surveys at Targets and East Douglas but has been fairly stable at 15 Miles South and has declined at Peel after peaks in October 1992 and 1993 (Tables 8 and 9; Figure 5).
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Logbooks
Commercial CPUE averaged approximately 20 scallops per metre of dredge per hour on the eight fishing grounds analysed for the majority of the study period (Figure 6). However, several seasons stand out as exceptions. During 1991/1992, there was a rise in CPUE on three of the six grounds for which we had data; Bradda Inshore, Bradda Offshore and Targets. This pattern was most marked at Targets, where catch rates rose approximately 50% (from 19.5 to 30.4). At the two Bradda grounds, the rises were more moderate, approximately 40% at Bradda Offshore and 15% at Bradda Inshore. The most striking change in catch rates, however, occurred during the fishing seasons of 1997/1998 and 1998/1999. On all eight of the fishing grounds studied catch rates rose over this period, ranging from an approximate 15% increase at East Douglas to 35–50% rises at Bradda Offshore, Targets, Peel, 15 Miles South and Laxey, to a near doubling at Bradda Inshore (16.4–31.2) and Chickens (23.8–41.6). Catch rates dropped during the 1999/2000 season, but remained above average on most grounds, and on some grounds (Bradda Offshore, 15 Miles South and Laxey) increased again in 2000/2001.
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Linking the data and predicting scallop catches
Spat settlement as a predictor of the abundance of 2-year-old scallops
During the October scallop surveys, the queen dredges often caught a considerable number of 2-year-old P. maximus compared to the scallop dredges. The link between spat settlement each year and the relative density of these scallops 2 years later was examined through regression analysis. There were no significant relationships between these data, either at site (fishing ground) level or overall (Table 10). The only exception was that the strong spat settlement in 1994 was well represented by a high abundance of 2-year-old scallops at Chickens in October 1996. On the other grounds the density of this year class fell within the normal range.
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The abundance of 2 year olds as a predictor of the abundance of older scallops
Most scallops (P. maximus) around the Isle of Man reach legal size (110 mm SL) at between 3 and 5 years old (Brand et al., 1991a). The relative density of the 2-year-old scallops caught in the queen dredges therefore had the potential to act as a predictor of the abundance of commercial sized scallops 1, 2 or 3 years in advance. When data from all grounds and years were combined there were strong and significant linear relationships between the density of 2-year-old scallops and both 3- and 4-year-olds (R2=0.85 and 0.73) caught in scallop dredges 1 and 2 years later (Table 11; Figure 7). This relationship became substantially weaker (R2=0.32), but was still significant, when extended to 5-year-old scallops caught 3 years later.
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There were also significant linear relationships between the relative density of 2-year-old and 3- and 4-year-old scallops at each site (with one exception; see Table 11). At only one site (Bradda Offshore) was there a significant relationship between the density of 2-year-old and 5-year-old scallops. The general pattern was for the strength (R2) and slope of these relationships to decrease with age, but these relationships varied considerably between different grounds (see Table 11 and Figure 8). The slope of these relationships between 2-year-old and older scallops can be used as an estimate of loss (mortality and emigration) of scallops over time, as a decrease in the density of older scallops results in a more gradual slope. On that basis, loss rates for each year between 2 and 5 years old were consistently highest at the Bradda Inshore ground. However, patterns of loss were not always consistent as age increased within sites. For example, at the Bradda Offshore and East Douglas grounds, loss rates appeared relatively low between 2 and 3 years old but high between 3 and 4 years old. At Chickens and Peel there appeared to be little loss between 3 and 4 years old, but high loss between 4 and 5 years. At 15 Miles South loss of scallops gradually increased with age, but at Laxey there was little change in the slope of the relationship over the age range examined.
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The relationship between scallop survey and commercial catch rates
There was a significant overall relationship between scallop survey and commercial CPUE (Table 12; Figure 9), although one point (Chickens, 1998/1999 fishing season) had a large influence on this relationship. When this point was removed the regression was still significant but became much weaker (R2=0.07, p=0.04). In most cases, CPUE was between 20 and 30 scallops per metre of dredge per hour for both the scallop survey and logbooks, but scallop survey catch rates were not an accurate predictor within this range. However, there were still significant relationships between scallop survey and commercial CPUE on three of the eight fishing grounds examined (Bradda Inshore, Chickens and Laxey), (Table 12). At Bradda Inshore and Chickens the strength of these relationships was only moderate (R2=0.38 and 0.55, respectively), but at Laxey there was a relatively strong relationship between all available scallop survey and commercial catch rates (R2=0.78). In all cases where there were significant relationships, the slope of the lines (<1) indicated that scallop survey catch rates were higher than those reported in the logbooks. CPUE values of less than 20 were more commonly observed in the survey data than the logbooks, however, causing positive y-intercepts except at Bradda Inshore.
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Spat settlement as a predictor of commercial catch rates
Regression analysis revealed that significant linear relationships between spat settlement and commercial catch rates were most common when a lag time of 4–5 years was used (Table 13). This produced significant relationships at Bradda Inshore, Chickens, 15 Miles South and when all grounds were combined. All of these regressions were strongly driven by the high spat settlement in 1994 and the corresponding rise in catch rates on all grounds during the 1998/1999 fishing season. Although not statistically significant, comparisons of patterns of spat settlement (Figure 3) and commercial catch rates (Figure 6) also suggested that the strong settlement in 1989 caused a rise in commercial catch rates on the Bradda Inshore, Bradda Offshore and Targets grounds in the 1991/1992 fishing season (i.e. after a lag of only 2–3 years). However, poor settlement on the collectors did not necessarily result in poor catches. The maintenance of good catch rates on most grounds during the 1999/2000 and 2000/2001 seasons (Figure 6) has occurred despite relatively poor spat settlement between 1995 and 2000 (Tables 2 and 3; Figure 3).
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Discussion |
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Despite the long history of exploitation of P. maximus populations around the Isle of Man (65 years) and high annual rates of exploitation, the fishery has not only been remarkably stable between 1991 and 2001, but catch rates have actually improved. This is in contrast to the downward trend in catch rates observed up to 1991, which suggested the fishery was in continuous decline (Brand et al., 1991a; Brand and Allison, 1994). Although current catch rates are far below those observed during the early days of fishery, (up to 150 scallops per metre of dredge per hour—Brand et al., 1991a) and scallop populations are now dominated by young individuals, (<5 years old), our results suggest that current levels of exploitation are sustainable. The reliance of the fishery on recruitment each year may place it at considerable risk, however, emphasising the potential importance of being able to predict future recruitment levels.
Results from our scallop surveys conducted between October 1991 and 2001 illustrate the high rate at which P. maximus is being harvested during each fishing season. Post-season relative densities of legal sized scallops (>110 mm SL) were almost always around 0.3 scallops per 100 m2, regardless of pre-season density. In most cases this represented a 40–50% decline in scallop density (occasionally as high as 70%) over the course of a single season. This decline also incorporates natural mortality, although annual natural mortality for scallops less than 5 years old is thought to be relatively low around the Isle of Man, with instantaneous coefficients in the region of 0.15–0.20 (Brand et al., 1991a). Previous studies, using a variety of methods, have found similarly high exploitation rates for P. maximus populations around the Isle of Man. For example, tag-recapture data indicated seasonal exploitation rates on the Bradda Inshore ground of 33% in 1965/1966 (Gruffydd, 1972), 37% in 1983/1984 (Brand and Murphy, 1992) and 41% in 1987/1988 (Allison, 1993). The patterns of exploitation we observed also indicate that fishermen operate in a relatively efficient manner, concentrating on high density patches of scallops and fishing them down until they reach uneconomic levels (presumably equivalent to the post-season densities we observed) before moving to other grounds. In further support of this idea was the fact that the low relative densities observed on the 15 Miles South and Peel sites, particularly in recent years, generally changed little over the course of each fishing season. These sites currently support a much higher proportion of older scallops (>5 years old) than the other sites analysed (unpublished data), also suggesting relatively low exploitation rates.
Recent work on the efficiency of the scallop dredges used on the Bradda Inshore ground indicates that they catch approximately one-third of P. maximus (>110 mm SL) present on the seabed (Beukers-Stewart et al., 2001). Post-season relative densities of 0.3 scallops per 100 m2 (>110 mm SL) can therefore be converted to actual densities of 0.9 scallops per 100 m2. It should be noted that dredge efficiency may vary according to the substrate type on each fishing ground (Dare et al., 1993), but post-season densities were undoubtedly very low. As scallop densities were as high as 20 per 100 m2 on the Bradda ground in 1965/1966 (Gruffydd, 1972), and therefore surely much higher prior to exploitation of the stocks, it is possible that the spawning stock biomass has now fallen to a level close to the threshold which ensures regular recruitment (Brand et al., 1991a). Such recruitment overfishing has recently been demonstrated in several other marine invertebrate fisheries (Lipcius and Stockhausen, 2002; Peterson, 2002). Given the reliance of the Isle of Man scallop fishery on the strength of each recruiting year-class, even a single season of low or failed recruitment could be disastrous.
The resilience, to date, of Isle of Man populations of P. maximus to such a long history of consistently high exploitation rates may largely be due to external supplies of larvae rather than reliance on self-seeding. The source of larval supply to these populations is a matter of some debate (Heipel et al., 1998). Given the 4–6 week larval life of P. maximus (Brand et al., 1980) and indications of a prevalent northward current in the Irish Sea (Ramster and Hill, 1969), several authors (Macleod et al., 1985; Murphy, 1986) have suggested that scallop larvae settling around the Isle of Man originate from extensive beds between Anglesey and the south of the Island. However, more recent computer simulations of current patterns (Backhaus and Hainbucher, 1987) suggest that currents can change according to weather conditions, resulting in a general southward flow during June and July, the main spawning period for P. maximus (Brand et al., 1980). Further studies, possibly utilising a combination of hydrodynamic modelling, surveys of larval distribution and genetic discrimination are therefore required to elucidate the source of larval supply for Isle of Man stocks of P. maximus. Once parental stocks are identified, we suggest they should be at least partially protected, through the use of areas closed to fishing (e.g. Murawski et al., 2000; Bradshaw et al., 2001), as a safeguard for the future of the fishery.
Aside from two exceptionally good years, our data suggest that settlement of P. maximus spat was low but constant around the Isle of Man. These data continue a trend observed since the spat collection programme commenced in 1975 (see Brand et al., 1991b). Our method of measuring spat settlement (only retrieving collectors at the end of each settlement season) may underestimate total settlement, but fortnightly measures of spat settlement in several years (see Brand et al., 1980) were also low. The comparatively high and constant recruitment of juvenile scallops indicated by our surveys of the stock and commercial catch data therefore suggests very high survival rates when there is a low density of spat present at the end of a settlement season. This would also explain the overall stability and longevity of the Isle of Man fishery for P. maximus, relative to many other more variable scallops fisheries (see Orensanz et al., 1991). An alternative explanation is that our spat collectors were placed in unsuitable locations or that spatial coverage was inadequate, resulting in an underestimation of spat settlement. Although our analysis of spat settlement was based on relatively few locations, wider ranging studies have found even less settlement at other locations, particularly on the east coast of the Island (Brand et al., 1991b; Wilson, 1994). The sites used in this study were chosen on the basis of consistent settlement and proximity to several important fishing grounds around the south and west of the Island. Ideally, spat collectors would have been placed directly over fishing grounds; however, the high risk of loss due to fouling on commercial boats or adverse weather conditions precluded the use of such locations.
A large number of factors may affect the survival of scallop spat (predator density, the availability of suitable substrate, food availability, indirect fishing effects etc.), all of which vary spatially and temporally (Brand et al., 1980; Thouzeau, 1991; Peterson and Summerson, 1992; Harvey et al., 1995; Peterson et al., 1996). For example, studies on the bay scallop Argopecten irradians, in North Carolina (Peterson and Summerson, 1992; Peterson et al., 1996) found strong relationships between spat settlement and recruitment at some sites and in some years but not others. They attributed this variation to differential effects of predation on newly settled scallops. Therefore, it is perhaps not surprising that in our study there were no significant relationships between spat settlement and the abundance of P. maximus 2 years later. However, the two exceptional years of spat settlement were well represented in the commercial catch rate data. For instance, the excellent catch rates observed on several grounds in 1998/1999 (especially Chickens) were among the highest recorded since the logbook scheme began in 1981 (this study and Brand and Allison, 1994). These results indicated that mortality of spat was density-dependent (given that spat settlement was 15 times higher than average in 1994 whereas resultant commercial catch rates were only 90% higher than other years) but that density-dependent effects were not strong enough to disrupt relative settlement patterns from persisting into adult populations (Caley et al., 1996).
Interestingly, recent work utilising both age structure information (Brand et al., 1991a; Allison, 1993) and genetic analysis (Heipel et al., 1998, 1999) has suggested that scallop recruitment to the east and north of the Isle of Man may be supplied by a different parental stock from the other fishing grounds. The strong spat settlement observed in 1989 was very localised; by far the highest settlement was observed at the Niarbyl site and this was most noticeably reflected by a rise in catch rates at Targets, one of the closest fishing grounds. Growth rates of scallops at Targets are among the highest around the Island (Allison, 1993), also explaining why catch rates peaked after a lag of only 2–3 years. In contrast, the effects of the strong spat settlement in 1994 were seen on all of the fishing grounds analysed, including Laxey, the furthest ground from the spat collection sites. The lower spat settlement in the other years gave little indication of either the distribution or magnitude of future scallop stocks. Similarly, inconsistent effects of spat or puereli settlement have been seen in other studies of scallops (Fraser, 1991; Peterson and Summerson, 1992; Peterson et al., 1996) and lobsters (Booth et al., 2001). Hence, although our spat settlement data were not adequate for predicting scallop catches at a fine scale, they did provide notice of exceptional recruitment events, up to 4 years in advance, that were very important for the fishery.
The strong relationships observed between the abundance of 2 year olds and older scallops were more promising for the development of a detailed predictive model. Data from the eight fishing grounds and 11 years of sampling were all combined in our initial analysis. The decline in the strength of the relationship when extended to 5-year-old scallops was not unexpected, as most 5-year-old scallops had been available to the fishery for at least a year and therefore their patterns of abundance were likely to have been disrupted. When these relationships were examined at the site level, there were strong indications that patterns of total mortality varied both spatially and temporally. In some cases these observations could be related to known patterns of fishing effort and scallop biology. For example, the Bradda Inshore fishing ground has regularly been subject to the highest levels of fishing effort since the fishery commenced in 1937 (Brand et al., 1991a). Correspondingly, loss rates of scallops consistently appeared highest at this site. In addition, the majority of scallops at Bradda Inshore reach commercial size when 3 years old (compared with 4 years old at most other grounds; Wilson and Brand, 1995), explaining the high loss of 3-year-old scallops relative to other sites. These patterns of spatial variation illustrate the need for any predictive model to analyse each fishing ground separately whenever possible. Prairie (1996) states that the predictive power of regression analyses with an R2
0.65 is low, but increases rapidly above this level. Almost all of the grounds analysed in this study passed this criterion when using the abundance of 2-year-old scallops to predict the abundance of 3 and 4 year olds.
There is potential to further refine these predictive relationships by collecting more data and by investigating other explanatory variables such as environmental factors and indirect fishing mortality (e.g. McLoughlin et al., 1991; Agnew et al., 2000; O'Brien et al., 2000). Environmental conditions such as water temperature and plankton levels (food availability) are know to influence recruitment success in many other fish and shellfish stocks (Wolff, 1987; Agnew et al., 2000; O'Brien et al., 2000; Caputi et al., 2001) and may be at least partly responsible for recent increases in recruitment and catch rates of P. maximus around the Isle of Man (unpublished data). Indirect fishing mortality (mortality of scallops discarded or disturbed but not caught by fishing) has also been identified as a major issue in several other scallop fisheries (McLoughlin et al., 1991; Myers et al., 2000). Ongoing research on the Isle of Man fishery for P. maximus has shown that discarded and disturbed scallops may attract predators (Veale et al., 2000) and be less able to escape them than undisturbed scallops (Jenkins and Brand, 2001; Maguire et al., 2002). Indirect fishing mortality could therefore have affected the patterns of scallop abundance observed in this study and requires further quantification. With the refinement of relationships between the abundance of P. maximus at different ages and on different grounds, it would be possible to apply information on site-specific growth rates (which are already known) to these data to accurately predict future numbers of legal sized scallops.
The other way in which predictions could be improved is to increase the relevance of our surveys of the stock to commercial catch data. It is commonplace for the results from fisheries independent surveys to differ from commercial catch per unit effort data (Hilborn and Walters, 1992; Brand and Allison, 1994; Harley et al., 2001). As our data suggests, fishermen often move from one high density patch of fish or shellfish to another, maintaining CPUE at a relatively high or stable level while stocks are fished down. In contrast, in our analysis CPUE from the scallop surveys was often higher than from commercial boats. However, this may be the result of fishermen failing to subtract travelling, setting and/or hauling time from their estimates of fishing time, thereby biasing CPUE values downwards (personal observation). Alternately or in addition, our scallop survey could, in some cases, be focused on small patches of scallops present in higher density than on the remainder of the fishing ground. Unlike some more random sampling programmes, our surveys of the stock were designed to provide an indication of commercial catch rates by concentrating on the most productive fishing areas. Therefore, the main reason for the disparity between the two sets of results is probably the limited spatial and temporal coverage provided by our stock surveys (2 days fishing on each ground per year) compared with up to and above 100 days per year from the logbooks. For the smaller fishing grounds such as Bradda Inshore and Laxey our stock surveys are probably adequate, as illustrated by the significant relationships on these grounds between CPUE from the surveys and logbooks. On the Peel and 15 Miles South grounds, however, the surveys indicated low densities and exploitation rates in recent years while the logbooks suggested these were still productive fishing grounds. Surveys on these grounds produced average seasonal CPUE values of less than 15 scallops per metre of dredge per hour on several occasions, which were never seen in the logbook records. It therefore appears that fishermen may now be exploiting different areas, within these fishing grounds, than those covered by our surveys. On these and several of the other grounds we now plan to expand our stock surveys to ensure more relevance to the commercial fishery.
Overall, our results support the continuation of our data collection programmes and the further development of a model capable of predicting future scallop catch rates around the Isle of Man. Such a model would provide early indications of strong year-classes before they enter the fishery as well as warning of recruitment failures. Consequently, fishermen and managers could adjust their strategies accordingly. However, the ability of fisheries managers to ensure the sustainability of the fishery will depend on the extent to which management/protection of the local spawning stock guarantees continued recruitment success. Identification of sources of larval supply to P. maximus populations around the Isle of Man, along with examination of the factors affecting survival of newly settled scallops, are priorities for further research into this fishery.
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Acknowledgements |
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This study was funded by the Department of Agriculture, Fisheries and Forestry of the Isle of Man Government. Many thanks to the boat crews of the RV "Roagan" and RV "Sula" for making the study possible and to the countless people who have helped collect this data over the years, especially Graham Hughes, Kate Prudden, Dave Pennington and Ulli Wilson. Ross Huggett, John Stead and Clare Mullen also processed several years' worth of spat collectors. Reinhard Kype was largely responsible for revitalising our database to enable extraction of the data and for producing Figures 1 and 2. Drafts of the manuscript have been considerably improved by comments from Richard Nash and two anonymous reviewers.
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