ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on December 7, 2006
ICES Journal of Marine Science: Journal du Conseil 2007 64(2):298-308; doi:10.1093/icesjms/fsl028
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Collapse of the fishery for Iceland scallop (Chlamys islandica) in Breidafjordur, West Iceland
1 University of Iceland, Askja, Sturlugata 7, 101 Reykjavík, Iceland
2 Marine Research Institute, PO Box 1390 Skulagata 4, 121 Reykjavík, Iceland
Correspondence to J. P. Jonasson: tel: +354 525 5229; fax: +354 575 2001; e-mail: jonasp{at}hafro.is
Jonasson, J. P., Thorarinsdottir, G., Eiriksson, H., Solmundsson, J., and Marteinsdottir, G. 2007. Collapse of the fishery for Iceland scallop (Chlamys islandica) in Breidafjordur, West Iceland. – ICES Journal of Marine Science, 64: 298–308.The stock index of the Iceland scallop (Chlamys islandica) in Breidafjordur on the west coast of Iceland has declined drastically in recent years. Total fishing mortality was very high throughout the study period from 1993 to 2003, a period characterized by a steady increase in summer sea surface temperature, in 2003 reaching the highest estimated level of the previous century. Between 1998 and 2005, estimates of chlorophyll a (food availability) fluctuated with periods of low chlorophyll followed by a reduction in muscle weight and high natural mortality. High levels of natural mortality were observed in the main fishing area in the southern part of Breidafjordur. There the stock index had been declining since 1994. Recruitment to the fishable stock was highly variable during the study period, with low recruitment towards the end of the 1990s. Subsequently the fishery has been on relatively few year classes, and the stock has been fragile because of several years of poor recruitment and high natural mortality. Consequently, the stock appeared unable to withstand the fishing pressure and declined to historically low levels, leading to a halt to fishing in 2004.
Keywords: Chlamys islandica, fishery collapse, Iceland scallop, mortality, recruitment, temperature
Received 20 April 2006; accepted 6 November 2006; advance access publication 7 December 2006.
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Introduction |
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Population fluctuations have long been recognized as common phenomena of both marine and terrestrial animals (Elton, 1924). In scallop stocks, population size can be influenced by numerous factors, including variability in recruitment and catastrophic mortality from various sources (Orensanz et al., 1991). Fish and shellfish stocks have been classified into steady, cyclical, irregular, and spasmodic stocks according to their pattern of fluctuation (Caddy and Gulland, 1983).
The Iceland scallop (Chlamys islandica) is distributed within the Subarctic transitional zone at maximum sea temperatures of 12–15°C (Sundet, 1988; Hovgaard et al., 2001) and at depths <100 m (Wiborg, 1963). It is relatively long-lived with a maximum observed age of at least 23 y (Vahl, 1981). Long-lived Arctic and boreal scallops, such as the Iceland scallop, may fall into the steady stock group (Orensanz et al., 1991), but fluctuations in populations of Iceland scallop caused by changes in temperature and/or salinity (Wiborg, 1963), predation (Brun, 1968), and heavy fishing have been observed at several locations in the North Atlantic (Hovgaard et al., 2001).
In Iceland, the Iceland scallop has been fished since 1969 (Eiriksson, 1997). Population size decreased in all major subpopulations in Icelandic waters towards the end of the 1990s. Stock biomass indices for small scallop stocks in the northwest decreased by 45–80%, and the greatest decline was in the area where fishing was minimal (Marine Research Institute, MRI, Reykjavík, unpublished data). The stock size index of the largest scallop population in Iceland, in Breidafjordur, declined by 70% during the period 2000–2003. Landings (total weight) there peaked at 12 700 t in 1986, decreased slightly in the following years, then remained relatively stable at 8000–9000 t during most of the 1990s. Then, between 2000 and 2003, the stock collapsed and annual landings decreased from 8600 to 800 t. As a result, fishing was stopped in 2004 (Anon., 2005).
The objective of the current study was to look for possible causes for the dramatic decline in the stock of Breidafjordur Iceland scallop between 1999 and 2003. Data from stock surveys, specific sampling, and fishery logbooks were explored in order to analyse stock biomass, natural and fishing mortality, recruitment, and muscle condition. Available environmental data (chlorophyll and temperature) were also analysed and are discussed in relation to the observed changes in the stock.
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Material and methods |
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Survey data
Data were collected during the annual scallop surveys conducted by the MRI in the inner part of Breidafjordur, West Iceland, in March/April of the years 1993–2003 (Figure 1). On each survey, some 120 fixed standardized tows were taken. From 1993 to 1997, a 470 kg sledge dredge 1.5 m wide was used. In 1998, this was substituted with a 835 kg roller dredge 1.2 m wide (Garcia et al., 2006). Both dredges were equipped with 60 mm steel rings. Earlier experiments on the sledge dredge had revealed that its efficiency was 20% (i.e. e = 0.2). Comparative experiments between the roller and sledge dredge showed that the catch of scallops in roller dredge tows was on average 30% higher than in sledge tows, so e for the roller dredge was set at 0.26 (n = 46, MRI, unpublished data). Here, instead of using the average difference between the dredges, a single parameter regression was forced through the origin (r = 0.94). The regression had a slope of 0.70, corresponding to a fresh estimate for e of 0.285.
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Each survey tow covered approximately 0.4 nautical miles and the tow speed was 4 knots. For the analysis, all tow lengths were standardized to 0.4 nautical miles. For each tow the total catch was weighed and a random subsample of approximately 25 kg taken. In each subsample, all live scallops were weighed and the height of about 100 was recorded. The remaining scallops were counted and the numbers of cluckers (dead scallops attached on their hinges, both damaged and whole shells) were noted.
The survey area was divided into subareas, based on a grid of squares of equal size. Squares positioned on the main scallop grounds were split into two subareas (Figure 1). The total region fished was divided into a northern area (subareas 31–42), north of Bjarneyjarall (a trench bisecting the fjord from west to east), and a southern area (subareas 2–14), south of the trench. The size of the scallop beds in each subarea was based on estimates conducted at the beginning of the surveys in the early 1970s, where the total area was estimated to be 72 km2.
Stock biomass indices were estimated from
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| (1) |
s the average biomass per subarea, 
s the size of the scallop beds per subarea (km2), dw the width of the dredge (m), tl the tow length (nautical miles), and e the dredge efficiency.
Fishery data
Catch per unit effort (cpue) (landings per hour fishing) within each subarea was based on logbook catch reports, which are mandatory in the fishery. The cpue data go back to 1972, but with precise information from around 1985.
Fishing mortality and natural mortality
Fishing mortality was calculated by two non-model methods for the four major subareas in the fishery (12.1 and 12.2 in the south, and 42 and 32.2 in the north):
- A Beverton and Holt length-based fishing mortality (BHF) (Quinn and Deriso, 1999) was calculated from
where in each subarea s, K is a growth constant from the von Bertalanffy growth function (VBGF) (Ricker, 1975) [Equation (6)], L
(2) 
the asymptotic shell height (SH) from the VBGF, 
s the mean SH beyond lc (here 60 mm), and Ms is the calculated natural mortality [Equation (4)].
- Equilibrium fishing mortality (YF) (Quinn and Deriso, 1999) was calculated from
where in each subarea s, Y is the landings of scallops (t) for a 1-y period (
(3) 
= 1) and B is the estimated biomass [Equation (1)] in tonnes at the beginning of the period. The method does not consider indirect fishing mortality (non-yield) from fishing gear.
Natural mortality was based on the occurrence of cluckers in survey tows (Dickie, 1955):
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| (4) |
The geographical distribution of abundance and natural mortality of scallops from the surveys was plotted for the years 1994 and 2001–2003. In the plots of natural mortality, stations with <5 kg of scallops or where fewer than five scallops were measured were excluded. Data were interpolated spatially using a kriging method (Kaluzny et al., 1998).
Environmental and biological data
Owing to the lack of a complete sea surface temperature (SST) data series for Breidafjordur, SST was estimated on the basis of the relationship of SST at Flatey (an island in the middle of Breidafjordur) and the air temperature at Stykkisholmur (Figure 1). All available monthly mean SST data from Flatey (May–August of 1990–2001; n = 35) were used (Jonasson et al., 2004; r2 = 0.942, p < 0.001).
Estimates of chlorophyll a (Chl a) in Breidafjordur were derived from the NASA SeaWiFS project. The information consisted of Level 3 data, which are statistical data products derived by mapping Level 2 GAC data to a fixed global grid whose resolution elements are approximately 9 x 9 km (Campbell et al., 1995). In all, 50 monthly mean values were obtained from the region bounded by 65°02'–65°29'N and 22°41'–23°38'W. Mean values from March to September were calculated for each year of the period 1998–2005.
Measurements of adductor muscle wet weight were initiated in autumn 2000 when processing plants first experienced low meat yields. For those measurements, scallops were sampled from 65°17'5 N 22°52'5W in subarea 32.2 and from 65°05'3 N 22°42'7W in subarea 11 (Figure 1). Sampling was during the months September–December of 2000–2005. Muscle wet weight (W) of scallop was fitted to shell height (SH) by the equation
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| (5) |
Age determination and recruitment
Age was determined from shell height using Bhattacharya's method (Sparre and Venema, 1998). Graphs were used for visual identification of frequencies perceived to belong to one age group. General additive model (GAM)-smoothed data from 1993 to 2003 in subarea 12.1 (south) were used for this procedure. The VBGF (Ricker, 1975) was fitted to the mean shell height at age by linear regression (Crawley, 2002), also using data from subarea 12.1. The VBGF was formulated as
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| (6) |
the asymptotic shell height (108.1), K the growth constant (0.139), and t0 the intercept of the growth curve on the age axis. The VBGF fitted well the estimated mean age of the scallops (Figure 2).
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For analysis of year-class strength, the divergences from the mean shell height frequency indices of 1993–2003 were calculated for subarea 12.1 (south), during the same period, based upon a common method (Sund, 1930). For each year, the height frequency was weighted with the total numbers of scallops, estimated as
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| (7) |
where N is the total number of scallops (in thousands), the average number per station, a the size of the subarea (km2), dw the dredge width (m), tl the tow length (nautical miles), and e the dredge efficiency. The data were smoothed with a GAM model using a cubic B-line smoother with 20 degrees of freedom (Venables and Ripley, 1997).
Stock fluctuations in relation to biotic and abiotic factors
Multiple regression was used to analyse the effects of several biotic and abiotic factors on the number of harvestable scallops (
60 mm) in all subareas for which data were available. The dependent variable was log-transformed to attain normality (Shapiro–Wilk normality test, p = 0.7238). The starting model included the log-transformed numbers of 45–60 mm scallop recruits 2 years earlier, the natural mortality determined from the surveys, the mean summer temperature of the previous year, the YF for the preceding year, the BHF for the same year, and the effort of the previous year standardized by the number of harvestable scallops. Independent variables that did not contribute were removed by subtracting the least significant term in each step (Crawley, 2002).
All statistical analyses were performed using Version 6.0 S–PLUS software (MathSoft, 2001), except for Bhattacharya's method, which was performed with the Version 1.0 FISAT II statistical program (Gayanilo et al., 2002).
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Results |
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Stock size and the fishery
The stock index of Iceland scallop in Breidafjordur was relatively stable from 1993 to 1999, but it declined sharply from 2000 to 2003 (Figure 3). In 2003 (22 000 t), it was at a historically low level of some 30% of the average stock size during the 1990s.
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The main trends in stock size differed between subareas (Figure 4). In subarea 12.1, the major fishing area in southern Breidafjordur, the stock index declined gradually from 1994. In subarea 32.2, the major fishing area in the northern part of Breidafjordur, the stock index fluctuated during the period 1993–2000, peaked in 1997, then steadily declined to a historical low in 2003 (Figure 5). In other subareas, stock size generally only declined (or was observed to do so) after 2000, although there was a steady decline in subareas 13 and 3.1 from 1993 and 1994, respectively.
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Cpue, pooled for all areas in Breidafjordur, was relatively stable during the period 1986–1990, but it increased considerably from 1991 to 1996 (Figure 3). During the years 1996–1998, it was relatively high, but it then declined sharply from 1998 to 2003. The increase in cpue in the early 1990s coincided with changes in the scallop fishing gear, when the fleet changed from sledge dredges to the more efficient roller dredges.
Fishing effort was significantly higher during the period 1986–1993 than from 1994 to 1998, but it also increased again from 1999 to 2002, i.e. following an opposite trend to that in cpue (Figure 3). Throughout the period 1993–2000, the total allowable catch (TAC) was relatively stable at about 8000–8500 t (Anon., 2005). At that time, the recommended annual TAC was 10% of the total estimated biomass from stock surveys; since 1994, the national TAC and the landings have been in accord with the recommendations.
Fishing mortality and natural mortality
The estimates of the Beverton and Holt length-based fishing mortality (BHF) were relatively high in the main fishing areas between 1993 and 2003 (Figure 6). The estimates were stable in subarea 32.2 in the north, but in subarea 12.1 and 12.2 (south) and 42 (north), there was a slow but gradual decline in BHF, followed by an increase in area 42 from 2001 and in area 12.1 from 2002. The equilibrium fishing mortality (YF) was stable and substantially lower than BHF in all subareas (Figure 6).
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The natural mortality observed on scallop grounds in Breidafjordur during the period 1993–2003 increased in most areas after 2000 (Figures 5 and 7). In general, natural mortality was high in subarea 12.1, increasing from 0.1 in 2001 to 0.4 in 2003. In other subareas in the south, the increase was not so clear, although there were years with natural mortality >0.15 in subareas 2 and 13. In the north, natural mortality was relatively low or approximately 0.05 from 1993 to 2000, then increased to 0.1 during the years 2001–2003.
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Condition of Iceland scallop in relation to environmental parameters
The estimated mean SST at Flatey fluctuated periodically during the last century, with values above average in the years 1930–1950 and mainly below average from 1960 to 1990 (Figure 8). Mean SST then increased in 1990 and 1991, but declined again in 1992 and 1993. Since then, temperature has been gradually increasing towards a level similar to that of the 1930s; the maximum mean was >9°C in 2003 and 2004.
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Estimates of mean summer Chl a data in Breidafjordur are available for 1998–2005 (Figure 9). The means fluctuated, with relatively low values in 1999 (1.73 mg m–3) and 2005 (1.78 mg m–3), and the highest measured value in summer 2003 (2.60 mg m–3).
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The fit of the relationship between scallop height and muscle wet weight was low, especially for the years 2001–2003, and muscle weight was not isometric with shell height in any year (p < 0.001). The number of scallops in poor condition increased from 2000 to 2002 on both northern and southern grounds, decreased in 2003, but increased again in 2004 and 2005 (Figure 9). Muscle weight during those years fluctuated in a manner similar to that of Chl a. Although not statistically significant, the correlation coefficients were 0.55 and 0.58 (p = 0.26 and 0.22, d.f. = 4) for northern and southern scallops, respectively.
Mean shell height and recruitment
In general, the average shell height of scallops in the surveys increased from 1993 to the end of the decade, but then declined (Figure 10). Maximum shell height in subareas 12.1 (71.2 mm) and 12.2 (69.8 mm) was in 1999. In subarea 32.2, shell height peaked in 2000 (69.5 mm), then declined slower than in other subareas. Maximum shell height in subarea 42 was in 2001 (69.1 mm).
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Recruitment to the fishable stock (
60 mm) was highly variable during the study period in subarea 12.1, with low recruitment towards the end of the 1990s (Figure 11). Similar fluctuations were observed in other subareas (Jonasson et al., 2005). Relatively strong year classes entered the fishable stock between 1993 and 1996 (year classes 1987–1990), but then medium and weak year classes dominated until 2003. In 1993, the survey catch was dominated by shells of approximately 70 mm, i.e. mostly 7–9 years old, according to a length-based conversion (Figure 2), but 80 mm (10 years old) scallops were rare (Figure 11). The fishable stock then consisted, therefore, mainly of three year classes (1984–1986). During the period 1994–1996 the strong year classes of 1987–1990 grew and filled the gap observed around 80 mm in 1993. Medium or weak year classes from 1991 to 1992 appear to have entered the fishable stock in 1997 and 1998, and small year classes from 1994 and 1995 seem to have recruited to the fishable stock in 1999 and 2000. As a result, a gap in shell height from 60 to 70 mm was formed then. During 2001 and 2002, small year classes continued to enter the fishable stock, resulting in a scarcity of older age groups. In 2003, a year class from 1997 and a reasonably large year class from 1998 appear to have recruited.
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Stock fluctuations in relation to biotic and abiotic factors
The relative abundance of harvestable scallops was significantly affected by the numbers of juvenile scallops (45–60 mm) 2 years earlier, BHF, standardized effort, and natural mortality (marginally). Juvenile abundance had a positive effect on the number of harvestable scallops, whereas other factors had negative effects. The number of juveniles contributed most to the regression sum of squares (Table 1). Overall, the multiple regression explained some 66% of the variation in stock size abundance, and a good fit of the model was demonstrated by the apparent lack of structure in the deviance residuals relative to predicted values. Temperature and YF were not statistically significant.
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Discussion |
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The Iceland scallop stock in Breidafjordur has declined considerably since 2000. The period has been characterized by a steady increase in SST, increasingly poor condition of the scallops, and limited recruitment to the fishable stock.
Fluctuations in the stock size of Iceland scallop have differed between subareas, perhaps because of spatially different patterns of recruitment and exploitation (Beukers-Stewart et al., 2003), but possibly also through variable predation intensity or spread of disease. The effect of dredging on the seabed can also vary locally, but a new dredge introduced to the fishery around 1990 increased fishing efficiency substantially. The new dredge was heavier, could be towed faster, and was easier to operate than its predecessor. Consequently, cpue increased in all subareas for several years after the new dredge was introduced, but the total catch was stable, restricted by catch quotas. The survey data have the disadvantage of containing information from two different dredge types, because estimated comparisons are likely to include errors and thus bias stock estimates in one or the other direction.
Fishing mortality was relatively high throughout the study period. The equilibrium fishing mortality was low compared with the Beverton and Holt fishing mortality, but it only represents landings and is negatively biased in the presence of non-yield fishing mortality. The difference between the two estimates may indicate a substantial indirect fishing mortality, which can accompany scallop dredge fisheries (Caddy, 1989). Heavy gear may impose high levels of incidental damage to scallops, whether they are retained, pass through the ring or inter-ring spaces, or are run over by the dredge (Caddy, 1989). In the present investigation, the mean difference between the Beverton and Holt F (BHF) and equilibrium F (YF) was 0.284–0.439 over a period of 11 y (Figure 6). In an exploited scallop area in Canada, indirect fishing mortality was estimated to be as high as 0.364, based on cluckers and crushed scallops (Naidu, 1988). That area had been fished with a heavy dredge, similar to that used in Breidafjordur post-1990. In the presence of indirect fishing mortality, a rotational harvest strategy can provide equal or greater yield and maintain a higher spawning biomass (Myers et al., 2000). As the dredge is only a "semiquantitative tool" (Caddy, 1989), rotational harvesting would benefit from more precise and accurate estimates of scallop density and size distribution, which can be monitored by a technique such as underwater photography (Stokesbury, 2002).
In the present study, natural mortality was generally higher in the southern than in the northern area, with an extremely high value (0.41) in subarea 12.1 in 2003. The rates of natural mortality in subarea 2 in 2001 and in subarea 13 in 2002 were also very high, 0.22 and 0.29, respectively. The same adjustment factor was used here for tow-induced disarticulation as described by Naidu (1988) for tows of 0.25 nautical miles. Naidu noted that on longer tows, more scallops would be expected to disarticulate. The values of natural mortality presented here could therefore be underestimated, because the average tow length was about 0.4 nautical miles. In contrast, part of the estimated natural mortality could be dredge-induced mortality. In heavily fished areas in Canada, natural mortality has been reported to be as high as 0.21 (Naidu and Cahill, 1984).
Along with the decline in the scallop population, the sea temperature in Breidafjordur increased, and in 2003 and 2004, the mean reached the highest estimated values in 100 years. However, nothing is known about the status of the scallop stock during the warm years of the 1930s. The Iceland scallop fishery in Breidafjordur started in 1970 (Eiriksson, 1997) and therefore has until comparatively recently been prosecuted during relatively cool periods. An experimental study by Jonasson et al. (2004) showed that scallops collected during late summer can tolerate temperatures up to 13°C, at least for up to 21 d, but there is considerable mortality at 14°C. The rising temperature in Breidafjordur during recent years has therefore brought the summer maximum temperature close to the apparent temperature tolerance of the stock, e.g. 12.2°C in August 2003 (Jonasson et al., 2004). However, an increase in natural mortality had been observed already during the 2001 spring survey, so the direct effect of a high summer temperature is unlikely to have been the only factor inducing natural mortality of the stock. In contrast, Wiborg (1963) assumed that Iceland scallop located outside fjord sills in northern Norway were depleted at intervals owing to variations in temperature and/or salinity. Further, the drastic decline in the southernmost population of Iceland scallop in Hvalfjordur, Southwest Iceland, in 1983 was suggested to have been the result of elevated sea temperatures the previous year (Eiriksson, 1997). On several occasions, a high sea temperature has been associated with mass mortality of Placopecten magellanicus in Canada (Dickie and Medcof, 1963).
Chl a values provide an estimate of food availability to Iceland scallop. The highest mean value of Chl a in 2003 was 50% higher than the lowest value in 1999, illustrating a considerable annual difference in food availability in Breidafjordur. Chl a data derived from the NASA SeaWiFS project fit well the in situ Chl a data in shallow waters (Tang et al., 2003). Thorarinsdottir (1991) collected monthly Chl a samples 8 m deep in Breidafjordur and recorded mean values from March to September of 1990 and 1991 of 1.82 and 1.51 mg m–3, respectively. Those values fall in the lower ranges of the values presented here (1.73–2.60 mg m–3). The mean Chl a 2 m deep near a scallop ground in the mouth of Hvalfjordur from March to September 1997 was 2.63 mg m–3 (Eydal, 2003), close to the higher values observed during the current study.
The adductor muscles of scallops were in an abnormally poor condition at the time of the increase in scallop mortality (2000–2002). Muscle weight increased in 2003, coinciding with high values of Chl a, but decreased again with a low primary production in 2004 and 2005. At that time, two coccidia parasites were identified in Iceland scallop from Breidafjordur, one of which causes infection of muscle tissue (Kristmundsson et al., 2004). The prevalence of infection was about 90%, with severe infections in large scallops. The poor condition of scallop muscles in 2001 and 2002 could be attributed to this infection, although a more complex interaction of infection, food availability, and temperature would seem a more plausible explanation. In other shellfish species, parasite prevalence has been related to elevated sea temperature (Cook et al., 1998). Further, Yungkul and Powell (2004) proposed that malnourishment leading to death of surf clams (Spisula solidissima) was caused by an environmental shift that led to a mismatch between food supply and demand.
The increase in average shell height of Iceland scallop towards the end of the 1990s can misleadingly be interpreted as resulting from declines in exploitation rate (Caddy, 2004). The underlying cause was several years of weak and average recruitment to the fishable stock, which led to a relatively high proportion of older scallops in the total stock. The strong relationship between recruitment and harvestable biomass further supports the effect of variable recruitment in stock fluctuations, although fishing mortality and, consequently, effort also contribute to stock fluctuations. If other Iceland scallop beds in the Atlantic are compared, there is clear variability in recruitment between areas. In the Iceland scallop fishery off Jan Mayen and Svalbard, which collapsed through overfishing, the proportion of 65 mm scallops declined (Anon., 1988). A decline in older age groups would be expected in a selective overfishing scenario, but this trend was not observed in the present study. Better-than-average recruitment was rare in the Svalbard region after the depletion of the grounds during 1986 and 1987, good recruitment being first detected only in 1996 (Anon., 2002). In West Greenland, the Iceland scallop population is mainly old scallops. The area is characterized by low levels of recruitment, and scallop beds can be depleted quickly by fishing (Pedersen, 1994). In contrast, Naidu and Anderson (1984) suggested that the presence of several consecutive year classes of Iceland scallop on Canada's St Pierre Bank indicated relatively stable recruitment. Moreover, growth rates are considerably slower among Iceland scallop stocks off Svalbard and Greenland than off Norway, Iceland, and Canada (Pedersen, 1994), so making them less tolerant to fishing.
It is difficult to estimate the effects of size and age composition of the spawning stock on the variable year-class strength observed in the present study. Density-dependent responses of fish populations (particularly involving recruitment) are often obscured by variability, presumably because of fluctuating environmental (density-independent) factors (Sissenwine, 1984). Dickie (1955) demonstrated that fluctuations in catch and landings of P. magellanicus in the Bay of Fundy were caused largely by variable year-class strength and that recruitment to this stock was correlated with temperature, which influenced wind-driven transport during the pelagic larval period. Spatial and temporal patterns in settlement of benthic invertebrates can be strongly linked to transport by wind-driven currents (Bertness et al., 1996).
Several factors may enhance the recruitment and survival of spat. Addition of dead scallop shells to scallop grounds could have a positive impact, supporting an increase in the number of juvenile scallops and other invertebrates in the area (Guay and Himmelman, 2004). Other things being equal, recent natural mortality could therefore be a positive influence on future recruitment. Marine protected areas (MPAs) have been shown to increase scallop biomass manifold and to increase recruitment outside the protected areas. This was evident for P. magellanicus on Georges Bank, where scallop biomass increased 14-fold in protected areas during the 4 y of closure, although with moderate or below-average recruitment during the same period (Murawski et al., 2000). With increasing biomass in MPAs, there can be also a higher proportion of older animals (Bradshaw et al., 2001; Beukers-Stewart et al., 2005). Increased importance of gamete production with age has been shown for P. magellanicus by Langton et al. (1987) and for Iceland scallop by Vahl (1984). A high density of scallops may also increase fertilization rate (Claereboudt, 1999).
In summary, the fishable scallop stock in Breidafjordur in the late 1990s consisted of few year classes, and as the total (indirect and direct) fishing mortality was high, the stock was vulnerable to several years of poor recruitment. Moreover, there was high natural mortality, probably because of disease and unfavourable environmental conditions. Poor recruitment combined with intensified fishing and high natural mortality seems to have led to the collapse of the stock. In future and in order to predict future catch levels, the biomass of spat-fall and juvenile scallops should be estimated (Beukers-Stewart et al., 2003), and the current results can be used to manage fishing effort better. Finally, there would seem to be a strong case for the establishment of MPAs in Breidafjordur as well as the development of a rotational harvesting strategy for the stock.
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Acknowledgements |
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We thank the administrators of the NASA SeaWiFS project and the Distributed Active Archive Center at the Goddard Space Flight Center for allowing us access to the SeaWiFS data. SeaWiFS activities are sponsored by NASA's Mission to Planet Earth Program. We also thank Ingibjorg Jonsdottir, Kristinn Saemundsson, Lorna Taylor, G. Skuli Bragason, and Unnur Skuladottir for their various help and two anonymous reviewers for very constructive comments. Hlynur Petursson, Birgir Stefansson and the crew of the RV "Drofn" are thanked for assistance in data sampling.
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References |
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