Collapse of the fishery for Iceland scallop (Chlamys islandica) in Breidafjordur, West Iceland

298 Collapse of the fishery for Iceland scallop (Chlamys islandica) in Breidafjordur, West Iceland Jónas P. Jonasson, Gudrun Thorarinsdottir, Hrafnkell Eiriksson, Jon Solmundsson, and Gudrun Marteinsdottir 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. Keywords: Chlamys islandica, fishery collapse, Iceland scallop, mortality, recruitment, temperature. Received 20 April 2006; accepted 6 November 2006; advance access publication 7 December 2006. J. P. Jonasson and G. Marteinsdottir: University of Iceland, Askja, Sturlugata 7, 101 Reykjavı́k, Iceland. G. Thorarinsdottir, H. Eiriksson, and J. Solmundsson: Marine Research Institute, PO Box 1390 Skulagata 4, 121 Reykjavı́k, Iceland. Correspondence to J. P. Jonasson: tel: þ354 525 5229; fax: þ354 575 2001; e-mail: . Introduction 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 –158C (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, Reykjavı́k, 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. Material and methods 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 ¼ 0.2). # 2006 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: 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. 299 Collapse of the fishery for Iceland scallop in Breidafjordur, West Iceland where By is the total biomass (t), n the number of subareas, x̄s the average biomass per subarea, as 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): BH Figure 1. The study area in Breidafjordur, West Iceland. Each square containing a subarea is labelled. Squares on the main scallop grounds are divided into two subareas. Survey stations are marked with dots. Stations used for temperature recordings (Flatey and Stykkisholmur) are marked with triangles. 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 ¼ 46, MRI, unpublished data). Here, instead of using the average difference between the dredges, a single parameter regression was forced through the origin (r ¼ 0.94). The regression had a slope of 0.70, corresponding to a fresh estimate for e of 0.285. Each survey tow covered approximately 0.4 nautical miles and the tow speed was 4 k (...truncated)