Increase in natural mortality of the Icelandscallop (Chlamys islandica) in West Icelandand collapse of the fishery in the early 2000s

No abstracts are to be cited without prior reference to the author.

The Iceland scallop (Chlamys islandica) fishery started in Breidafjordur, western Iceland, in 1970 and was conducted until 2003 when a fishery closure was recommended by the Marine Research Institute, Reykjavík. The annual landings totalled ca. 9000 t from 1993 to 2000. Survey indices declined drastically between 2001 and 2008, resulting in indices in 2008 amounting to only 13% of the average for 1993–2000. The downward trend in the stock abundance is considered to be mainly owing to mass mortality, caused by an unknown apicomplexan parasite in adult scallops, with nearly 100% prevalence observed in larger shells (>60 mm shell height). The adductor muscles were most heavily infected and gonad development was hampered in infected individuals. The Iceland scallop is distributed within the subarctic transitional zone where maximum sea temperatures of 12–15°C have been recorded. In Iceland, the annual bottom sea temperature during the period 1970–1995 ranged from 0 to 10°C on the various scallop grounds. The period from 1993 to 2003, however, is characterized by a steady increase in summer sea surface temperature, reaching the highest estimated level of the previous century in Iceland in 2003. More recent temperature data from the Breidafjordur scallop grounds have shown recordings from 11 to 12+°C at 15–25-m depths in August–September for most of the last decade. An experimental study demonstrated that scallops collected during late summer can tolerate temperatures up to 13°C, at least for up to 21 days, but there is considerable mortality at 14°C. The rising temperature in Breidafjordur in recent years has therefore brought the summer maximum temperature close to the apparent temperature tolerance of the stock (e.g. the 12.2°C measured in August 2003). It does not appear, however, that the direct effects of temperature are the sole factor responsible for the dramatic decline in Iceland scallop stocks in recent years. Other factors that are often temperaturedependent, such as disease, may be equally or even more responsible. Thus, the increased temperature since around the mid-1990s may have made the scallops more susceptible to infection. Furthermore, it could have created more favourable conditions for the apicomplexan parasite to proliferate inside the shells, resulting in the observed mass mortality and recruitment failure in the Iceland scallop stock.