Observations On Long-Term Changes In Prevalence Of Fish Species With Southern Biogeographic Affinities In The Northern North Sea

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Recent observations made in the scientific and popular literature have suggested that the characteristics of both marine and terrestrial plant and animal communities are changing rapidly due to increasing global air and sea temperatures. It is well known, for example, that the Lusitanian planktonic copepod, Calanus helgolandicus, almost completely replaced its more boreal congener, Calanus finmarchicus in the northern North Sea between the late 1960s and late1990s. The exact causative mechanisms underpinning this dramatic shift are still poorly understood, but global temperature change is likely to be involved. In this paper we examine the hypothesis that the piscean species composition of the northern North Sea is changing in response to climate. To do this we use a rich dataset on fish abundance collected by trawl during research surveys carried out by Fisheries Research Services, Aberdeen. The surveys cover water depths of less than 200m around all of Scotland’s coasts, and span a period of 79 years (1925-2003). The data exist in the form of length-frequencies for 348 different species, while additional information (e.g. age, sex, weight and stage of sexual maturity) is available for the commercially important component (e.g. cod, whiting & haddock). Our detailed analyses of the data suggest that the northern North Sea is currently experiencing waves of immigration by exotic, southern species (eg. red mullet, anchovy & pilchard) which are unprecedented in the context of the 79 year history of our extensive databases. Changes in the size structure of the invading species also indicates that some of the recent arrivals are actually ‘recruiting’ on northern North Sea grounds. Finally, the striking long-term spatial changes in fish species abundance and composition observed are discussed in the context of sea surface temperatures which have risen over the last decade.