Long-term increases in prevalence of North Sea fishes having southern biogeographic affinities

Observations made in the scientific and popular literature suggest that the characteristics of both marine and terrestrial ecosystems are changing rapidly due to increasing global air and sea temperatures. Here, we examine the hypothesis that fish species with more 'southern' distributions are increasing in the northern North Sea over time. In order to do this, 2 important databases on fish abundance collected by trawl on research cruises are interrogated. When combined, the databases cover both the entire North Sea and the Scottish west coast and span a period of 80 yr (1925 to 2004). The data take the form of length-frequencies for all species caught (>300 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). The trawl data suggest that the North Sea is experiencing waves of immigration by exotic, southern species (e.g. red mullet, anchovy and pilchard). The purpose of this paper is to describe and document these changes.