Patterns of change in the size spectra of numbers and diversity of the North Sea fish assemblage, as reflected in surveys and models

Trends were analysed over two decades in the size spectra of numbers and diversity of the North Sea fish assemblage. In trawl survey data, the abundance spectrum was smoothly linear each year. Both slopes and intercepts increased significantly over the period, reflecting the effects of fishing. The diversity size spectrum was curvilinear, with diversity increasing among smaller sizes and decreasing linearly over larger sizes. The slope of the linear component of the spectrum varied with a multi-year pattern, but without an overall trend. The much greater stability of the diversity spectrum compared with the abundance spectrum suggests that the fish community structure has remained fairly stable over the period, despite significant increases in harvesting on component populations. To explore the hypothesis that the regulation of the com-munity structure arises from trophic interactions, the same community metrics were calculated from the output of a multi-species virtual population analysis of the major exploited fish predators and prey, parameterized with extensive catch data and feeding habits. Although many fewer species were included in the modelled assemblage than in the survey data, overall patterns were very similar. Annual abundance spectra were linear and slopes increased significantly and fairly smoothly over the 20 years, indicating significant effects of fishing on the size composition of the exploited fish assemblage. The annual diversity spectra were more dome-shaped than in the survey data. The shape showed no overall trend, but diversity of smaller size classes showed a different temporal pattern from the diversity of intermediate and large size classes. The patterns in modelled output are consistent with, but do not prove, the hypothesis that trophic interactions are an important factor in the fish community structure in the North Sea.