S. Jennings Æ S.P.R. Greenstreet Æ L. Hill Æ G.J. Piet J.K. Pinnegar Æ K.J. Warr Long-term trends in the trophic structure of the North Sea fish community: evidence from stable-isotope analysis, size-spectra and community metrics Received: 2 April 2002 / Accepted: 18 June 2002 / Published online: 27 August 2002 Ó Springer-Verlag 2002 Abstract Fishing has wide-ranging impacts on marine ecosystems. One of the most pervasive signs of intensive fishing is ‘‘fishing down the food web’’, with landings increasingly dominated by smaller species from lower trophic levels. Decreases in the trophic level of landings are assumed to reflect those in fish communities, because size-selective mortality causes decreases in the relative abundance of larger species and in mean body size within species. However, existing analyses of fishing impacts on the trophic level of fish communities have focused on the role of changes in species composition rather than size composition. This will provide a biased assessment of the magnitude of fishing impacts, because fishes feed at different trophic levels as they grow. Here, we combine body size versus trophic level relationships for North Sea fishes (trophic level assessed using nitro- gen stable-isotope analysis) with species–size–abundance data from two time-series of trawl-survey data (whole North Sea 1982–2000, central and northern North Sea 1925–1996) to predict long-term trends in the trophic structure of the North Sea fish community. Analyses of the 1982–2000 time-series showed that there was a slow but progressive decline in the trophic level of the de- mersal community, while there was no trend in the trophic level of the combined pelagic and demersal community. Analyses of the longer time-series suggested that there was no trend in the trophic level of the de- mersal community. We related temporal changes in trophic level to temporal changes in the slopes of nor- malised biomass size-spectra (which theoretically repre- sent the trophic structure of the community), mean log 2 body mass and mean log 2 maximum body mass. While the size-based metrics of community structure showed long-term trends that were consistent with the effects of increased fishery exploitation, these trends were only correlated with trophic level for the demersal commu- nity. Our analysis suggests that the effects of fishing on the trophic structure of fish communities can be much more complex than previously assumed. This is a con- sequence of sampled communities not reflecting all the pathways of energy transfer in a marine ecosystem and of the absence of historical data on temporal and spatial changes in the trophic level of individuals. For the North Sea fish community, changes in size structure due to the differential effects of fishing on species and populations with different life histories are a stronger and more universal indicator of fishing effects than changes in mean trophic level. Introduction The principal effects of fishing on the size and species composition of multispecies communities are well known. Spatial comparisons between areas subject to different fishing intensities, and temporal comparison within areas where fishing effort has increased over time, indicate broadly predictable effects. As fishing mortality rises, the mean size of individuals in the community falls, and species with larger body sizes form a smaller pro- portion of community biomass. These effects have been demonstrated in fisheries from the tropics to the Arctic (reviews: Gislason 1994; Jennings and Kaiser 1998; Hall 1999; Gislason and Sinclair 2000). Marine Biology (2002) 141: 1085–1097 DOI 10.1007/s00227-002-0905-7 Communicated by J.P. Thorpe, Port Erin S. Jennings (&) Æ J.K. Pinnegar Æ K.J. Warr Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Lowestoft, NR33 OHT, UK E-mail: S.Jennings@cefas.co.uk Tel.: +44-1502-524363 Fax: +44-1502-513865 S.P.R. Greenstreet Fisheries Research Services, Marine Laboratory, Aberdeen, AB11 9DB, UK L. Hill IPIMAR, Avenida de Brasilia, 1449-006, Lisbon, Portugal G.J. Piet Netherlands Institute of Fisheries Research, PO Box 68, 1970 AB, IJmuiden, The Netherlands