Publication Abstract

Large-scale forcing factors influence size-based metrics of Celtic Sea fish communities

J.L. Blanchard, N.K. Dulvy, J.E. Ellis, J.K. Pinnegar and S. Jennings

Fundamental ecological processes are highly size dependent in aquatic ecosystems. Ecosystem-based management requires the development of appropriate indicators that permit the detection of anthropogenic impacts on ecosystem structure and function. Size–based community metrics are attractive for several intrinsic and extrinsic reasons: 1. at an individual level size is inherently related to biological processes such as growth, predation risk and competitive ability and at a population level size is related to productivity, 2. Fishing is highly size-selective resulting in predictable changes in size structure of exploited communities, and 3. size-based metrics are easily calculated from readily available survey data. Recent studies have questioned whether size-based indicators can differentiate among fishing, environmental and ecological effects. We describe patterns in size metrics aggregated according to taxonomic, trophic, functional and spatial groupings in a strongly environmentally influenced exploited ecosystem.

We analysed data from the Celtic Sea English groundfish survey series (1987-2003) and constructed several proposed size-based metrics. Also we consider whether fishing and environmental influences are discernible from these size metrics. We show that the overall size structure of the Celtic Sea fish community has changed over time. A decrease in larger fishes over time was accompanied by an apparent increase in smaller fishes <10 cm. Furthermore, these size metrics appear to be highly influenced by a large-scale environmental signal (North Atlantic Oscillation index). The degree to which this occurs is dependent upon the taxonomic, trophic, functional and spatial groupings. The environmental signal was most apparent in the pelagic group but was less obvious in demersal and higher trophic level groups. Our observations have a bearing on the development of ecosystem-based indicators of anthropogenic impacts, particularly discerning fishing impacts from climatic influences.

Reference:

J.L. Blanchard, N.K. Dulvy, J.E. Ellis, J.K. Pinnegar and S. Jennings, 2003. Large-scale forcing factors influence size-based metrics of Celtic Sea fish communities. ICES CM 2003/N04. ICES Copenhagen.