Publication Abstract

Ecological and environmental factors influence size-based metrics of Celtic Sea fish community structure.

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

Ecosystem-based management requires the development of appropriate indicators that enable the detection of anthropogenic impacts against the background of natural variation. Size -based community metrics are potentially useful indicators because: 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 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 can be 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 calculated for functional and spatial groupings of fishes in the Celtic Sea, an ecosystem that is strongly influenced by the environment.

We calculated size-based metrics using data from the Celtic Sea English groundfish survey series (1987-2003). 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 (10-15 cm). These size-based metrics appear to be influenced by environmental signals. The degree to which this occurs is dependent upon the functional and spatial grouping of the data. Our results show that it can be difficult to disentangle fishing and environmental impacts on size-based indicators.

Reference:

J.L. Blanchard, N.K. Dulvy, J.E. Ellis, J.K. Pinnegar, and S. Jennings. 2004. Ecological and environmental factors influence size-based metrics of Celtic Sea fish community structure. ICES CM 2003/N: 04 Copenhagen