Community Research and Development Information Service - CORDIS

Demersal fish community analyses

Two quarter three (Q3) groundfish survey data sets were analysed, the ICES IBTS, which uses a GOV otter trawl, and the Dutch DBTS. A method for estimating catchability at length for each species sampled in the GOV otter trawl is described. Catchability of different species in the GOV trawl varied markedly. Application of catchability coefficients to the IBTS data suggest that the total biomass of the demersal fish community of the North Sea varied between 3 and 7 million tonnes, with peaks occurring in 1999 and 2000.

Cluster analysis of the species abundance suggested the presence of three or four distinct community types across the North Sea. The diversity indices applied to the data were Hill's N0 (species richness), N1 (exponential of Shannon-Weiner Index) and N2 (reciprocal of Simpson's Index). For both IBTS and DBTS data sets it was necessary to aggregate approximately 20 trawl samples in order to arrive at diversity statistics that were representative of the actual diversity. However, this runs the risk of confounding (point) and (transect) diversity. Analysis suggested that where the aggregation of 20 samples could be achieved within a search radius of less than 50km then the estimate consisted entirely of α diversity, but where search radius exceeded 50km the estimates included an increasing component of β diversity.

Spatial patterns of Hill's N1 and N2 indices varied between IBTS and DBTS, suggesting they were strongly affected by the variable catchability of the two gears. Application of catchability coefficients to IBTS abundance data generated a different spatial pattern explicable on the basis of environmental parameters. This suggested species diversity was highest in the shallower, mixed, more productive southern North Sea. However, species richness and species diversity of the targeted fish component (log2 weight classes 8 and above) was highest in the northern region when catchability was accounted for, but when based on the raw abundance data diversity appeared highest in the south-eastern region.

A method for estimating growth productivity of the fish assemblage, based on the von Bertalanffy growth function and weight at length relationships, has been presented. A linear regression method for estimating the required parameter values for each species is described. The maps of biomass density, production density and P/B ratios are, once again seriously affected by the catchability of the gear. Productivity in the prey-fish component of the assemblage (Log2 weight-classes 3 and 4) was highest in the northern North Sea.

Once the MAFCONS consortium have made use of these data and been written up into a number of scientific papers, making the results available to a wider audience, the databases containing these data will be made available to the general public via our website.

Reported by

School of the Environment and Society
Singleton park
SA2 8PP SWANSEA
United Kingdom
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