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The ecological disturbance caused by fishing

Huston's Dynamic Equilibrium Model considers the effect of disturbance on species diversity. In order to test this model as a predictor relating fishing activity and ecological consequences it was therefore necessary to have some means of relating fishing activity to the level of mortality generated by the disturbing event. Therefore, indices of disturbance should be based on a quantification of total mortality, including all target and non-target organisms, induced by all fishing activity in a given area over a period of time, including some measure of impact from habitat alteration.

In the context of modelling species diversity and disturbance, the assessment of disturbance must be based on the alteration of population mortality caused by the specified perturbation. The components of mortality to target fish and invertebrates caused by fishing include landings, discards and trawl escapees; whereas the mortality of non-target fish and invertebrates must be modelled on data that quantify levels of fishing activity, namely fishing effort data.

Because of the fundamental differences in mobility and spatial distribution between fish and benthic invertebrates, two separate but complimentary approaches were made. Consequently a major review was made of the knowledge of ecological disturbance caused by fishing and the final results have been the production of two modelling approaches that utilise the fishing effort data to estimate mortality within fish and benthic invertebrate communities. From these, maps of ecological disturbance to these components were generated for use in the testing of Huston s model.

The invertebrate mortality models were based on recent meta-analysis studies to determine "per fishing event" mortality rates for a range of benthic invertebrate fauna. Recent small scale studies of the Dutch beam trawl fleet have suggested that the distribution of fishing activity follows a Poisson distribution, such that tow velocity, tow duration and gear width for each fishing metier can generate a mean fishing frequency in each ICES rectangle from fishing effort data. This can then be coupled with the "per fishing event mortality" to provide the total benthic mortality in each rectangle to be estimated.

The fish mortality model utilised swept area estimates combined with estimates of local density, making assumptions about catchability, to determine the number of fish taken in each fishing event. The number of events per rectangle was then estimated from effort statistics and the model generated annual rates of fishing mortality for each species recorded. Estimates of discards for each commercial species were obtained from annual stock assessments and these were then used to raise landings to total catch, with corrections applied to account for non-target species. Such total catches in each ICES rectangle were then converted to "exploitation" rate indices by dividing them by estimates of the abundance of each fish present.

Reported by

School of the Environment and Society
Singleton park
United Kingdom
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