Publication Abstract

A size-based model to predict the impacts of bottom trawling on benthic community structure

D. E. Duplisea, S. Jennings, K. J. Warr, T. A. Dinmore

Bottom trawling causes widespread physical disturbance to the sediments in shallow shelf seas, and some areas of seabed are swept by trawls five or more times each year. The resultant mortality of benthic fauna is strongly size dependent. We used empirical data to demonstrate that the frequency of bottom trawling disturbance in the central North Sea had a greater effect on the size structure of the fauna in a soft-sediment benthic community than other environmental variables such as sediment particle size and depth.. This analysis provided compelling reasons to predict the impacts of trawling on benthic community structure using a size-based model. Accordingly, we simulated the impacts of trawling disturbance on a benthic community consisting of thirty seven size classes of organisms in three broad categories: meiofauna, soft bodied macrofauna and hard bodied macrofauna and ranging in size from 1 µg to 80 g shell free wet weight. The model produced a P/B vs body mass relationship consistent with that reported in previous empirical studies and allowed us to predict the impacts of trawling frequency on the size-spectra of organisms in benthic communities. Our outputs were broadly consistent with the results of empirical studies. We used sensitivity analyses to investigate the effects of changes in the minimum trawling induced mortality, and the range in the size of animals affected by trawling mortality, on the biomass of infauna of intermediate and large body size. If the minimum mortality per trawl pass for these animals was high, then the model predicted that they would be extirpated at the mean levels of trawling effort observed in central North Sea beam trawl fisheries. However, since empirical data show that intermediate and large animals persist, we suggest (i) that trawling disturbance in real fisheries is sufficiently heterogeneous to provide refuges that are unimpacted or infrequently impacted by trawling and (ii) that some of the largest bivalves may burrow so far into the sediment that trawling induced mortality is reduced. Our analyses suggest that fishery management measures that do not reduce total effort but do lead to effort displacement and increased homogeneity of trawling disturbance (e.g. temporary closed areas) may have adverse effects on the persistence of intermediate and large size classes of benthic fauna.

D. E. Duplisea, S. Jennings, K. J. Warr, T. A. DinmoreBottom trawling causes widespread physical disturbance to the sediments in shallow shelf seas, and some areas of seabed are swept by trawls five or more times each year. The resultant mortality of benthic fauna is strongly size dependent. We used empirical data to demonstrate that the frequency of bottom trawling disturbance in the central North Sea had a greater effect on the size structure of the fauna in a soft-sediment benthic community than other environmental variables such as sediment particle size and depth.. This analysis provided compelling reasons to predict the impacts of trawling on benthic community structure using a size-based model. Accordingly, we simulated the impacts of trawling disturbance on a benthic community consisting of thirty seven size classes of organisms in three broad categories: meiofauna, soft bodied macrofauna and hard bodied macrofauna and ranging in size from 1 µg to 80 g shell free wet weight. The model produced a P/B vs body mass relationship consistent with that reported in previous empirical studies and allowed us to predict the impacts of trawling frequency on the size-spectra of organisms in benthic communities. Our outputs were broadly consistent with the results of empirical studies. We used sensitivity analyses to investigate the effects of changes in the minimum trawling induced mortality, and the range in the size of animals affected by trawling mortality, on the biomass of infauna of intermediate and large body size. If the minimum mortality per trawl pass for these animals was high, then the model predicted that they would be extirpated at the mean levels of trawling effort observed in central North Sea beam trawl fisheries. However, since empirical data show that intermediate and large animals persist, we suggest (i) that trawling disturbance in real fisheries is sufficiently heterogeneous to provide refuges that are unimpacted or infrequently impacted by trawling and (ii) that some of the largest bivalves may burrow so far into the sediment that trawling induced mortality is reduced. Our analyses suggest that fishery management measures that do not reduce total effort but do lead to effort displacement and increased homogeneity of trawling disturbance (e.g. temporary closed areas) may have adverse effects on the persistence of intermediate and large size classes of benthic fauna.

Reference:

D. E. Duplisea, S. Jennings, K. J. Warr, T. A. Dinmore (2002) A size-based model to predict the impacts of bottom trawling on benthic community structure. Canadian Journal of Fisheries and Aquatic Sciences; 59: 1785 - 1795pp