Symposium Abstract: Shrimp and Crab Trawling Impacts on Estuarine Soft-Bottom Organisims
L. B. Cahoon, M. H. Posey, W. H. Daniels, and T. D. Alphin
This project addressed some possible impacts of trawling for crabs and shrimp in North Carolina estuaries on populations of organisms associated with soft-bottom habitats. The organisms of interest included benthic microalgae, demersal zooplankton, and macrobenthic infauna, encompassing the lower trophic levels in the benthic food chain and the essential trophic coupling that supports estuarine fishery production. The approaches used in this project included sampling before and after experimental trawling at several estuarine locations, sampling in areas actively trawled and areas closed to trawling, and sampling during several seasons over two years to address seasonal and inter-annual effects. Sampling began in February, 1999, and ended in November, 2000 at six locations in the Pamlico River Estuary. Experimental trawling had no significant effect on the biomass of benthic microalgae, no consistent effect on the abundance of demersal zooplankton, and only a slight but non-significant effect on the abundances of benthic macrofaunal animals. Benthic microalgae were significantly more abundant in untrawled locations than in trawled locations, with strong seasonal variation as well. Abundances of demersal zooplankton were not significantly or consistently different between untrawled and trawled locations. There were higher abundances of benthic macrofauna in trawled locations than at untrawled locations, but only at certain times of the year. Species dominance was fairly consistent between trawled and untrawled areas, with only a few exceptions. While inter-annual variation and substrate did have an effect, seasonal variation was far stronger and seemed to have an overriding effect. We conclude that direct, negative impacts of trawling activity on these soft-bottom organisms are small relative to other sources of population variability. The soft-bottom communities we studied experience considerable natural disturbance in these broad, shallow estuarine ecosystems. Although trawling per se does not seem to have a consistent effect on estuarine soft-bottom benthos, there are interesting differences between trawled and untrawled habitats that merit further investigation.