MODIFICATION OF MARINE BENTHOS BY TRAWLING: TOWARD A GENERALIZATION FOR THE DEEP OCEAN?

Abstract

Anthropogenic disturbance of deep-sea benthic systems, especially by fish- ing, has increased markedly in the last 40 years. Deep-sea mining and extraction of fossil fuels can occur at extraordinary intensity at individual sites, but the large number of fishing vessels and their mobility probably makes commercial trawling the most pervasive of our marine activities to depths of up to about 1200 m. Knowledge of the effects of trawling on soft-sediment, benthic communities is, however, limited to shallow, coastal systems, mostly at small spatial scales. We extend that knowledge to deeper systems at the scale of commercial fishing by assessing the effects of bottom trawling in northeastern New Zealand. We characterized the invertebrate catch of 66 research trawls spread along 220 km of continental slope in depths of 200-600 m (encompassing about 2400 km 2 ). At each site, we indexed the intensity of previous trawling using trawl-by-trawl fishing returns. A suite of multivariate analyses revealed that fishing activity was negatively associated (after ex- cluding the effects of depth and location) with invertebrate species richness and diversity and with the density of several taxa. Our models explained up to two thirds of the variation in the invertebrate catch of research trawls. After excluding the effects of depth and location, we attributed 11-40% of total variation to fishing. Concordance among the results of several multivariate methods based on different analytical approaches and assumptions reveals a strong and coherent pattern in the data that is consistent with the predicted and observed effects of trawl fisheries and other sources of physical disturbance. We infer that trawling probably changes benthic community structure and reduces biodiversity over broad spatial scales on the continental slope as well as in coastal systems. Such effects would have important implications for local and regional biodiversity and for the development and management of sustainable fisheries.