Publication Abstract

Trawling disturbance can modify benthic production process

S. Jennings, T. Dinmore, D. Duplisea, K. Warr and J. Lancaster

The disturbance caused by towed fishing gears has widespread impacts on the marine environment and is well known to modify benthic (sea-bed) habitats and community structure.

It has been suggested that frequent trawling disturbance favours smaller benthic species with faster life histories because they can withstand the mortality due to trawling and benefit from reduced competition as populations of larger species are depleted. Since these smaller species are more productive, trawling disturbance may be responsible for 'farming the sea', by increasing rates of benthic production and energy cycling, with knock-on benefits for consumers, including fish populations.

We compared the size structure and production of benthic invertebrate communities across sites that were subject to known levels of trawling disturbance in two regions of the North Sea. Size structure was described using normalised size-spectra, and the gradients and intercepts of these spectra were related to levels of trawling disturbance. Production was estimated from a new allometric relationship between body mass and the P:B ratio of marine invertebrates. The general validity of the relationship was confirmed using a phylogenetic comparative approach.

In the region of the North Sea that was deeper and subject to a greater range of trawling disturbance (Silver Pit region, 27 fold across sites), trawling led to a significant decrease in the total biomass of infauna. The abundance of larger individuals was depleted more than smaller ones, as reflected by the positive relationship between the gradient of the normalised size spectra and trawling disturbance. The effects of trawling were not apparent in the epifaunal community or in the infuanal and epifaunal communties in another region where the range of trawling disturbance was lower (Hills region, 10 fold across sites).

In the Silver Pit, total infaunal production fell with increasing trawling disturbance while relative production (as P:B ratio) of the infaunal community rose significantly. Elsewhere, there were no consistent trends. The results show that intense trawling disturbance can lead to increases in relative production, but these increases were largely attributable to the dominance of smaller animals in the trawled community and did not compensate for the loss of biomass and production of larger animals.

We conclude that trawling disturbance does not lead to significant increases in the total production of benthic communties. We suggest that recent increases in the biomass and production of small invertebrates in the southern North Sea are more likely to be attributable to increases in primary production or eutrophication than to the effects of trawling disturbance.

Reference:

S. Jennings, T. Dinmore, D. Duplisea, K. Warr and J. Lancaster, 2001. Trawling disturbance can modify benthic production process. Journal of Animal Ecology, 70: 459-475