Trawling disturbance can modify benthic production processes

1. Trawling disturbance has wide-ranging impacts on the marine environment and is well known to modify benthic habitat and community structure. This h as led to speculation about the positive and negative impacts of trawling o n ecosystem processes such as production. 2. Existing theory suggests that frequent trawling disturbance may lead to the proliferation of smaller benthic species, with faster life histories, b ecause they can withstand the mortality imposed by trawling and benefit fro m reduced competition or predation as populations of larger species are dep leted. Since smaller species are more productive, trawling disturbance may 'farm the sea', with knock-on benefits for consumers, including fish popula tions. 3. We conducted the first large-scale studies of trawling effects on benthi c production across quantified gradients of trawling disturbance on real fi shing grounds in two regions (Silver Pit and Hills) of the North Sea. There were 27- and 10-fold differences in levels of beam trawl disturbance among the Silver Pit and Hills sites, respectively. 4. Size structure was described using normalized size-spectre, and the slop es and intercepts of these spectra were related to levels of trawling distu rbance. Production was estimated from the size spectra, using a new allomet ric relationship between body mass and the production to biomass (P:B) rati o of marine invertebrates. The general validity of the relationship was con firmed using a phylogenetic comparative approach. 5. In the Silver Pit region, trawling led to significant decreases in infau nal biomass and production. The abundance of larger individuals was deplete d more than smaller ones, as reflected by the positive relationship between the slope of the normalized size spectra and trawling disturbance. The eff ects of trawling disturbance were not significant in the epifaunal communit y. In the Hills region, where the range of trawling disturbance was lower, trawling disturbance did not have significant effects on biomass or product ion. 6. In the Silver Pit, relative infaunal production (production per unit bio mass) rose with increased trawling disturbance. This was attributable large ly to the dominance of smaller animals in the disturbed communities. The in crease in relative production did not compensate for the loss of total prod uction that resulted from the depletion of large individuals. There was som e evidence for the proliferation of small polychaetes at moderate levels of disturbance, but at higher levels of disturbance their biomass and product ion fell. 7. We conclude that reported increases in the biomass and production of sma ll infaunal invertebrates in the North Sea are attributable largely to rece nt increases in primary production that were driven by climate change, and not to the effects of trawling disturbance.