Publication Abstract

Indicators of the ecological impact of bottom-trawl disturbance on seabed communities

J.G. Hiddink, S. Jennings and M.J.Kaiser

Recent political commitments to an Ecosystem Approach to Fisheries (EAF) require that managers take account of the environmental impacts of fishing in fisheries management plans. Bottom trawling can have significant impacts on benthic communities and habitats, and indicators are required to report on the sustainability of these impacts and on progress in relation to management targets. We develop linked state and pressure indicators of bottom trawl impacts on benthic communities. The state indicator measures the proportion of an area where benthic invertebrate biomass (B) or production (P) is greater than 90% of pristine benthic biomass (B _0.9) or production (P_0.9). The pressure indicator measures the proportion of the area where trawling frequency is sufficiently high to prevent predicted B or P reaching predicted B_0.9 or P_0.9. A validated size-based model of the benthic community in the North Sea was used to estimate the time to 90% recovery of the unimpacted biomass, depending on the habitat. Recovery was fast on sand and gravel in the south-western North Sea (1-3 y), while recovery in the northern North Sea was slower (>7 y). Detailed satellite monitoring of fishing vessels (VMS) was used to estimate trawling intensity. Based on modelled recovery times and current trawling intensity, we calculated that in 2003 53.5% of the southern North Sea was trawled too frequently for biomass to reach B_0.9 and 27.1% of the North Sea was trawled too frequently for production to reach P_0.9. In 2003, in 56% of the southern North Sea benthic biomass and in 27% benthic production was below B_0.9 and P_0.9 due to trawling. Modelled recovery times were validated by comparing them with literature estimates of recovery time, and the model and literature estimates were comparable (2.5 to >6 years). The advantages of using the area affected by trawling as a pressure indicator are that it is conceptually easy to understand, responds quickly to changes in management action, can be implemented at a relevant scale for fisheries management and that the necessary effort distribution data are centrally collected by national fishery inspectorates. However, since policy commitments typically relate to state, the response of state to pressure would still need to be reported to demonstrate progress in relation to these commitments.

Reference:

J.G. Hiddink, S. Jennings and M.J. Kaiser (2006). Indicators of the ecological impact of bottom-trawl disturbance on seabed communities. Ecosystems, 9: 1190-1199