Can bottom trawling indirectly diminish carrying capacity in a marine ecosystem?
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- Shephard, S., Brophy, D. & Reid, D.G. Mar Biol (2010) 157: 2375. doi:10.1007/s00227-010-1502-9
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Bottom trawling is associated with reduced biomass and production in the marine benthic community. Abundance of hard-bodied organisms such as bivalves, crustaceans and echinoderms typically declines in favour of soft-bodied opportunists such as polychaetes. Trawling effects vary with habitat; impact and recovery time are typically greater for more complex substrates/communities and those with lower rates of natural disturbance. Benthic organisms represent the prey base of a large component of the demersal fish assemblage. Hence, trawling-induced change in benthic community structure and function may exert an indirect effect on feeding success and growth of important commercially exploited fishes such as plaice Pleuronectes platessa. We present habitat-specific mixed effects models of plaice length as a function of age, bottom-trawling effort, population density and near-bottom temperature, with sampling year and area, and fish sex incorporated as random effects. Across an observed gradient of trawling effort in the Celtic Sea, plaice on gravel habitat showed significant declines in length at age while plaice on sand habitat showed significant increases in length at age. Contrasting trawling effects likely reflect dietary differences between habitats. Plaice on sand substrates are known to consume predominately polychaetes, which may proliferate at moderate trawling intensity in this habitat. Conversely, plaice on gravel substrates are reported to consume more of the fragile organisms such as echinoderms and bivalves that show marked declines with bottom trawling. An indirect effect of trawling on prey availability and growth of demersal fish has substantial implications for fisheries sustainability via reduced ecosystem carrying capacity and production of commercial fish.