Modification of marine habitats by trawling activities: prognosis and solutions Michel J Kaiser 1 , Jeremy S Collie 2 , Stephen J Hall 3 , Simon Jennings 4 & Ian R Poiner 5 1 School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Anglesey, LL595EY, UK; 2 Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA; 3 Australian Institute for Marine Science, PMB 3,Townsville MC, Queensland 4810, Australia; 4 The Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Lowestoft, NR330HT, UK; 5 CSIRO Division of Marine Research, PO Box120, Cleveland, Queensland 4163, Australia F I S H and F I S H E R I E S, 2002, 3 , 114^136 114 # 2002 Blackwell Science Ltd Abstract Fishinga¡ects the seabed habitat worldwide onthe continental shelf.These impacts are patchilydistributedaccording tothe spatialand temporalvariation in ¢shinge¡ortthat results from ¢shers’behaviour. As a consequence, the frequencyand intensityof ¢shing disturbance varies among di¡erent habitat types. Di¡erent ¢shing methodologies vary in the degree to which they a¡ect the seabed. Structurally complex habitats (e.g. sea- grass meadows, biogenic reefs) and those that are relatively undisturbed by natural per- turbations (e.g. deep-water mud substrata) are more adversely a¡ected by ¢shing than unconsolidated sediment habitats that occur in shallow coastal waters. These habitats also have the longest recovery trajectories in terms of the recolonization of the habitat by the associated fauna. Comparative studies of areas of the seabed that have experi- enced di¡erent levels of ¢shing activity demonstrate that chronic ¢shing disturbance leads to the removal of high-biomass species that are composed mostly of emergent seabed organisms. Contrary to the belief of ¢shers that ¢shing enhances seabed produc- tionand generates food for target ¢sh species, productivity is actuallyloweredas¢shing intensity increases and high-biomass species are removed from the benthic habitat. These organisms also increase the topographic complexity of the seabed which has been shown to provide shelter for juvenile ¢shes, reducing their vulnerability to preda- tion. Conversely, scavengers and small-bodied organisms, such as polychaete worms, dominate heavily ¢shed areas. Major changes in habitat can lead to changes in the com- position of the resident ¢sh fauna. Fishing has indirect e¡ects on habitat through the removal of predators that control bio-engineering organisms such as algal-grazing urchins. Fishing gear resuspend the upper layers of sedimentary seabed habitats and hence remobilize contaminants and ¢ne particulate matter into the water column. The ecological signi¢cance of these ¢shing e¡ects has not yet been determined but could have implications for eutrophication and biogeochemical cycling. Simulation results suggest that the e¡ects of low levels of trawling disturbance will be similar to those of natural bioturbators. In contrast, high levels of trawling disturbance cause sediment systems to become unstable due to large carbon £uxes between oxic and anoxic carbon compartments. In low energy habitats, intensive trawling disturbance may destabilize benthic system chemical £uxes, which has the potential to propagate more widely through the marine ecosystem. Management regimes that aim to incorpo- rate both ¢sheries and habitat conservation objectives can be achieved through the appropriate use of a number of approaches, including total and partial exclusion of towed bottom ¢shing gears, and seasonal and rotational closure techniques. However, the inappropriate use of closed areas may displace ¢shing activities into habitats that Correspondence: Michel J Kaiser, School of Ocean Sciences, University of Wales-Bangor, Menai Bridge, Anglesey, LL595EY, UK Tel.: þ44 01248 383751 Fax: þ44 01248716367 Received11Jan2002 Accepted23Apr2002