Assessing and predicting the relative ecological impacts of disturbance on habitats with different sensitivities

1. Methods for assessing habitat sensitivity to human impacts are needed to gauge the sustainability of existing impacts, develop spatial management plans and support meaningful environmental impact assessments. These methods should be quantitative, validated, repeatable and applicable at the scales of impact and management. 2. Existing methods for assessing the sensitivity of marine habitats to human impacts have tended to rely on expert judgement and/or scoring systems. They are neither validated, quantitative nor repeatable. 3. We have developed a method that meets the criteria for assessing the sensitivity of seabed habitats to physical disturbance, and delineating and mapping habitat sensitivity at large spatial scales (>105 km2). The method assumes that sensitivity is related to the recovery time of production or biomass, as predicted using a size-based model that takes account of the effects of natural disturbance. 4. As trawling disturbance is a major and widespread direct human impact on shelf seas, this was used as an example of anthropogenic physical disturbance. We mapped habitat sensitivity to trawling in 9-km2 boxes across an area of 125 000 km2 in the North Sea. 5. Habitat sensitivities varied widely, and a trawling frequency of 5 year-1 in the least-sensitive habitat had the same ecological effect as a trawling frequency of 0.3 year-1 in the most-sensitive habitat (based on production). When trawling effort was held constant but redirected to the least-sensitive habitats, the existing impacts on production and biomass were reduced by 36% and 25%, respectively. 6. Synthesis and applications. The method described in this paper enables managers to predict the implications of changing patterns of human impact on seabed habitats when establishing spatial management plans. In the context of fisheries management, this will support the identification and selection of fishing grounds that minimize the adverse ecological effects of fishing; the selection of closed areas (both representative and highly sensitive); the comparison of management options that might reduce the overall environmental impacts of fishing; and any future steps towards the application of environmental impact assessment in advance of fishery development

Reference points and reference directions for size-based indicators of community structure

Size-based community and ecosystem metrics, such as mean body mass and the slopes of size spectra, have been proposed as indicators to support the Ecosystem Approach to Fisheries Management (EAFM). These metrics show relatively consistent responses to size-selective exploitation, and " unexploited" indicator reference points may be predicted with models of size-structured foodwebs. Whereas unexploited reference points provide a baseline for assessing the relative magnitude of fishing impacts, target or limit reference points are needed to guide management. Values for target or limit reference points are difficult to justify on scientific grounds. However, given that fishing impacts in most ecosystems need to be reduced to meet the objectives of the EAFM, we argue that reference directions provide alternative medium-term management targets. We show that the power of surveys to detect trends consistent with reference directions depends on the range of body size classes included in the analysis. Selection of different size ranges will weight metrics to respond to the release of small fish from predation, the depletion of larger individuals as a consequence of exploitation, or both. Such weightings may not be consistent over time, because the differential vulnerability of larger species, within-population changes, predator-prey relationships, and the effects of competition depend on contemporary rates of fishing mortality and the history of exploitation. The power of the surveys investigated is poor on time scales of less than 5-10 years. Therefore, size-based indicators provide better support for medium-term, rather than year-on-year, management decision making

The role of marine protected areas in environmental management

Marine protected areas (MPAs) are one of several tools used to meet management objectives for the marine environment. These objectives reflect political and societal views, and increasingly reconcile fishery and conservation concerns, a consequence of common high-level drivers, such as the World Summit on Sustainable Development. The contribution of MPAs to meeting objectives should be assessed in conjunction with other tools, taking account of the management systems of which they are part. Many of the same factors determine the success of MPAs and other management tools, such as quality of governance and the social and economic situation of people using marine goods and services. Diverse legislation governs MPA designation. Designation could be simplified by prearranged and prenegotiated agreements among all relevant authorities. Agreements could specify how to make trade-offs among objectives, interpret scientific advice, ensure effective engagement among authorities and stakeholders, deal with appeals, and support progressive improvement. The jurisdiction and competence of fishery management authorities mean that they are well placed to contribute to the design, designation, and enforcement of MPAs. Their strengths include well-established procedures for accessing scientific advice, the capacity to work across multiple jurisdictions, experience with MPA management, and access to vessels and personnel for enforcement