Setting Maximum Mortality Limits for Harbour Porpoises in Dutch Waters to Achieve Conservation Objectives

Harbour porpoises in the North Sea are protected under national and EU legislation, and under the regional agreement ASCOBANS, within the UN Convention on Migratory Species. As yet, no methods exist to quantify the cumulative impacts of direct anthropogenic mortality (e.g. by-catch) and indirect effects of human activities (e.g. contaminants) on porpoise populations. However, procedures do exist for assessing whether total anthropogenic mortality may result in failure to meet conservation objectives. While the ultimate aim is to reduce the number of such deaths to zero, ASCOBANS has an interim objective to reduce anthropogenic mortality to a level that will allow recovery of populations. Several different criteria have been proposed as limits to anthropogenic mortality that may still allow conservation objectives to be met. These criteria include simple percentages of the best population abundance estimate and more complex procedures that account for uncertainty and other information about the population. In this paper, we report new estimates of abundance for porpoises in Dutch waters, and we apply several methods to calculate maximum anthropogenic mortality limits from these estimates. We considered whether these mortality limits would meet the objective of the ASCOBANS agreement and other international obligations, and how these limits may be applied at a national level rather than the biological population level. The current simple approach of a limit of 1.7% of the best abundance estimate may not satisfy conservation objectives. We recommend the use of management procedures for setting mortality limits that take into account available data including associated uncertainties and biases, and whose performance has been extensively tested through simulation.