Increasing the harvest for mussels Mytilus edulis (L.) without harming oystercatchers Haematopus ostralegus (L.).

Abstract

Oystercatchers Haematopus ostralegus (L.) arriving on their wintering grounds at the end of summer require there to be 6-8 times more mussel Mytilus edulis (L.) biomass available on their feeding grounds than they will consume over the winter if the birds’ normal high survival rate until spring is to be maintained: that is, their ‘Ecological Requirement (ER)’ is considerably larger than their ‘Physiological Requirement (PR)’. The ratio ER/PR was termed the ‘Ecological Multiplier (EM) and has been applied to a number of shellfisheries to calculate the Total Allowable Catch (TAC). The high value of the EM, however, has meant that mussel fisheries have suffered from much reduced harvests and thus economic difficulties. This paper proposes two methods by which the TAC could be increased without any predicted impact on the birds. In the ‘roll-over’ approach, the surplus biomass remaining at the end of one month is harvested during the next. In the ‘delayed start’ approach, the EM is not set at the beginning of autumn but at the beginning of the winter, which is when birds begin to starve. The two approaches can be applied together and would enable many more mussels to be harvested than is currently allowed without reducing oystercatcher survival. In the test case presented here, the TAC over the winter could be increased from 5% to between 35% and 45% of the standing crop of mussels present in September when the birds arrive.