Groundfish Exploitation Rates Based on Life History Parameters

The problem considered is how to choose a fixed exploitation rate that will provide a high yield at low risk, when the investigator has no knowledge of the yield curve or the spawner–recruit relationship of the stock. Commonly employed expedients are F 0.1 and F = M, but these have little empirical or theoretical support. Calculations made with a range of life history parameter values typical of demersal fish and a range of realistic spawner-recruit relationships show that yield will be at least 75% of maximum sustainable yield so long as the spawning biomass is maintained in the range of about 20–60% of the unfished level, regardless of the form of the spawner–recruit relationship. A relative spawning biomass in this range can be achieved by choosing a fishing mortality rate that will reduce the spawning biomass per recruit to about 35% of the unfished level. This is the level of fishing mortality that maximizes the minimum yield among all of the spawner–recruit relationships considered ("maximin yield" rate, F mmy ). The actual value so calculated depends on the relative timing of growth, maturity, and recruitment to the fishery. In practice, F mmy appears to be very close to F 0.1 except where recruitment and maturity schedules do not coincide.