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 F0.1 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 F0.1 except where recruitment and maturity schedules do not coincide.