The single cohort model shows that we delay the harvest of a cohort beyond the age at which the cohort's growth rate equals the discount rate when harvesting costs are proportional to the harvest rate (Clark 1990). With more than one cohort, one needs to determine if cohorts of various age classes should be harvested at the same time and rate or selectively at different times and harvest rates. This paper establishes that knife-edge selectivity can be optimal in some cases, but is dominated by selective harvesting when either harvesting is costless, gear congestion costs are relatively large, or it is feasible and desirable to harvest older cohorts at a larger harvest rate than younger cohorts. It is furthermore shown that in most cases it is optimal to harvest fish at an earlier age than the single cohort model predicts, but it might even be optimal to harvest fish that grow faster than the discount rate. In some cases, it may be optimal to utilize two gears to harvest two cohorts, even if gears have different operating costs.

Additional Metadata
Keywords Age selection, Dynamic optimization, Fishery economics, Renewable resources, Resource management
Journal Marine Resource Economics
Schott, S. (2001). The optimal selection and harvest of multiple cohorts. Marine Resource Economics, 16(2), 109–126.