Exploitation frequently reduces the mean age of a fish population, particularly where a lack of property rights stimulates producers to ignore the user cost of harvest. The authors demonstrate that harvesting a young fish bears a significant efficiency cost when the multiple benefits accruing to its protection are recognised. This is magnified when the full complement of year classes within a fish population is considered. These findings identify the importance of protecting older year classes using rights-based management and age/size restrictions, although their successful application can be problematic. In addition, the importance of incorporating more detail in bioeconomic models of multiple-cohort fisheries is highlighted, as underestimating the magnitude of user costs associated with the cropping of younger fish will promote recommendations for inefficient harvest levels. These factors are demonstrated in an application of an optimal control model to the New Zealand longfin eel (Anguilla dieffenbachii) fishery.