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
