Longfin eels need reserves: modelling the effects of commercial harvest on stocks of New Zealand eels

Freshwater eels in New Zealand are heavily exploited by the commercial fishery and, to a lesser extent, by customary fisheries. A model was developed to investigate the effect of the New Zealand commercial eel fishery on escapement of migrating longfin (Anguilla dieffenbachii) and shortfin (A. australis) eels. Exploitation rates of 5% and 10% per year reduced the spawning per recruit of A. dieffenbachii females by 83% and 96.5%, respectively, below unexploited levels. The model suggests that New Zealand longfin eels may be severely recruitment overfished and only absence of fishing in some productive areas is likely to be effective in maintaining at least 50% of spawning per recruit. Being younger at spawning, eels are less susceptible to overfishing. Using the current minimum legal weight (220 g), the spawning per recruit of A. australis females was reduced by only 48% at an exploitation rate of 10% per year.