Characteristics of a Stocked Eel Population in a New Zealand Hydro Reservoir

Juvenile eels have been transferred annually into the seven hydro-electric reservoirs upstream of Karapiro Dam on the Waikato River, New Zealand, since the summer of 1993-94. The total numbers of elvers transferred as well as the commercial harvest of eels (>220 g) from each of the developed hydro-reservoir fisheries has been monitored. Records appear to indicate that the transferred juveniles have a limited survival rate. To elucidate the issue, an intensive survey and tagging study of the sub-adult eel population in one of the hydro-reservoirs, Lake Arapuni has been ongoing since 2011. The results to date show that only 5% of the transferred elvers develop to a harvestable size. Growth rates are extremely variable and have declined significantly over time. Males now dominate the populations in the reservoir  and may be out-migrating before reaching a harvestable size. This is in contrast to the surrounding tributaries where large females dominate the population.