There is a significant body of evidence that global stocks of freshwater eel (family Anguillidae) are in decline. Of the 22 known species or subspecies of freshwater eel, eight have been listed as threatened or near threatened on the IUCN Redlist. It is likely that the often arduous catadromous migration requirement of anguillid eels contributes to the vulnerability of this family to anthropogenic disturbances.
Short-finned eels (Anguilla australis) are an important component of Tasmania’s freshwater fish fauna. While they are adept at negotiating a wide range of natural barriers, large dams and weirs impact upon both upstream and downstream migration. Trevallyn dam is located in the downstream reaches of Tasmania’s largest water catchment, and blocks the migratory path of eels between the Tamar estuary and the South Esk River. The dam spills infrequently and downstream migrating eels have little option but to pass through Trevallyn power station to reach the estuary.
Hydro Tasmania deployed an adaptive resolution imaging sonar (ARIS) on the intake of Trevallyn power station and implanted migrating eels with Juvenile Salmon Acoustic Telemetry System (JSATS) tags to obtain information on migrating eel behaviour around the dam, power station intake, and tailrace. The study showed a high level of eel activity around the intake during the migration season, the majority of this activity was nocturnal, migration peaks were related to catchment hydrology, and the majority of tagged eels eventually passed through the power station.
The study showed that there is significant potential to facilitate downstream migration at Trevallyn, as eels showed aversion to entering the power station intake, and did not necessarily enter it upon their initial encounter. Hydro Tasmania is currently designing a downstream bypass system to safely pass eels downstream by capitalising on the results of the migration study.