Lake Manapouri - Biodiversity

Biodiversity

The area is well renowned for its fishing and high water quality. Both Lake Manapouri and Lake Te Anau lie within the ultra-oligotrophic index on the trophic state index with clear highly oxygenated waters of very low biological productivity. Both lakes along with the connecting Waiau River contain the New Zealand longfin eel and introduced brown trout and rainbow trout as well as some Atlantic salmon. The area is however free from macrophyte Lagarosiphon major an exotic and invasive species which has infiltrated many other New Zealand lakes.

Lake Manapouri provides 73% of New Zealand’s longfin eel lake habitat protected from commercial fishing. However the lake has experienced a decline in numbers of longfin eels due to the construction of the hydro- electric dam blocking eel migration. The outflow passage is blocked by the hydro-electric dam which means that eels can be killed when passing through the turbines. A vertical slot fish pass was installed in 1999 and trap and transfer of elvers (young eels) was started in summer 1998/1999. Since the trap and transfer operation commenced in 1998/99 more than 407,000 elvers have been transferred upstream of the Maraoa Control structure. Catch totals in the trap and transfer system can vary in correspondence to surface water temperatures falling below 15 °C (288 K). The trap and transfer technique was successful in improving fish distribution however operations had to be stopped in 2004 due to the concerns that transferring elvers to upstream habitats would spread the exotic invasive algae Didymosphenia geminata. The operation now transfers elvers just past the dam due to these concerns. There is concerns also that the trap and transfer operation does not trap enough silvers eels (eels of breading age) with an average of 200-400 annually. This however equates to only one silver eel per hectare within the lake which is much lower than standards recommended overseas. More research is needed to understand if trap and transfer is the best option in Lake Manapouri.

Submerged vegetation within Lake Manapouri is mostly native species. Lake Manapouri has a high diversity of submerged vegetation. The rare Hydatella inconspicua an endemic shallow water plant was found in many of the Fiordland lakes including Lake Manapouri in 1998, this was the first reporting of it being in the South Island. The finding meant that the plant was no longer considered endangered.