Otago Regional Council scientists need to reassess its science around minimum flows for the Low Burn if threatened longfin eels are to survive in the catchment, an environmental scientist says.
Longfin eels were classified as in gradual decline and needed a good-quality food source in summer if they were to migrate, Dr Terry Broad, who is the council's natural resources committee Ngai Tahu representative, said.
"Just setting a minimum standard for their habitat is very unlikely to provide eels the protein required to complete their life cycle."
A council report advising on the flows needed for aquatic systems in the Low Burn, a 50.2sq km catchment running from the Pisa Range to Lake Dunstan, suggested habitat requirements for native fish such as the longfin eel were well below those for trout, with connection to Lake Dunstan the most important requirement for the eels and koaro (the juveniles of which are one of the five species of whitebait).
"It is likely that a flow that provides connectivity to Lake Dunstan will also provide sufficient habitat for native fish species in the Low Burn," the report, which was to inform future minimum flow consultation, said.
Dr Broad, who studied longfin eels for his PhD, questioned the comments, which he believed inferred the eels would be fine in very low flows or pools.
"I felt the assumption eels need low flows is quite wrong. The flows need to be higher."
His research had shown the eels, like trout, preferred free-running water with a good source of food, such as invertebrates.
While they could survive in lower flows during the day, when they rested, they fed at night, requiring greater flows.
It was similar in winter as eels "hibernated" when the water temperature got below 10degC-15degC so could cope with low flows.
"In summer, they feed well and need a highly productive habitat to put on weight to grow and gain surplus weight to migrate."
If they could not put on the additional weight, they did not migrate to breed.
Declining habitat and overfishing were two of the main problems scientists believed could be leading the species to extinction, Dr Broad said.
The council needed to assess low flows on the basis of what provided for the sustainability of the eel which would also provide for the spawning and juvenile rearing of brown and rainbow trout, and anything left over could be used for irrigation.
While the catchment was considered to be "significantly overallocated", it was likely a large portion was not taken, the report said.
However, the present level of allocation in the Low Burn catchment was having some impact on flow variability.
Cr Duncan Butcher recommended the report go back to council scientists for them to address Dr Broad's concerns, to which the committee agreed.
Dr Broad has passed on his research to the scientists for consideration.
Longfin eel facts
- Loss of wetlands and fishing practice reduced numbers
- Commercial fishery now under quota management system
- After 25 to 80 years, they migrate to the Pacific to breed and die
- Mainly nocturnal and prefer habitats with plenty of cover
- Can grow up to 2m long and weigh up to 10kgHunt by sense of smell
- One of biggest freshwater eels in world
