Juvenile Trout Mortality In A New Zealand River Subject To Persistent And Extreme Abstraction

To assess the impacts of water abstraction on fish populations, we studied juvenile trout movement and mortality during summer low flows in the Lindis River, Central Otago, New Zealand. Water abstraction in this river results in a residual summer flow <= 25% of the MALF for 75 - 100 days each year. The movements of > 1000 juvenile brown and rainbow trout (age 0+ and 1+) were tracked over two summer low flow periods using passive integrated transponder (PIT) tags. PIT tagging occurred at the onset of extreme low flow conditions, which resulted in 20 - 35% of the study reach drying up during the study period each season. Our mark recapture analysis shows that annual low flow events, exacerbated by water abstraction, result in the death of > 60% of the trout population within six weeks. We also found evidence that increased predation pressure, due to reduced habitat cover, was the primary cause for the unusually high mortality rates. These results show that predictions of trout population responses to water abstraction based on physical habitat alone can be overly simplistic, by failing to account for ecosystem effects such as increased vulnerability to predation at low flows.