Juvenile trout survival and movement during the summer low flow abstraction period in the Lindis River, Central Otago

Abstract

Survival and movement of juvenile brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) were investigated during low flow conditions in the Lindis River, Central Otago. High levels of abstraction from this river result in drought-like events on an annual basis. Over two years, during the summer low flow period >1000 juvenile trout were marked with passive integrated transponder (PIT) tags. Their movements were tracked using mobile PIT tag antenna. To detect emigration and losses to an irrigation channel stationary PIT tag antenna were also positioned on the main-stem river and on the channel. After PIT tagging the study area was subjected to extreme low flow conditions in both years. Mark-recapture model analysis indicated that only a small proportion of the sample population survived the low flow events; 0.34 (95 % CI: 0.21-0.50) in year one and 0.29 (95% CI: 0.18-0.39) in year two. Water temperatures in flowing water remained within salmonid tolerance levels. Low survival rates were primarily attributed to high levels of predation and the lowest survival probabilities were associated with reduced cover for juvenile salmonids. Fish that did die from heat stress in disconnected and drying pools were immediately removed by scavengers, indicating that any dead fish observed during prolonged low flow conditions are likely to represent a small proportion of the total mortality experienced. Potential fish movement from stressful habitat during the both years was constrained by drying reaches, and an irrigation intake structure. Some sample fish displayed considerable movement along the remaining fragmented river corridor, presumably in an attempt to find refuge habitat.