The influence of natural variation in discharge on juvenile brown trout population dynamics in a nursery tributary of the Motueka River, New Zealand

Abstract

The effects of natural flow variation on juvenile brown trout population dynamics were investigated by biannual sampling over 5.5 years in the Rainy River, a tributary of the Motueka River. A large flood in late March (50-year return period) substantially reduced the density (by 66%) and biomass (by 73%) of 0+ trout over autumn-spring, but the cohort responded with compensatory survival to achieve similar density and biomass by spring as in other years. A lowflow event in February-April (return period >8.4 years), when 7-day low flows fell to 56% of the 7-day mean annual low flow (MALF) and were less than the MALF for 46 days, had no adverse affect on the population. We found no evidence for density-dependent growth. However, there was strong evidence for a two-phase self-thinning response in density, with no self-thinning occurring over summer (i.e. the 0+ population remained below carrying capacity) until a threshold mass of 22.08 g (length=123.7 mm) was attained in autumn after which severe selfthinning took place over autumn to spring. The results indicate that over spring-autumn the population is insensitive to flow reduction and that over autumn-spring the effects of high (and probably low) flow events on local abundance and biomass are offset by compensatory (densitydependent) survival. However, effects on the contribution of migrants to the downstream population remain unknown. The study identified ecological redundancy, which could be exploited for flow allocation. Significantly, it has shown that minimum flows equivalent to the MALF (often advocated by New Zealand conservation and fisheries management organisations) are not always necessary for sustaining juvenile trout populations.