Water temperature and food availability influence brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) population dynamics in the Cardrona River: implications for flow regime management

Abstract

Reduced surface discharge negatively influences freshwater fish populations. Under the current flow regime management, the lower Cardrona River has experienced annual low flow events over the summer months. This low flow period has potential impacts on the wellbeing of the brown and rainbow trout populations in the river, as out-migrating juveniles may become stranded and subsequently experience high mortality rate during summer low flow. To estimate the impact of summer low flow on juvenile trout out-migration and to investigate how food availability and water temperature drive out-migration, brown and rainbow trout were sampled at three sites in the upper, middle and lower reaches of the Cardrona River, Central Otago, New Zealand in November 2016, January 2017 and April 2017; before, during and after a low flow event. The density of juvenile trout was estimated at three sampling locations along the Cardrona River via single pass electric fishing. The results suggest juvenile trout were restricted at the lower reach site due to a surface disconnection in January. Furthermore, the discrepancies between observed total biomass and biomass predicted by a temperature-based bioenergetics model based on empirical data indicate energetic constraints may drive juvenile downstream movement from the upper site to the lower reach site from January. Trout growth rates in response to water temperatures were estimated using a bioenergetics model. The model outcomes suggest the water temperature at the upper reach site was the least suitable for trout growth while the middle site was the most suitable. The water temperature at the lower site increased dramatically over the summer months during flow reduction, which rendered the site unsuitable for trout growth and indicated likely mortality. Food availability in the form of invertebrate drift and trout daily consumption were estimated and compared. The results indicate ample food supply at the middle reach site and limited food availability at the upper site relative to trout energy consumption. The lower site experienced a sharp increase in energy production in January followed by a dramatic decrease in April. This study identifies water temperature and food availability as two potential drivers that initiate trout out-migration. However, additional sampling efforts in late autumn to early winter at all sampling sites are necessary to advance the understanding of trout population dynamics in the Cardrona River. In terms of water management, this study emphasizes the importance of continuous flow to trout movement over summer. A revised management plan is needed to improve fish passage at the lower Cardrona