Measuring the effects of an overwinter dewatering on egg-to-fry survival of brown trout in a Montana stream

In Montana and other parts of the arid west, water diversion can create environmental challenges for downstream fish populations. Diverting water out of a stream in the autumn and winter has particular implications for autumn-spawing fish whose eggs are in the gravel all winter, such as brown trout and bull trout. This study was designed to examine how the year-round diversion of water by a local mining industry affects redd building and egg-tofry survival of wild brown trout in a Montana creek. To measure redd-building activity, I had access to fifteen years of annual redd-count data at this site. To measure egg-to-fry survival, I counted fertilized brown trout eggs into meshlined baskets and placed these baskets into artificial redds. I removed these baskets after hatch in April and tallied the number of live fry, dead fry, live eggs and dead eggs. While the eggs were developing in the gravel, I measured parameters critical to egg development and survival that could be affected by dewatering, including: intergravel dissolved oxygen (IGDO), intergravel temperature, stream temperature, rate of flow through the gravel, and depth and water velocity at each artificial redd. There was no autumn or winter dewatering during the course of this study, so I cannot comment on the effects of a dewatering. The survival rate for the first two years was similar, 38% and 35%. Survival was considerably lower the third year, measuring only 8.5%. This may be due to a longer, colder winter the third year. The data collected and methods developed in this study will be useful for a future study in the event of an autumn or winter dewatering