Effects of extreme floods on trout populations and fish communities in a Catskill Mountain river

Summary

1. Extreme hydrologic events are becoming more common with changing climate. Although the impacts of winter and spring floods on lotic ecosystems have been well studied, the effects of summer floods are less well known.

2. The Upper Esopus Creek Basin in the Catskill Mountains, NY, experienced severe flooding from Tropical Storm Irene on 28 August 2011, and peak discharges exceeded the 0.01 annual exceedance probability (>100 year flood) in some reaches. Three years of fish community data from pre-flood surveys at nine sites were compared to data from 2 years of post-flood surveys to evaluate changes in fish communities and populations of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss).

3. Basinwide, fish assemblages were not strongly impacted and appeared highly resilient to the effects of the flood. Total density and biomass of fish communities were greater at most sites 10-11 months after the flood than 1 month prior to the flood while richness and diversity were generally unchanged. Community composition did not differ significantly between years or between the pre-and post-flood periods.

4. Although the density of mature brown trout was low at most sites (mean density = 146 fish ha-1), young-of-the-year brown trout reached their highest density (mean = 2312 fish ha-1) during 2012. In contrast, rainbow trout densities declined substantially during the 5-year study and the 2012 year class was small (mean density = 222 fish ha-1).

5. Late summer floods may be less damaging to stream fish communities than winter or spring floods as spawning activity is negligible and early life stages of many species are generally larger and less susceptible to displacement and mortality. Additionally, post-flood conditions may be advantageous for brown trout recruitment.