Population Assessment and Habitat Use of Brown Trout Following Severe Overwinter Predation from Common Mergansers in a Western New York Stream

Abstract

Heavy predation by Mergus merganser (Common Merganser) during severe winters of 2013–2014 and 2014–2015 resulted in substantial reductions of wild Salmo trutta (Brown Trout) in open-water, groundwater-fed reaches of Oatka Creek in western New York State. New York State Department of Environmental Conservation considered the need for habitat manipulation to reduce the severity of future overwinter-predation by mergansers in Oatka Creek Park (OCP) but lacked data to make an informed decision. Thus, this study sought to estimate population abundance, density, and year-class distribution of Brown Trout; quantify the availability of cover and habitat; and identify habitat features used by Brown Trout in OCP. We recorded data for 100 Brown Trout (101–512 mm TL) during spring 2016, autumn 2016, winter 2017, and spring 2017. Despite the absence of mergansers in ensuing warmer winters, trout population indices decreased as the study continued, likely affected by high streamflow during sampling events. Year-class distributions typical of a small stream, however, suggest that the population is recovering. Velocity refuges and structural cover were the primary factors determining habitat selection. Large woody debris was the most favored cover type by all Brown Trout; however, boulders were also important, especially during low streamflow. Large trout (>300 mm) showed a strong preference for deepwater habitats with slow currents and high densities of woody debris and boulders, while small trout (<200 mm) preferred shallower complex habitats with slow currents, course substrates, and high cover densities. Quality trout habitat and instream cover is abundant throughout OCP, but the abundance of highly complex overwinter habitats capable of providing protection from mergansers may be limited. Adding complex structural cover to areas favored by smaller trout (<300 mm) could increase habitat complexity and likely reduce the severity of future overwinter predation.