Seasonal habitat use and survival of brown trout (Salmo trutta) in Oatka Creek, Monroe County, New York

Abstract

Heavy predation by common mergansers during the severe winters of 2013-2014 and 2014-2015 resulted in dramatic brown trout reductions throughout the spring-fed reaches of Oatka Creek in western New York. Management agencies are considering habitat manipulation to reduce the severity of overwinter merganser predation on the wild brown trout population in Oatka Creek Park (OCP; Monroe County) but currently lack data to make an informed decision. My study sought to 1) quantify the availability of trout cover and habitat in OCP, 2) estimate the population abundance, density, seasonal survival rate, and year-class distribution of brown trout in OCP, and 3) identify habitat features used by brown trout and evaluate the seasonal importance of each feature. Data were recorded for 100 brown trout (101-512 mm total length; TL) during spring 2016, autumn 2016, winter 2017, and spring 2017. Trout density in OCP was estimated at 10.6-11.4 trout per km2. Despite the absence of mergansers, brown trout population metrics decreased as the study continued; however, variable sampling conditions, especially discharge, were likely responsible. Relatively normal year-class distributions suggest that the population is recovering. The relative abundance of large trout (400+ mm TL) was greater than expected, which may be a result of low trout densities (i.e., reduced competition and increased resource availability may have enhanced growth and survival rates). Velocity refuges and structural cover were the primary factors determining habitat use throughout the study. Large woody debris was the most favored cover type; however, boulders were also important, especially during low streamflow, as they provide cover in deeper midstream channels. Large trout (300+ mm TL) showed a strong preference for slow, deep pools with high densities of woody debris and large boulders, while age-0 trout (TL < 125 mm) preferred slow, shallow-water habitats with course substrates (i.e., cobble and boulders) and high densities of complex cover (i.e., boulders, LWD, and turbulence). Quality trout habitat and instream cover is abundant throughout OCP, but the availability of complex overwinter habitats capable of providing protection from piscivorous birds may be limited. Adding structural cover to areas favored by small trout (TL < 200 mm) would increase habitat complexity and likely reduce the severity of overwinter predation.