Using brown trout otoliths to understand growth patterns in the Upper Clark Fork River

Age structure and growth rates of fish are key characteristics of assessing the status of fish populations. For example, growth rate estimates can be used to makes inferences regarding habitat quality. Unfortunately, aging fish can be difficult as age estimates from nonlethal approaches including fin rays and scales can be biased and/or difficult to accurately read. Otoliths are calcium carbonate formations found inside a fish’s head and are often used to age fish because they are easily read and unbiased. Otoliths deposit minerals continuously and develop growth rings much the same way trees do. We used otoliths to age fish and describe growth patterns in brown trout (Salmo trutta) along the Upper Clark Fork River (UCFR). Montana Fish, Wildlife and Parks (MFWP) have previously developed size at age estimates for brown trout on the UCFR using fin rays. We will compare size at age curves derived from otoliths to validate these previously derived estimates and describe the growth patterns for these fish. Brown trout were chosen because they are important to anglers. Brown trout were collected from seven different sites along the UCFR from Rock Creek to Warm Springs Creek. Otoliths were extracted from each individual fish, sanded flat to see their growth rings, then pictures and measurements were taken. Differences between otolith and fin ray estimates of size at age will be investigated by comparing differences in aging technique and controlling for key environmental conditions, such as temperature and flow for the years of growth. To describe growth patterns across the fish collection sites, we will compare variation in growth trajectories among individuals, sampling sites, and broad river sections (upper, mid, and lower) to explore the scale of variation and identify areas of high growth