Use of otolith microchemistry to identify Yellowstone cutthroat trout and lake trout natal origins and movement patterns in Yellowstone Lake, Wyoming

Abstract

The Yellowstone Lake Yellowstone cutthroat trout population has declined as a result of drought, whirling disease, and the introduction of lake trout. Little is known about the recruitment patterns of cutthroat trout and lake trout in this system. Otolith microchemistry is uniquely suited for answering these questions by matching the chemical signatures found in otoliths to the same signatures found in the water fish occupy. My first objective was to identify and compare the primary spawning streams contributing to historic (1997) and recent (2013) cutthroat trout recruitment. I analyzed the chemical signatures (87 Sr:86 Sr, Sr:Ca, Ba:Ca, Mg:Ca, and Mn:Ca) of 22 cutthroat trout spawning streams and the same signatures from the natal region of cutthroat trout otoliths. There was low variation among the chemical signatures of many spawning streams, thus streams were grouped into 9 clusters using a cluster analysis. Relative recruitment to each cluster was assessed using random forest models with a classification accuracy of 84.4% for known-origin cutthroat trout fry otoliths and 79.0% for simulated otolith signatures. There was a significant difference in the proportions of recruitment between historic and recent cutthroat trout spawning clusters (X 2 = 15.40, p = 0.03). The majority of historic (0.84) and recent (0.77) recruitment occurred in the same three stream clusters, with the most notable change being a decrease in recent recruitment in the stream cluster containing Pelican Creek and an increase in recruitment in tributaries in the upper Yellowstone River drainage. The second objective was to identify the spawning locations and movement patterns of lake trout within Yellowstone Lake. I analyzed the 87 Sr:86 Sr, and Sr:Ca signatures from 8 locations throughout Yellowstone Lake and the same signatures in 20 lake trout otoliths. I did not find sufficient variation within the lake water chemistry to differentiate lake regions and there was no significant differences found within in the lake trout otolith transects. This study can be used to inform future spawning stream conservation and restoration by directing managers towards spawning streams of increasing or decreasing importance. This study also highlights some of the strengths and limitations of using microchemistry studies in freshwater system.