Using Otolith Chemistry to Detect Trace Element Signatures in Juvenile Steelhead from Lake Michigan Tributaries

Lake Michigan tributaries sustain quality steelhead trout (Oncorhynchus mykiss) populations that attracts many anglers to Michigan and other Great Lakes States. Migratory salmonid populations are managed by the Michigan Department of Natural resources. The amount of naturally produced steelhead in Lake Michigan averages about 30% of the total population. The objective of our study was to discriminate steelhead populations from different streams based on otolith microchemistry. Age-1 steelhead of the 2012 cohort were collected from 14 Lake Michigan tributaries (10 Lower peninsula MI, 3 Upper Peninsula, MI, 1 Wisconsin). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used to determine trace element signatures (Mg, Mn , Cu, Zn, Sr, Ba, and Pb)  in the otoliths.  Distinct trace element signatures differentiated fish from most streams. A random forest algorithm was able to correctly classify 89% of all steelhead to the correct stream of origin after using a bootstrapping resampling procedure. Of the 14 sites, 6 classified with 100% accuracy Sites that had miss- classifications were often placing fish to streams that were within 50 miles of the correct stream of origin. Our findings demonstrate the ability to use naturally occurring trace element signatures to differentiate natal tributaries of Lake Michigan steelhead.