Contribution of drifting fry to trout diets in the Big Manistee River, Michigan

Although juvenile brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) feed primarily on aquatic invertebrates, non-invertebrate prey may be important.  The Big Manistee River, Michigan supports spawning runs of game and non-game species, including lake sturgeon (Acipenser fulvescens) as well as large trout populations.  Our objectives for this study were 1) to determine whether trout prey upon drifting larval sturgeon, 2) determine selectivity in trout diets, and 3) compare diets between both trout species.  Drift and fish samples were collected on 22 May, 12 June, and 14 June 2007 to evaluate trout diet during the period that larval sturgeon drift.  Larval sturgeon were removed from each sample and ultimately released while other organisms were kept for analysis.  Boomshocking was conducted immediately after drift sampling at 0100 h.  Preliminary diet analysis indicates that no larval sturgeon were included in trout diets during the sample period.  However, numbers of drifting larval sturgeon were lower than in previous years.   In contrast other larval fish accounted for 19.7% and 20.1% of the drift on June 12^th and 14^th, respectively, and 25.3% and 75.8% of brown and 85.3% and 74.7% of rainbow trout diets, respectively.