Potential Impact of Burbot on the Fisheries of Flaming Gorge Reservoir

We examined the potential effects of recently introduced burbot (Lota lota) on the food web and recreational fisheries of Flaming Gorge Reservoir. We assessed the relative abundance, age structure, and feeding habits of burbot in the reservoir.  Abundance varied spatially with greatest catch rates of burbot in the Open Hills area of the reservoir and lowest catch rates in the Canyon. Catch rates were highest in spring and fall and lowest during summer months when surface temperatures exceeded 17C.  Stomach content analyses indicated that crayfish dominated the diets of burbot during all seasons and at all locations.  Burbot smaller than 500 mm total length feed almost exclusively on invertebrate prey.  Approximately 30% of diets of burbot larger than 520 mm TL were comprised of forage fish.  Juvenile Utah chub were the most commonly consumed forage fish.  These chubs were consumed mostly during fall.  Analyses of stable isotopes of Nitrogen and Carbon indicated that burbot likely compete with smallmouth bass in the reservoir.  Growth rates of burbot were high for individuals with bioenergetics simulations suggesting that burbot are feeding at near maximal rates given their size and the water temperatures they occupy.  Comparison of growth rates between 2008 and 2009 suggested that growth decreased during that time.  Whether this decrease was related to year-to-year differences in climate, or to density dependent factors was not clear.  Bioenergetics analyses indicated that a population of 50,000 individuals would likely consumer 20 metric tonnes per year of forage fish and over 100 metric tonnes of total food.  This value suggests that the current burbot population will likely impact forage fish populations in the reservoir.  A critical question of how burbot diets would change if crayfish numbers declined remains unanswered.