Seasonal Variations in Relative Weight of Lake Trout (Salvelinus namaycush), Kokanee Salmon (Oncorhynchus nerka), Rainbow Trout (Onocorhynchus mykiss), and Brown Trout (Salmo trutta) in Blue Mesa Reservoir, Colorado

Blue Mesa Reservoir is the largest body of water in Colorado and is located on the western slope of the Rocky Mountains at an elevation of 7520 feet. Blue Mesa Reservoir contains recreationally important populations of l ake trout (Salvelinus namaycush), kokanee salmon (Oncorhynchus nerka), rainbow trout (Onocorhynchus mykiss), and brown trout (Salmo trutta). A management challenge in recent years has been the overpopulation of lake trout, which has led to a steep decline in abundance of kokanee salmon (a preferred food source) followed by a steep decline in body condition of the lake trout. Relative weight, Wr, is commonly used to assess body condition and prey availability of fish in a variety of ecosystems, but its seasonal variability has not been well documented in cold water species. Relative weight is 100 times the actual weight of a fish divided by expected weight (or standard weight) of that species at the measured total length. The present study uses multi-year, multi-season length and weight data provided by the Colorado Department of Parks and Wildlife to quantify seasonal variations in mean relative weight in these four species. It is found that lake trout consistently lose significant body condition between the spring and the fall of a given year, with a mean loss in relative weight of 11.7 +/2.4. In contrast, kokanee salmon tend to gain significant body condition between spring and fall with a mean gain in relative weight of 5.2 +/2.9. Rainbow trout tend to lose body condition between summer and fall with a mean loss of relative weight of 7.4 +/1.7. Brown trout tend to gain body condition between spring and fall with a mean gain of 5.6 +/1.8. These findings can be interpreted in terms of the feeding habits, forage availability, and metabolic activity of each species.