Influence of Variable Prey Abundance On Walleye Diets and Growth in a Large Missouri River Reservoir

In Lake Oahe, South Dakota, rainbow smelt are the primary prey of walleye, but their abundance varies substantially from year to year.  Recently, gizzard shad have become established in Lake Oahe and their abundance has increased considerably.  To evaluate the influence of gizzard shad on walleye diets and growth in Lake Oahe, we compared recent estimates of walleye diets and growth in 2008 – 2010 and made comparisons to diets and growth during periods of both high and low smelt and shad abundance.  In 2008, gizzard shad were the dominant prey item of walleye, representing about 60% of the diets by weight.  However, by 2009, gizzard shad declined appreciably in the diet (22%) and were absent from walleye diets by 2010.  Conversely, rainbow smelt abundance represented 12%, 27% and 90% of walleye diets by weight in 2008, 2009 and 2010, respectively.  Walleye growth was slowest when gizzard shad were dominant in the diets (2008) and increased every year thereafter as rainbow smelt became more prevalent in the diets.  Although only available to walleye during short periods in late summer, when abundant, gizzard shad may provide an important subsidy to Lake Oahe walleye during periods of low rainbow smelt abundance.