Cause for the Decline of Large Rainbow Trout in a Tailwater Fishery: Too Much Putting or Too Much Taking?

We studied causes for the decline of large (>40 cm total length, TL) rainbow trout Oncorhynchus mykiss and the transition from put–grow–take to put‐and‐take conditions in Lake Taneycomo, a popular tailwater in Missouri. In this fishery, rainbow trout densities have been sustained against increasing fishing pressure over the last 30 years mainly by increasing stocking rates of catchable‐size fish (23–33 cm TL). Short residence times of rainbow trout resulting from heavy fishing pressure and high vulnerability to angling are generally considered the main reason for this speciesˈ limited growth after stocking in cold tailwaters. However, we found that increasing stocking rates in Lake Taneycomo have been paralleled by food base degradation, leading to submaintenance consumption rates for rainbow trout (i.e., poor growth conditions). A less prominent role of residence times was evident in the decline of poststocking growth and fewer large fish. In contrast to resident brown trout Salmo trutta , high diet similarity was observed across all sizes of rainbow trout collected during the 3‐year study, hinting that this species may have a relatively limited capacity to partition food resources intraspecifically in some cold tailwaters. This limitation may contribute to the tendency of rainbow trout to fail to reach large sizes in these environments. Our findings indicate that when poststocking growth of rainbow trout is desired in cold tailwaters, greater attention to energetic conditions is warranted.