Modelling the effect of cormorant predation on stocked walleye

A simple model was developed for evaluation of the effects of double-crested cormorant (Phalacrocorax auritus) predation on stocked walleye (Stizostedion vitreum) in the Great Lakes. The model was applied to obtain general relations for stocking and for cormorant predation. If there is no stocking, maintenance of a sizable walleye population is necessary for reproduction, but when the walleye population is maintained by stocking, yield can be increased by fishing the population below the level which produces the maximum sustainable yield without stocking, and the loss in reproduction is made up by stocking. If there is no predation on the stocked walleye, stocking can greatly increase yield. If there is cormorant predation, in the long run the number of cormorants increases with stocking but the number of walleye does not, and stocked walleye are feeding the cormorants.