Predation on fish by cormorants and pelicans in a cold-water river: a field and modeling study

We studied diets of double-crested cormorants (Phalacrocorax auritus) and American white pelicans (Pelecanus erythrorhynchos) collected on the North Platte River, Wyoming, from May to August. Rainbow (Oncorhychus mykiss) and cutthroat (O. clarki) trout 10-16 cm long were stocked in late June - early July. Cormorants ate mainly suckers (Catastomus spp., 61% of numbers, 85% of fresh mass) before trout stocking; but trout consumed increased from 17% (12% of mass) in samples before stocking to 60% (82% of mass) after stocking. Pelicans ate mostly suckers (64% of numbers, 84% of mass) and minnows (Rhinichthys cataractae, Pimephales promelas; 21% of numbers, 1% of mass) throughout the study; trout were 1% (0.1% of mass) of the pelican diet before stocking, and 22% (14% of mass) after stocking. A bioenergetics model estimated that these birds consumed up to 80% of trout stocked in 1994. In August 1993, cormorants ate mostly trout (86% of numbers, 98% of mass), suggesting that many stocked trout survived until autumn. However, the much smaller fraction of trout eaten by cormorants in spring 1994 before stocking (mean 17% of numbers, 13% of mass) than after stocking (60% of numbers, 82% of mass) suggests that stocked trout declined substantially during winter. Because overwinter survival might have limited trout recruitment between years, it is unclear whether bird predation was additive or compensatory.