Double-crested Cormorant Abundance and Foraging Pressure in Response to Three Rainbow Trout Stocking Strategies at Central Washington State Lakes

Populations of double-crested cormorants (Phalacrocorax auritus) have been increasing or rebounding from historical levels, causing management concerns in some areas about their effects on managed fisheries. We studied double-crested cormorant abundance and foraging activity on 12 lakes in central Washington State managed by three different rainbow trout (Oncorhynchus mykiss) stocking strategies. We compared mean double-crested cormorant abundance and percent of birds actively foraging among treatments for three time periods: the 36-week summer season, the 34 days after individual fish stocking events, and a non-stocking time period (whole season minus the 34-day stocking event). Mean double-crested cormorant abundance did not vary among treatments within stocking periods or across the whole season, but was significantly higher in the 34-day stocking period in all treatments. Double-crested cormorants spent a greater proportion of time foraging while on lakes stocked with larger size trout ( >76 mm) than on lakes stocked with fry only. This variation in time spent foraging among lakes and treatments suggests that a simple abundance record may not accurately estimate consumption of fish stocks by double-crested cormorants, especially in areas with a high concentration of individually stocked waters. Based upon bird abundance, foraging behavior and a fixed daily intake of fish (504 g/ bird/day), we estimated that double-crested cormorants consumed a higher biomass of larger size trout, but a greater percentage of the fish in fry only stockings. Based upon biomass (kg) consumed by double-crested cormorants and a regional estimate of the cost of raising trout, monetary loss was least on the fry size class of trout. Results of this study suggest that fry-sized trout are the most cost effective and receive the least amount of foraging pressure from double-crested cormorants, possibly because of foraging efficiency and food preferences during the breeding and brood-rearing season.