Predation on fish and intersite variation in the diet of common garter snakes, Thamnophis sirtalis, on Vancouver Island

Diets of garter snakes (Thamnophis) often vary in space or time in response to variations in prey abundance. We compared the diet of the common garter snake (T. sirtalis) on Vancouver Island at fish-rearing facilities (hatcheries) and at nearby natural sites where fish were present but less abundant. Snakes of all sizes fed on fish at hatcheries, but fish were rarely eaten at natural sites, where amphibians or earthworms were the major prey types. Any particular characterization of the diet of this species therefore must be site specific. Although snakes exhibited intersite variation in diet, there was no evidence of temporal variation in diet at any site. The proportion of snakes with food in their stomachs varied among sites (perhaps indicating differences in frequency of feeding among sites) and was correlated with mean relative body mass of snakes. This suggests that some sites are more productive than others for snakes, but rigorous tests of whether snake populations are food-limited have not been done.