Activity patterns of garter snakes, Thamnophis sirtalis, in relation to weather conditions at a fish hatchery on Vancouver Island, British Columbia

Temperature is one of many factors that may have an important influence on activity of ectotherms. We investigated the influence of weather conditions on activity of common garter snakes, Thamnophis sirtalis, at a fish hatche ry on Vancouver Island where the water in which they forage is cold (<14 de grees C). We used reduced major axis regression to partition the relationsh ip between air temperature and number of snakes seen into subsets, and the uppermost subset of data was used to describe the upper boundary of the ove rall relationship. At the hatchery site, the maximum number of snakes obser ved increased in relation to maximum daily air temperatures, with the highe st numbers of snakes being observed on days when maximum daily air temperat ures exceeded 25 degrees C. By contrast, at a nearby reference site where s nakes do not consume fish, the maximum number of Snakes observed decreased in relation to maximum daily air temperatures, with very few snakes being o bserved on days with high maximum daily air temperatures. Radiotagged snake s were observed swimming in hatchery channels only on days when maximum dai ly air temperatures were high. Body temperatures of these snakes decreased extremely rapidly when the snakes entered the water, and remained low while they were in the hatchery channels. The body temperature of one snake that captured a fish while swimming in the channels was just over 14 degrees C. We hypothesize that hot weather conditions permit snakes at hatchery sites to achieve high body temperatures rapidly subsequent to foraging activity.