The Herpetological Journal

pp. 155-161
Authors: Sewell, David; Baker, John M.R. & Griffiths, Richard A.

Abstract: Grass snakes (Natrix natrix) were monitored for nine years on a site in eastern England restored for an amphibian reintroduction. Counts of snakes increased between 2004 and 2012 from 1.25 to 3.83 snakes per survey visit. Grass snake counts were positively correlated with the number of common frog spawn clumps each year and peak counts of pool frogs. During surveys and incidental encounters 137 adult males, 161 adult females, 131 juveniles and 44 hatchlings were captured and individually photographically identified. Captures of hatchlings were erratic and recapture rates were low, so they were excluded from the analysis. Annualised capture data were analysed in the capture-recapture programme MARK, using the Cormack-Jolly-Seber model. The top ranked model gave an apparent annual survival rate of 0.66 (95% CI=0.543–0.755) and an individual detection rate of 0.17 (0.118–0.245). Population estimates based on this model ranged from 53 (95% CI=37–76) to 576 (95% CI=400–831) over the nine years of study. Grass snake population estimates were equivalent to densities of 4.8 to 52.4 individuals ha^-1. Nevertheless, it is unlikely that these snakes were permanently resident within the study area, and annual survival may therefore be underestimated. A more plausible explanation for the large population estimates is that the snakes were temporarily resident within a patch of high quality habitat and moved through home ranges that included the study site.

Keywords: MONITORING, CORMACK-JOLLY-SEBER, NATRIX NATRIX, POPULATION ESTIMATE