ADRIAN HAILEY* AND P. M. C. DAVIES

ABSTRACT

Viperine water snakes were studied by mark-recapture in a small river in Eastern Spain from 1981 to 1983, a period of drought. Observed growth rates between captures were low and highly variable. Growth rings were counted in skull bones; narrow rings could not be found, and the observed rings were probably from periods of faster growth or normal growth before the drought. The rings predict that males and females reached maturity in their third and fifth years, respectively. The effect of drought was greater on growth than on reproduction. Movements of snakes captured several times at long intervals suggest that the home ranges of mature males and females were 170m and 250m of river, respectively, although some individuals moved 1 km between years. A review of published data showed that widely foraging grass and garter snakes have greater displacements between captures than the more sedentary water snakes.

There were estimated to be 1060 and 390 adult males and females in the main study area, which was a 2km length of river which included most of the permanent unpolluted water in the river system. Annual survival of adult snakes remaining within the study area was estimated as 0.53 ± 0.16 and 0.71 ± 0.25 in males and females, respectively. A simple model of the dynamics of this population was developed from results on survival, recruitment, growth and fecundity, as a hypothesis for further study. The model suggested that: 1) There was dispersal of adult males but not adult females - this was supported by the population size structure in peripheral areas where mature females were scarce and which were probably stocked by dispersal. 2) The number of adult males and females was stable or increasing, and juvenile survival of about 0.65 year^-1 would be necessary for total population stability.