Comparative Demography of Clemmys marmorata Populations in the Trinity River of California in the Context of Dam-Induced Alterations

Abstract

Western pond turtle (Clemmys marmorata) populations were examined on two forks of the Trinity River in northern Califoria, one of which is subject to the impacts of damming and one of which is free-flowing. Mark-recapture was conducted over a three-year period for each population, and the Jolly- Seber model was used to estimate population size. Size/age structures were compared with reference to potential impacts of damming on this species. The density of populations (turtles/ha) on the dammed fork (mainstem Trinity) did not differ significantly from that on the undammed fork (south fork Trinity). How- ever, both populations were much sparser than western pond turtle populations elsewhere. Higher-order streams, in general, do not appear to support high densities of western pond turtles, possibly because of the sparse distribution of resources. The mainstem population had an age structure dominated by adults relative to a population with a known stable age distribution, suggesting that damming of the mainstem Trinity may have negatively impacted juvenile turtles. Populations of western pond turtles (Clemmys marmorata) have experienced declines through-