Populations in the Trinity River of California in the Context of Dam-induced Alterations

-Western pond turtle (Clemmys marmorata) populations were examined on two forks of the Trinity River in northern California, 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 JollySeber 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). However, 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 m population had an age structure dominated by adults relative to a population with a known stable age distribution, Trinity may have negatively impacted j uvenile turtles. Populations of western pond turtles (Clemmys marmorata) have experienced declines throughout their range (Holland, 1992; Holland and Bury, 1998). Many of these declines can be attributed to habitat alteration resulting from agricultural development and urbanization. Dams and other water diversion structures, for example, have altered aquatic habitats throughout California. Specific impacts include conversion of lotic to lentic water, reduction in water quantity and quality, and changes in flow regimes. The mainstem Trinity River, dammed in 1963, has changed dramatically as a result. Slow-moving edgewater habitats have been eliminated, riparian vegetation has expanded and encroached on the banks, and seasonally flooded marshes have become dry. Overall, the river has become more channelized with straighter edges and a lower diversity of depths and flows (Hampton, 1995). These environmental changes have been implicated in the radical decline of a native frog species in the Trinity system (Lind et al., 1996). Little is known about the potential impacts of dam-induced alterations on western pond turtle populations. Comparison of population status in the dammed mainstem Trinity (upstream of its north fork confluence) and the undammed south fork offers an opportunity to assess these impacts. These adjacent forks of the river are similar in flow volume, channel size, vegetation, and land-use history. The south fork Trinity, 3 Present Address: Science, Engineering, & Diplomacy Fellow, USAID / G / ENV 3.08-36A, Ronald Reagan Building, Washington DC 20523. 4 To whom reprint requests should be addressed. suggesting that damming of the mainstem with its natural alternation of riffles and deep pools, resembles the mainstem in its pre-dam condition. On the mainstem Trinity, habitats preferred by western pond turtles were probably more abundant prior to installation of the dam (Reese, 1996). Several of the downstream changes produced by the dam, including reduction of water temperature, increased sedimentation, and decreased range of flow velocities (cf. Ligon et al., 1995; Mount, 1995), have decreased the quantity and quality of suitable aquatic habitat for this species. Ultimately, a decrease in habitat suitability manifests in demographic parameters, such as reduced population size, altered age structure, or lower growth rates. For example, elimination of shallow, slow-moving waters on which hatchlings depend (Reese, 1996; Holland and Bury, 1998) might create a skewed age distribution dominated by adults. A decrease in water temperature might reduce growth and / or juvenile survivorship. Although we have no historical information on densities of turtles at either the south fork or mainstem site, we can speculate that, due to their similarity, they may have historically hosted turtle populations with similar attributes. M A T E R I A L S A N D M E T H O D S Study Sites.-The mainstem study site consists of a 63-km section of river between the Lewiston/ Trinity dams and the confluence with the North Fork Trinity (Fig. 1). Wilson et al. (unpubl. data) divided this section into 16 unequal length segments, hereafter referred to as reaches. Our demography study occurred along three of these reaches (reaches 3, 10, and 11; Fig. 1), D. REESE AND H. WELSH