Habitat use of Northern Map Turtles (Graptemys geographica) in an altered system, the Susquehanna River, Maryland (USA)

Abstract

Abstract Background/Question/Methods Riverine turtle populations are declining worldwide and many of these species have been extirpated from large areas of their range. In Maryland, the northern map turtle (Graptemys geographica) occurs only in the lower Susquehanna River, which is the eastern edge of its range. The northern map turtle is a Maryland state endangered species and had not been seen in Maryland since the early 1990’s. The Susquehanna River is divided by a hydroelectric dam and is heavily used for recreational activities, both of which may negatively impact turtles. To determine the status of map turtles in Maryland, we performed visual encounter surveys for basking turtles at 12 sites from April to November 2008. Results/Conclusions Overall, we observed 0-38 map turtles per day. The largest consistent aggregations of adult turtles were found at an island complex 5.9 km below the dam. Some aggregations were quite large, with as many as 21 turtles observed basking on a single rock. Observed basking activity began by 12 April and stayed more or less constant through June. Observed basking activity was reduced between July and September, and then increased in mid-September continuing through early November. Most map turtles observed were adults, and the only evidence of ongoing reproduction and recruitment was two small juveniles found above the hydroelectric dam. The adult sex ratio was female biased throughout the study period, with daily sex ratios as high as 33F:2M. This may represent sampling bias (females are much larger than males and are thus easier to detect) or the actual population sex ratio in Maryland is dominated by females. We surveyed for nesting habitat both above and below the hydroelectric dam, by looking for predated nests at potential nesting beaches, but no depredated nests were found. Virtually nothing is known about what specific areas of the Susquehanna River and its tributaries are used for the various aspects of the map turtle’s life histories or what potential negative effects may be caused the by river impoundment and recreation activities. Given the absence of basic ecological data of map turtles in Maryland, we are using radio telemetry, diet analysis, and visual surveys to continue to gather information on nesting, feeding, mating, basking, and hibernation, and how these activities are affected by anthropogenic alterations to the Susquehanna River.