Spatial ecology of Northern Map Turtles (Graptemys geographica) in an altered river system

Background/Question/Methods

Riverine turtle species are declining worldwide and many populations have been extirpated due to anthropogenic stressors. Last officially recorded in Maryland in the early-1990’s, the Northern Map Turtle (Graptemys geographica) is listed as state endangered and is currently only known in the lower Susquehanna River in northern Maryland with a few historic records from nearby waterways. This river is divided by a hydroelectric dam, is heavily used for recreational activities, and has increasing shoreline development, all of which may negatively impact this population. Given the absence of basic ecological data of map turtles in Maryland, we used visual encounter surveys, trapping, and radio-telemetry to examine the spatial ecology of this population in the 1840ha portion of river below the Conowingo Hydroelectric Dam. We derived seasonal movement data from 14 adult females. We also calculated home ranges for those individuals using two methods: Minimum Convex Polygons (MCPs) and Fixed 95% Kernels. We quantified intensity of habitat use by using a combination of MCP size and Kernels (95% and 50%).

Results/Conclusions

There was a significant difference in average movement per day across seasons (F=22.4, df=2, P<0.0001) with the largest movements occurring during nesting (early-May to mid-July). There was no significant difference in size estimation between home range methods (F=0.42, df=1, P=0.53). 89.6% of the 95% kernels and 95.6% of the 50% kernels fell in the 230 ha area known as the island complex. This area has the highest concentrations of basking structures (rocks and logs/deadwood) which are important habitat characteristics for thermoregulation and in turn the viability of this population. All home-range portions outside this complex can be attributed to large movements for nesting (N=7), though not all females leave the island complex to nest (N=7). Our data suggest that impacts of dam operations on river water levels are dramatic and that high flows from the dam hinder basking activity and movements of turtles. Basking sites are submerged during high flows, drastically reducing the availability of suitable basking habitat. Dam activities have also hindered the natural deposition of sand, impacting the number of available nesting beaches. The remaining beaches are heavily inundated with human activity and force map turtles to nest in sub-optimal conditions (e.g. heavy canopy cover). We suggest the turtle’s reaction to sudden changes in river flow and depth and the management of public lands for optimal nesting conditions is crucial in mitigating impacts on this population.