Summary
Lentic (i.e., lake) and lotic (i.e., river) environments differ in several biotic and abiotic variables such as water velocity, productivity, thermal regimes, and depth. These variables can interact with important factors such as sex, body size, and life-history stage to shape the spatial ecology of aquatic animals such as freshwater turtles. We used radio-telemetry to study seasonal movement patterns and home-range size of juvenile and adult Northern Map Turtles (Graptemys geographica) both in a small lake and in a large river in eastern Ontario, Canada. Adult females in the lotic environment moved longer distances and had larger home ranges than conspecifics from the lentic environment. Males and juvenile females at each site had similar patterns of space use. A seasonal effect on movement was only apparent for adult females in the lotic environment in which adult females moved longer distances during the nesting season. Differences in swimming abilities resulting from a larger body size, in natal homing, or in nest site availability are potential factors explaining the site difference in the spatial ecology of adult females. Our findings illustrate the complexity of interactions shaping patterns of space use by aquatic reptiles.
Methodology
Study Areas
We conducted this study from May to September in 2005-06. The two study sites were located approximately 40 km apart in eastern Ontario, Canada (Fig. 1). Our lentic site was Lake Opinicon situated approximately 100 km south of Ottawa, Ontario. Lake Opinicon is a small (788 ha), shallow, mesotrophic lake. Lake Opinicon is part of the Rideau Canal waterway, and locks restrict access to other bodies of water by turtles. The mean depth of Lake Opinicon is 4.9 m (maximum = 9.2 m). Our lotic site was the St. Lawrence River between Mallorytown and Rockport, Ontario. Situated in the vicinity of a large island (Grenadier Island, area of 554 ha), this site covered an aquatic area of 2,890 ha and was entirely open, having no physical barrier restricting the upstream and downstream movements of turtles. The relatively large St. Lawrence River provides many habitat types ranging from rugged granite shores to dense cattail marshes. The water depth ranges from less than 1 m to more than 25 m.
Radio-Telemetry and Data Collection
Turtles were captured mainly with basking traps and by snorkeling near areas of aggregation. Sex was determined by size, carapace shape, and preanal tail length. Juvenile females were defined as females with a carapace length < 200 mm, a criterion based on the smallest gravid female captured. Subsets of captured individuals from three reproductive classes (juvenile females, adult females, and adult males) were fitted with radio-transmitters (Holohil SI-2FT 16 g, battery life of 28 months; SI 2FT 12 g, battery life of 18 months; and SB-2FT 6 g, battery life of 12 months) at each site. Over the two years of the study, 49 turtles (17 adult females, 15 juvenile females, and 17 adult males) were followed in Lake Opinicon, and 31 turtles (12 adult females, 9 juvenile females, and 10 adult males) were followed in the St. Lawrence River. We used stainless steel bolts to attach the transmitters to the rear marginal scutes of the carapace. Nontoxic aquarium silicone was used to cover the bolts and the transmitter edges for additional adhesion and for contouring to reduce snagging by macrophytes. Transmitters (including bolts, nuts, and silicone) represented maximally 5% of the turtle's body mass. Turtles were released at their site of capture the following day and were then tracked every 2-3 days using a telemetry receiver and a directional antenna. At each location, we re corded the Universal Transverse Mercator (UTM) coordinates (3D differential receiver status, NAD83 datum) with a GPSmap76 (Garmin International, Inc., Ola the, KS) at an estimated accuracy of <3 m.