Summary
- Basking is the primary mechanism used by many freshwater turtles to maintain their body temperature (Tb) in a range that maximizes physiological performance. Basking turtles are easily disturbed by motorboats, but the consequences of the increasingly popular use of motorboats on turtles is largely unknown.
- In this work, predictive models built from field and laboratory data were used to assess the effects of the frequency of basking disturbance by motorboats on Tb and metabolic rate (MR) of female northern map turtles (Graptemys geographica), a species of conservation concern.
- Simulations revealed that the effects of boat disturbance vary seasonally. In early May, a conservative estimate of the disturbance rate (0.15 per hour) resulted in a 0.34°C decrease in mean daily Tb, which translated to a 7.8% reduction in mean MR. In June, July and August, owing to warmer lake temperatures, the effect of disturbance was less marked and the observed disturbance rates (0.32, 0.96 and 1.23 per hour, respectively) reduced the mean MR of an adult female by 2.1%, 0.5%, and 0.4 %, respectively.
- Reduction in MR decreases the rate of energy assimilation, which could translate into sublethal effects on turtles, such as reduced growth and reproductive output.
- Motorboat usage is increasing in many areas and is probably affecting other species of freshwater turtles that use aerial basking. This study offers important insights on the implications of disturbances for species that bask.
Methodology
The simulation integrated direct measurements of basking behaviour and responses to disturbance. These observations were made between April and August 2014. In April and May 2014, 92 mature female northern map turtles were captured from Lake Opinicon while snorkelling at hibernation sites. To allow the recognition of individual turtles on the time‐lapse pictures (see below), unique identification numbers were painted on the carapace of each individual. The turtles were released at their site of collection within 24 h of their capture.
Time‐lapse cameras were placed at natural basking sites to measure how long it takes for undisturbed and for disturbed turtles to resume basking after entering the water. One camera was installed at each of five basking sites and set to take a picture every minute between 7:00 and 19:00 from mid‐May until basking ceased at each of the sites at the end of July to early August. A disturbance was defined as an event causing at least half of the basking turtles in the field of view to leave the basking site within one minute. Instances where only one turtle was present at a basking site were excluded. In addition, experimental disturbances were carried out using a 5.8 m aluminum jon boat with a 25 hp motor (Table 1). Disturbances involved approaching the basking site at 15–20 km h‐1 until half the turtles at the site entered the water. The boat was then immediately turned around, leaving the area at the same speed. The approach distance was typically 15–25 m before turtles entered the water. For each disturbance (including the experimental disturbances) captured by the cameras, the time it took for every numbered individual in the field of view to resume basking at the same location was recorded, with the set of times referred to as rdist. Because turtles also interrupt basking for reasons other than disturbances, the time lapse pictures were also used to record how long it took for seemingly undisturbed numbered turtles to resume basking after entering the water (rundist). Turtles entering the water were considered undisturbed if other basking turtles present at the site did not cease basking when the focal turtle did. Instances where turtles did not return to basking during the same day were excluded from the data set because turtles could have resumed basking at other basking sites not under camera surveillance. The total sample size of rdist is much larger than that of experimental disturbances because it includes disturbances not caused by researchers (Table 1), and the sample mean of rdist (mean = 24.3 min, SD = 56.3) was less than that of the mean time to return following experimental disturbance (mean = 29.1 min, SD = 37.1). Therefore, the distribution of rdist was used as a conservative estimate of the time to return to basking following motorboat disturbances. There was no significant difference in rdist (ANOVA: P = 0.884, F3, 127 = 0.218) and rundist (ANOVA: P = 0.878, F2, 143 = 0.1302) between months, thus a single distribution was fitted to rdist and a single distribution was fitted to rundist. To estimate actual disturbance frequencies in Lake Opinicon, the number of non‐experimental disturbances for numbered turtles was tallied on time lapse photographs and divided by the sum of the hours spent basking by numbered turtles (Table 2).