Summary
Recreational power boating is growing in popularity in North America. This activity is known to have lethal and sub-lethal effects on aquatic wildlife and freshwater turtles may be particularly sensitive to this activity. This study reports on patterns of traumatic injuries inflicted by powerboat propellers to northern map turtles (Graptemys geographica) from two sites differing in boat traffic intensity in Ontario, Canada. The relative vulnerability of turtles was assessed, in light of seasonal patterns in boat traffic, as a function of sex- and age-specific movement patterns, habitat use, and basking behaviour obtained by radio-telemetry. Population viability analyses (PVA) were also conducted to evaluate the potential demographic consequences of mortality induced by powerboats. The prevalence of propeller injuries was two to nine times higher in adult females than in adult males and juvenile females. Patterns of movement, habitat use, and aquatic basking indicated that adult females are more exposed to collisions with boats. PVA showed that boat-induced mortality in adult females could lead to rapid population extinction if the risk of mortality when hit by a boat is greater than 10%. The results of this study showed that recreational power boating is a serious threat to northern map turtles, even under moderate boat traffic. The need to adopt measures restricting boat traffic in areas important to turtles is discussed.
Methodology
Study sites and quantification of propeller injuries
Northern map turtles were studied in the St. Lawrence River and in Lake Opinicon in Ontario, Canada (Figure 1). The St. Lawrence River study site was located within St. Lawrence Islands National Park (hereafter SLINP) in the Thousand Islands at the border of Ontario and New York State. The Thousand Islands region is one of the most important sites for recreational boating in Ontario. The Canadian Coast Guard estimates that 65% (780 000) of the recreational powerboats in Ontario are used on the Great Lakes and on the St. Lawrence River (Great Lakes Commission, 2000). The SLINP study site covers an aquatic area of 2890 ha located around Grenadier Island. Lake Opinicon is a small lake (788 ha) that is part of the Rideau Canal waterway linking the Ottawa River to Lake Ontario, 30 km to the north-west of SLINP. Although recreational boating in Lake Opinicon is not as intensive as in SLINP, the Rideau Canal is also an important area for nautical tourism in Ontario with, on average, 5003 boats using the canal per year between 2004 and 2006 (Parks Canada, unpublished data). At both sites, map turtles were captured by snorkeling and with basking traps. All turtles were marked individually by drilling small holes in their marginal scutes and were examined for traumatic injuries caused by boat propellers. Only individuals displaying unambiguous and extensive propeller-induced damage to the carapace or plastron (see Figure 2 for examples) were included in the analysis.
Spatial ecology, aquatic basking, and boat traffic
To understand better sex- and age-specific exposure to propeller injuries, findings on the prevalence of injuries were compared with patterns of habitat use, movements, and basking behaviour. In parallel studies, radio-telemetry was used to quantify movement patterns (Carrière et al., in press) and habitat use (Carrière, 2007; Bulté et al., 2008a) in both populations. In the current study, patterns of aquatic basking are also reported. Basking while floating at the surface of the water (aquatic basking) is a common thermoregulatory behaviour in emydid turtles. Aquatic basking may put turtles at risk of being hit by a boat. The seasonal variation in aquatic basking as well as sex- and age-specific differences in this behaviour was investigated. Observations of aquatic basking were collected from 53 radiotelemetered map turtles (17 adult females, 18 juvenile females, and 18 adult males) from Lake Opinicon between 2004 and 2006. Although turtle behaviour at the time of telemetry location was noted in SLINP, approaching floating turtles inconspicuously at this site was often difficult due to waves, boat traffic, and the geography of the river, making observing aquatic basking difficult. Seasonal patterns in movement and aquatic basking were compared with seasonal patterns in boat traffic. Data on the number of boats visiting SLINP (boat docking records) between 2000 and 2008 (Parks Canada, 2008) and on the number of boats passing through Lake Opinicon on the Rideau Canal between 2005 and 2007 (Parks Canada, unpublished data) were used as indirect measures of boat traffic.