Summary
Turtles are caught as bycatch in commercial fisheries in both inland and marine waters. Turtle mortality associated with bycatch is concerning, as life-history characteristics of turtles, including high juvenile mortality and delayed sexual maturity, make them particularly susceptible to population declines following small increases in adult mortality. In eastern Ontario, Canada, freshwater turtles are encountered as bycatch in an inland commercial fyke-net fishery. Although some temperate turtle species can tolerate prolonged submergence, their ability to withstand submergence decreases as water temperatures increase such that turtles may experience severe physiological disturbances and mortality following prolonged forced submergence. The purpose of our study was to evaluate the sublethal physiological consequences and related behavioural impairments associated with fyke-net capture for 3 species of freshwater turtles (eastern musk turtle Sternotherus odoratus, northern map turtle Graptemys geographica and painted turtle Chrysemys picta). Individuals that were entrapped for 3 h at elevated water temperatures (23 to 29°C) displayed considerably higher blood lactate and lower blood pH compared to free-living individuals. This trend was consistent across species and sexes. Despite having the largest increase in blood lactate, musk turtles did not exhibit behavioural impairment from entrapment, whereas both map and painted turtles displayed low responsiveness to behavioural assessments following entrapment. Our results suggest that sub-lethal responses can be used to identify potential harm or fitness impacts even in the absence of immediate mortality. Assessment of behavioural impairments, which could compromise activity and potentially result in post-release mortality, is important for protected and at-risk species that exhibit high juvenile mortality and delayed sexual maturity.
Methodology
Fyke nets
We used fyke nets similar to those used in the local commercial fishery to collect turtles following the methods outlined by Larocque et al. (2012a). Nets were fished in pairs and were attached by leads (see Fig. 1 in Larocque et al. 2012a). Each net contained 7 steel hoops, which were 0.5 m apart and 0.9 m in diameter, with 2 throats located in the second and fourth hoop. Two wings and a lead were attached vertically to the mouth of each net. Wings were 4.6 m long by 0.9 m high, while each lead was 10.7 m long by 0.9 m high. All gear was fabricated with 2.54 cm square, 5.08 cm stretch, nylon mesh.
Experimental procedure
We used 73 turtles of 3 different species for both treatment and control groups (Table 1). To assess sex-specific differences, we used only painted turtles despite an approximately equal sex capture ratio of 1:1 in the other species to minimize any potential sub-lethal effects on females. This was a condition of our Scientific Collection Permit and Species-at-Risk Permit from the Ontario Ministry of Natural Resources. We used 2 groups to determine the physiological effects and behavioural impairments associated with entrapment in fyke nets: a treatment group in which turtles were submerged in a fyke net for 3 h, and a control group of free-living turtles. This submergence time was chosen to provide sufficient time for blood physiology to respond to capture stress while avoiding immediate mortality (Larocque et al. 2012b). It should be noted that our treatment length of 3 h is not representative of typical entrapment length within the commercial fishery in Ontario, as fishers are required to check their nets every 2 to 7 d, depending on the season. Our estimates of physiological disturbance are therefore very conservative.
Control turtles were ‘free-living’ individuals which were caught via dip-net or snorkeling and sampled immediately (within 3 min of capture) to obtain baseline blood physiology values and behaviour tests and were transported back to the laboratory for morphometric measurements. Turtles were held outdoors in ~700 l fibreglass tanks at ambient temperatures until they could be returned to their capture location. Tanks were supplied with lake water using a flowthrough system, and turtles were not fed, but were provided with basking platforms and exposed to ambient sunlight.