Summary
- Freshwater turtles are susceptible to drowning in commercial fishing nets and this is a major conservation concern. Methods to mitigate turtle bycatch mortality typically involve reducing the capture of bycatch using gear modifications. Another method to reduce mortality is to keep bycatch alive following capture.
- Using physiological measures of anoxia, this study determined whether providing air spaces using floats within fyke nets could prevent turtles from drowning. In a controlled setting, blood lactate and pH of painted turtles (Chrysemys picta) experimentally introduced into submerged nets, nets with floats, and nets that breached the surface were compared. While emulating commercial fishing practices – where turtles and fish voluntarily entered nets – catch rates and compositions as well as blood lactate in turtles captured were compared in submerged nets with and without floats.
- Painted turtles in submerged nets exhibited elevated blood lactate and pronounced acidosis compared with turtles from nets with floats and surfaced nets.
- Catch rates and compositions from emulated fishing were statistically similar in nets with and without floats; however, total fish catches were roughly one‐third less in nets with floats. The same pattern of physiological disturbance was observed with turtles captured in submerged nets with and without floats as in the controlled experiment.
- Overall, blood physiology indicated that anoxia occurred in turtles in submerged nets while nets with floats reduced physiological disturbance. However, variation in blood lactate levels when fishing fyke nets with floats suggests that turtles were experiencing slight anoxia and so the size of air spaces may be important in allowing access to air. Creating air spaces in fyke nets using floats is a simple and cost‐effective method to avoid the drowning of turtles.
Methodology
Study site
The study was conducted on Lake Opinicon (44° 34’ N, 76° 19’ W) approximately 100 km south of Ottawa, Ontario, Canada. Lake Opinicon is a 788 ha shallow warm-water lake with a mean depth of 2.8 m. The controlled experiments were conducted in August 2009 during which time lake temperatures ranged from 18–21 °C. Commercial fishing practices were emulated in spring (late April–mid-June) 2010 during which time lake temperatures ranged from 12.7–25.9 °C.
Fyke nets
The fyke nets used had similar dimensions to those used in the commercial fishery (Figure 2). Each fyke net contained seven 0.9 m diameter steel hoops positioned 0.5 m apart. There were two throats per net, located at the second and fourth hoops. Each net had two wings and a lead attached to the front hoop that measured 4.6 m long by 0.9 m high, and 10.7 m long by 0.9 m high, respectively. All the nets, wings, and leads were constructed with 5.08 cm stretch nylon mesh.
Controlled experiment
Controlled conditions were used to determine whether turtles use air spaces provided in nets. Using fyke nets, 30 painted turtles (Chrysemys picta; 12 females, 18 males; mean carapace length SD: 140.67 12.22 mm; mean mass SD: 353.53 95.24 g) were captured. There were three net treatments and painted turtles were randomly assigned to each. The submerged treatment was a fyke net set completely under water at a depth of 1.5 m. The float treatment was a fyke net set with an air space created by putting at least two floats (e.g. water jugs; Styrofoam) in the cod-end of the net (ensuring that the floats and nets breached the surface). The surfaced treatment was a fyke net set at a depth of 0.5 m in which about half of the net was submerged. For all treatments, the opening of the net was closed after the insertion of a turtle. Trials involved putting an individual painted turtle into the net for 4 h. Preliminary trials using video surveillance indicated that after 4 h at ~20 °C, painted turtles reduced activity and showed signs of anoxia. After 4 h, turtles were retrieved and, within 2–5 min, a blood sample was taken to measure blood lactate and blood pH. Ten trials were completed for the submerged treatment, while nine trials were completed for the surfaced and float treatments as one turtle escaped in each.