Summary
Turtles are vulnerable to population declines in response to even low levels of additional adult mortality, for instance bycatch mortality. Inland commercial fisheries that use passive gears such as fyke nets cause the drowning of some freshwater turtles. To reduce fisheries impacts on turtles, bycatch reduction devices (BRDs) successfully implemented in marine systems may be adapted to freshwater systems. We tested the efficacy of two BRDs designed to exclude turtles from fyke nets by comparing catch rates and composition to unmodified nets. We also tested the efficacy of a BRD designed to let turtles escape the net by comparing turtle and fish escape capacities to a large hole in the net. The exclusion device with bars across the net opening significantly reduced turtle catch rates, and both exclusion devices did not affect fish catch rates. With the escape device, all turtles escaped (using painted turtles, Chrysemys picta, as an experimental model) and most (88%) fish were retained while a large hole allowed 60% and 77% of turtles and fish to escape, respectively. The escape device was the most effective for avoiding turtle bycatch mortality while retaining fish. Implementing the escape device or a combination of both exclusion and escape devices would reduce turtle bycatch mortality within fyke net fisheries. However, evaluations are needed to test the effectiveness of escape designs on additional turtle species and in different environments
Methodology
Study area
Our study was conducted during spring of 2010 and 2011 (late April–mid June) and fall of 2010 (early September–mid October) in 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. Water temperatures ranged from 12.7 to 25.9 °C in spring and from 13.6 to 20.0 °C in fall.
Fyke net modifications
All fyke nets (modified and unmodified) had similar dimensions as those used in the local commercial fishery (Fig. 1a). 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 which 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 from 5.08 cm stretch nylon mesh.
Two fyke net modifications designed to exclude turtles were tested. Our first modification was exclusion bars that were constructed by attaching 1.27 cm diameter wooden dowels across the first hoop of the net (Fig. 1b). Eight dowels were positioned vertically across the opening of the net, all spaced 8.0 cm apart. All adult turtles encountered in Lake Opinicon except eastern musk turtles (Sternotherus odoratus), (i.e., painted turtles, northern map turtles, Graptemys geographica, and snapping turtles, Chelydra serpentina), have a carapace width larger than 8.0 cm and should be prevented from entering the net (if swimming upright) with this device.
Our second exclusion modification was an exclusion ring that was constructed by attaching a hose clamp at the first funnel of the fyke net. The hose clamp was shaped to be a rectangle (18 cm high by 7.5 cm wide), and attached such as to create a small narrow vertical slot. This rigid narrow slot was shaped to restrict turtles from entering the first funnel of the net, contrary to the unmodified yielding funnel mesh.
Finally, an escape modification was tested by attaching a chimney-like structure to the fyke net (Fig. 1c). This escape chimney was based on Fratto et al. (2008) in which a mesh tube (1.0 cm mesh) 15 cm wide by 28 cm long by 85 cm tall was attached to the net between the sixth and seventh hoop. At the attachment site, a hole was made in the net in which we attached a 19 mm diameter PVC pipe ring, with inner ring dimensions of 15 cm by 28 cm, to keep the entrance to the chimney open. Two steel wire rings were attached to the mesh tube to keep the chimney from collapsing. The top of the chimney also contained a 32 mm diameter PVC pipe ring, with inner ring dimensions of 15 cm by 28 cm that kept the chimney afloat and oriented towards the surface. At the top of the chimney, a 5.0 cm high by 15 cm long hole was made on one side of the mesh tube to allow turtles to swim out of the net.