Summary
Bycatch of turtles in passive inland fyke net fisheries has been poorly studied, yet bycatch is an important conservation issue given the decline in many freshwater turtle populations. Delayed maturity and low natural adult mortality make turtles particularly susceptible to population declines when faced with additional anthropogenic adult mortality such as bycatch. When turtles are captured in fyke nets, the prolonged submergence can lead to stress and subsequent drowning. Fish die within infrequently checked passive fishing nets and dead fish are a potential food source for many freshwater turtles. Dead fish could thus act as attractants and increase turtle captures in fishing nets. We investigated the attraction of turtles to decomposing fish within fyke nets in eastern Ontario. We set fyke nets with either 1 kg of one-day or five-day decomposed fish, or no decomposed fish in the cod-end of the net. Decomposing fish did not alter the capture rate of turtles or fish, nor did it alter the species composition of the catch. Thus, reducing fish mortality in nets using shorter soak times is unlikely to alter turtle bycatch rates since turtles were not attracted by the dead fish. Interestingly, turtle bycatch rates increased as water temperatures did. Water temperature also influences turtle mortality by affecting the duration turtles can remain submerged. We thus suggest that submerged nets to either not be set or have reduced soak times in warm water conditions (e.g., >20 °C) as turtles tend to be captured more frequently and cannot withstand prolonged submergence.
Methodology
We used fyke nets of a similar design as those used in the commercial fishery. Each fyke net contained seven 0.91 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 (4.57 m long and 0.91 m high) and a lead (10.67 m long and 0.91 m high) attached to the front hoop. All the nets, wings, and leads were constructed with 5.08 cm stretch diamond nylon mesh.
We used fishing practices commonly employed by commercial fishers in the area: we set nets in pairs by adjoining two fyke nets by their leads with the net openings facing each other and extending the wings at a forty-five degree angle from the entrance of the net. In the commercial fishery, fyke nets are set completely submerged along the substrate, in shallow vegetated waters 1–2 m deep and parallel to the shoreline. In our study, however, we placed two plastic floats in the cod-end of each net to create air spaces and prevent turtles from drowning while the net opening remained submerged (Vogt 1980; Larocque and others 2012b). Using floats in nets does not affect fish or turtle entry (Larocque and others 2012b). We set nets in ten areas parallel to the shoreline in vegetated shallows ranging from 1 to 1.75 m in depth (mean ± std. deviation: 1.26 ± 0.18 m). Distance from shore varied between 7 and 97 m. Nets were set for between 24 and 48 h to decrease the time that captured turtles remained in the net and to reduce captured fish mortality. We recorded water temperature when setting and lifting the nets as well as Universal Transverse Mercator (UTM) coordinates via GPS (GPS 76, Garmin). Distance to shore was determined for each net set from UTM coordinates using ArcMap v.9.3. Flow rates could not be used to accurately determine “bait plume” dispersals from decomposed fish due to changing wind speed and direction. Instead, for each sampling location we set one net pair per treatment spaced approximately 50 m apart to prevent interference between net pairs and potential “bait plumes”.
Treatments included nets containing: (1) no fish (control), (2) one-day decomposed fish, and (3) five-day decomposed fish. Decomposition durations were chosen to represent short (i.e., one day) and long (i.e., five day) soak times that commercial fishers’ use and are also durations that fish would be dead in their nets. Decomposed fish consisted of 1 kg of bluegill (Lepomis macrochirus) that was collected from Opinicon Lake with fyke nets. After cerebral percussion, dead bluegill were placed in a mesh bag and submerged in a tank containing slow, continuously circulating lake water for the duration of the decomposition period (one day or five days). As such, the decomposing fish were exposed to similar water temperatures as in the lake. Once the decomposition period was over, we removed the mesh bags from the tank and suspended them from the sixth hoop of each treatment net in the conjoined pair, so that the mesh bag was touching neither the top nor the sides of the net. After the 24–48 h set period, we lifted each of the net pairs and quantified the catch. All fish and turtles captured in the nets were identified to species and tallied.