Summary
The drowning of freshwater turtles following incidental capture in fishing gear has the potential to cause population declines. Fyke nets can be equipped with bycatch reduction devices that enable the escape of turtles before they drown. We employed quantitative and qualitative behavioral observations (with action cameras deployed underwater) to develop a new, collapsible, escape bycatch-reduction device that mounts internally in the terminal end of a fyke net. We also used behavioral observations to identify areas of the net most used by turtles, thus revealing the most logical placement for an escape bycatch-reduction device. When turtles were introduced into modified nets, escape was rapid (mean of 12.4 min), with 100% escape for map Graptemys geographica and musk turtles Sternotherus odoratus and 94% escape for painted turtles Chrysemys picta. Our preliminary field trials indicated that modified fyke nets decreased the capture rate of turtles relative to unmodified nets. Escape devices can be used as a key component of a bycatch reduction program and be particularly effective when paired with exclusion bycatch-reduction devices. The escape device developed in this study can potentially be used in the local fishery or modified for other fisheries. The use of behavioral observation to guide the development of bycatch reduction devices may provide an extra tool for managers to increase selectivity and maintain sustainable harvests of target fish.
Methodology
Nets
The fyke nets were constructed of seven 0.91-m structural hoops made from 0.64-cm steel rods and attached together with #15 knotted nylon, 2.54-cm square mesh (5.08-cm stretch; Christiansen's Nets Company, Duluth, MN). Throats or funnels on the second and fourth hoops directed organisms into the terminal end of the net and minimized escape. We set nets in pairs, connected mouth to mouth by a lead net 10.7 m long and 0.91 m tall. Each net had 4.6-m-long wings made of the same material set at ~45° angle from the lead. We set nets in shallow water (1-2.5 m). For collecting turtles and fish, we typically set nets for 24 h (see Larocque et al. 2012a for details), whereas we set the nets for 4 h for field trials of BRDs.
Documenting in-net behavior and activity
To quantify behavior, we conducted observations using a completely submerged net and underwater video recording. We observed and compared in-net position and activity for the four species of turtles. We conducted trials from 18 May to 20 June 2011 in a shallow (maximum depth 2 m) bay of Lake Opinicon. Water temperatures during the trials ranged from 16.5 to 24.5°C. We used male turtles to avoid potentially harming reproductive females (Brooks et al. 1991; Midwood et al. 2015). We collected males using unmodified fyke nets fished for 24 h. Following capture, we held turtles outside in 700-L fiberglass flow-through tanks with access to basking platforms for a minimum of 24 h before trials. We used 39 turtles of four species (10 painted, 10 map, 10 musk, and 9 snapping turtles) in these trials. We placed each turtle in the mouth of a sealed net and observed its behavior in real time and recorded it for 3 h using three underwater cameras (Figure 1).
If we did not observe a turtle moving for 15 min, or if it appeared to be in acute distress, we ended the trial and removed the turtle from the net immediately. The terminal end of the net was divided into quadrants (along the x/y axis as viewed from the side; Figure 1). To account for the unidirectional movements within hoop nets and the reduction in activity of most species beyond 2 h, we limited in-net behavioral analysis to the terminal end of the net between 5 and 120 min. We also compared the first and second hours to determine whether occupancy for each species varied between these two periods. We compared the occupancy of the four terminal-end quadrants by species using a chi-squared goodness-of-fit test and repeated G-tests of goodness-of-fit for comparisons between hours. To determine whether turtles changed their activity patterns over time, we completed 30 s of observation every 5 min from the video generated. We assigned a 1 if the subject was active (crawling, swimming, or pulling on the netting) and a 0 if inactive (sitting on the bottom or clinging to the netting without pulling). We used binary logistic regression to determine the relationship between activity and time for each species. We conducted all analyses using R statistical software (R Development Core Team 2012) and set alpha to 0.05.