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As silky shark populations continue to decline worldwide, these apex predators are still captured as bycatch in commercial longline and purse seine fisheries targeting tuna. Our research study investigated how fishing practices in the purse seine fishery affect the survival of this species and where effort should be placed to reduce silky shark mortality.
Purse seine fishermen often use man-made structures called fish aggregating devices (FADs) to concentrate tuna schools with the hope to improve their catch. These FADs not only attract tuna, but many other species as well with juvenile silky sharks caught in greater numbers when purse seines are set at FAD locations. Our research focused on investigating the survival of these juvenile silky sharks after their capture and release from the purse seine fishery while fishing at these FADs.
To determine silky shark survival after capture, we attached satellite tags to 28 sharks. These tags documented whether a shark was dead, alive, eaten, or lost its tag at the specified pop-off date of 100 or 360 days. Tags collected other data as well—the location of the shark at the time the tag popped off and the temperature and depth of the water that the shark swam through during its daily movements.
Additionally, sharks were assessed for condition both visually and by using lactate concentration—a biochemical marker collected from blood samples that indicates physiological stress. A total of 295 silky sharks were captured and sampled visually for condition with 87 of these sharks sampled for lactate concentration. Seventeen of the sharks sampled were also tagged to relate survival to lactate concentration.
Sharks were tagged and assessed for condition at different stages of the purse seine operations to determine survival rates throughout the fishing set. Purse seine fishing can take over 2 hours with sharks exposed to different amounts of stress throughout the operations. At the beginning, sharks may have little stress with the ability to move around freely; whereas later, they may be confined with limited ability for gas exchange and with more time struggling against the net. Purse seine fishing begins when a net, attached to a small skiff and a larger mother ship is pulled around a school of fish. The net is then pursed and tightened around the school by stacking it on the vessel deck. Once the fish are piled alongside the vessel into a “sack” they are brought onboard using a large dipnet called a brailer. To verify the concept that sharks have reduced survival at later stages of fishing, we tagged and sampled sharks at the following stages: 1) swimming at FADs prior to fishing, 2) “free swimming” inside the net, tightened to half of its original size, 3) entangled in the net, 4) landed in one of the first brails, and 5) landed in one of the last brails.
Satellite tagged sharks confirmed lactate concentration, as well as, shark condition staged visually, were good predictors of shark survival. When survival was extrapolated to all sharks sampled for lactate concentration, we observed 100% survival for sharks sampled at FADs prior to fishing and while “freely swimming” inside the net. Sharks that were entangled had 69% survival, while sharks released at the later stages of fishing of first brail and last brail had survival rates of 17% and 7%, respectively. Overall, sharks released after entanglement or brailing stages, had a mortality rate of 84% (survival rate of 16%).
Tagging data from this study indicated that day and nighttime vertical movements of juvenile silky sharks span the entire depth range of purse seine and longline fishing operations with a preference of silky sharks for the shallow, warm waters also preferred by tuna. Consequently, avoidance of shark habitat during normal fishing may not be possible. This study also illustrates that solely a no retention policy for silky sharks captured in the purse seine fishery will likely not improve their survival with an overall mortality rate of 84% for sharks released after entanglement in the net or after being landed from the brail. We recommend that mortality rates be documented in the purse seine fishery by fisheries observers visually assessing shark condition at the time of release, which our study showed was a good predictor of post-release survivorship. These estimates will improve mortality estimates in silky shark population assessments. This study also highlights the importance of releasing sharks early in fishing while they are still “free swimming” in the net through good handling practices and innovative solutions, such as release panels. Additionally, silky shark population status can be improved by concentrating conservation efforts on longline fisheries which capture greater numbers than purse seine fisheries with the potential for higher post-release survivorship of silky sharks through best handling practices. Read more about Shark Tagger survivorship studies in longline fisheries.
A research study was conducted to complement our previous work on shark survivorship in purse seine fisheries. This study tested whether fishing for silky sharks, using hook and line, inside the purse seine net while sharks are “free swimming” would be a viable method to release captured sharks. In addition, the post-release survivorship of Chilean devil rays, Mobula tarapacan, and whale sharks were assessed after capture and release using the recommended best handling practices for purse seine fishing.
The time available to fish for silky sharks within the purse net was limited by fishing operations; consequently, this method has a cap on the number of sharks that can be successfully released. This method was only possible for juvenile silky sharks. When fishing away from FADs, fishermen may capture less juveniles and greater numbers of adult silky sharks. Adult sharks were observed successfully foraging within the net during our study and, as a result, were uninterested in biting baited hooks.
Five of six tagged devil rays did not survive the fishing interaction with delayed mortality occurring between two and ten days. These animals were all captured at later stages of fishing with animals maneuvered from the brail into a cargo net for release into the water. This method proved to be difficult with these large, heavy, and unwieldly animals, with handling time sometimes over 10 minutes and visible signs of trauma for some animals. We recommend future conservation measures be focused on finding innovative ways to release rays while animals are still “free swimming” in the net or to avoid fishing in hot spots.
Good news for whale sharks—two whale sharks were tagged with both surviving the fishing interaction. These animals were not observed during fishing until the net was pursed into a smaller sack at a later stage of fishing; still, these animals were robust enough to survive the interaction. This study indicates that the current recommended handling practices are viable for releasing whale sharks at this stage of fishing operations in the purse seine fisheries.
This study researched silky shark movements to better understand their habitat use in relationship to the longline and purse seine fisheries that capture them as bycatch. Juvenile silky sharks were found to spend 100% of their time swimming within the shallow, warm waters of the mixed layer. Their temperature preferences overlap those of tuna and the longline and purse seine fisheries operations; as a result, avoidance of juvenile silky shark habitat during normal fishing operations may be difficult. In addition, juvenile silky sharks were observed crossing international boundaries; consequently, management efforts should be coordinated internationally for successful conservation of this species.