Hutchinson, M.R.; Itano, D.G.; Muir, J.A.; Holland, K.N. Post-release survival of juvenile silky sharks captured in a tropical tuna purse seine fishery. Marine Ecology Progress Series 521: 143-154, 2015. doi: 10.3354/meps11073
Why we care
Environmental conservation is difficult (if you’ve read my other OceanBites posts, you’ll know that this is something I think about a lot). To achieve conservation success in the long term, you must balance the interests of many different people who want to use a resource, know why a species is declining, and have a solution that is sustainable both economically and culturally.
This article deals with the middle issue listed above: knowing why a species is declining. One of the largest impacts humans have on ocean species is through our consumption of seafood. This obviously requires that species be harvested from the ocean, but harvesting methods often unintentionally negatively affect other, non-target species.
Identifying species that are caught as bycatch is unfortunately not enough information to craft a good conservation solution. For example, if you are fishing for bass in a pond but catching bluegill, should you change the type of bait you’re using? Or the time of day you’re fishing? What about the area of the pond where you’re casting your line? These issues get much more complicated when many boats with complex fishing techniques are operating in an area where a species of conservation interest lives.
These authors worked with silky sharks unintentionally caught in purse seine nets. These are circular nets deployed from the water’s surface around a school of target species and then pinched together below these fish like a coin purse, similar to pulling a drawstring to close a bag. Purse seiners sometimes deploy electronic devices, called Fish Aggregating Devices (FADs), into the ocean to attract fish. These are free-floating and each one sends data to fishermen on its location and how much biomass is in the water beneath it. Of the sharks, skates, and rays, silky sharks are by far the most often caught in FAD-associated purse seines. Scientists have previously shown that the survival in silky sharks caught in purse seine nets and then released is less than 20%. The silky shark is an important component of ocean food webs, but it is currently being overfished. Understanding mortality associated with purse seine fishing is an important step in managing silky shark populations.
The authors sampled sharks throughout the entire fishing process, including after the sharks were released, to determine which step was most responsible for shark mortality. Their results could be directly applied to shark management strategies in areas where purse seining is common.
The authors spent 41 days aboard a normally fishing purse seine vessel in the western and central Pacific Ocean. During this time, they observed sharks incidentally caught in 30 fishing sets associated with FADs and 1 set where no FAD was used. Prior to net deployment, they captured and sampled silky shark blood chemistry to determine chemical stress levels when sharks were exposed to only a minimum amount of handling that did not include being caught in purse seine nets. They then sampled sharks during 4 stages of the fishing process, when they were:
1) Encircled by a half-closed net but were still able to swim freely,
2) Entangled in the net but not hauled aboard the fishing vessel,
3) Landed on the fishing vessel and encountered as soon as the net was open (so they were on top of the individuals hauled in), and
4) Landed on the fishing vessel and encountered only after the net was initially opened (so they were on board the vessel longer and were subjected to more weight being put on top of them while aboard).
Sharks were released after sampling and were scored qualitatively for the way in which they swam away from the area. Live sharks were equipped with satellite tracking tags used to quantify post-release behaviors. Sharks that survived 10 days or more were considered to have survived their encounter with the fishing vessel.
295 juvenile silky sharks were captured while at sea and 28 sharks were fit with satellite tags. Of the chemicals analyzed in the sharks’ blood, lactate concentrations emerged as the most appropriate predictor of shark survival (based on lactate concentrations paired with satellite tag data). Since the authors were unable to tag every shark captured in this study, they used lactate concentrations as a proxy for survival for the 267 sampled but untagged sharks.
Silky shark survival rates were 100% for those sharks caught before fishing began and after having been encircled by the net (stage 1). Sharks that were entangled in the net but not brought on-board the vessel (stage 2) had a 68.7% survival rate, while survival rates declined to 16.7% for sharks landed with the catch (stage 3). Sharks encountered at the end of sifting through the catch (stage 4) only survived 6.7% of the time. Overall, sharks brought on board the boat (sampled during stages 3 – 4) and then released into the ocean only survived the incident 15.8% of the time.
This study also included an analysis of the differences between the number of sharks caught as reported by fisheries observers, vessel crew members, and the scientists. Satellite tag data on shark behavior was also aggregated and analyzed.
This study is one of the first to analyze in detail silky shark mortality associated with purse seine fishing. The trauma experienced by sharks hauled aboard the fishing vessel enormously increased the probability of shark death. Observations while SCUBA diving and recorded on video suggest that sharks become exhausted while struggling against the net during entanglement, making breathing while onboard much more difficult and leading to higher mortality rates
While avoiding sharks in the first place would be ideal, this study highlights both the importance of good release practices before a purse seine catch is hauled onto a fishing vessel and the ineffectiveness of release after a shark has been landed with the catch. Management efforts that focus on release solutions before sharks come onboard are therefore likely to be most effective in increasing silky shark survival and ultimately impacting shark abundance.