The fisher strains against the edge of the boat, rod in hand and leaning dangerously over the side as they fight an unseen animal that is hooked. The minutes tick by, sweat beading on their upper lip as the slowly reel the big shark up to the surface. It’s a blue shark (Prionace glauca), one of the most heavily exploited species of sharks in the world, both in commercial and recreational fisheries.
Like other fish, sharks react to capture and handling stress in a variety of ways depending the species (e.g. sub-lethal and lethal effects). Capture stress in sharks has most commonly been measured through blood-based biomarkers such as pH, glucose, lactate, and plasma electrolytes. And while the capture response of blue sharks in commercial fisheries has been comparably well-studied, there is a relative lack of information regarding the influence of handling and capture on the species in the recreational fisheries sphere. Thus, Beneath the Waves Research Associate and Virginia Tech PhD student Brendan Shea set out to fill in the gaps. “Working in New England, we’ve been fortunate over the years to collaborate with local commercial and recreational fisherman, many of whom target tuna using rod-and-reel and commonly report encountering blue sharks as bycatch. While blue sharks are typically released in such scenarios, we wanted to better understand how these interactions may influence their stress physiology, and if there may be potential effects to health or fitness,” said Shea. “To answer this question, we set out to capture blue sharks in the same fashion in which they are captured as bycatch in the tuna fishery.”
Over the course of several months, a team of researchers captured 20 blue sharks using both stand-up fishing and spinning rods. “We wanted to know if the act of fighting on the line was stressful or exhausting to the shark, so as soon as the shark took the bait, we started a timer. The timer continued to run as we brought the shark to the boat, secured it, and finally, collected a blood sample, at which point the timer was stopped,” Shea explained. “With hook times ranging from less than 1 minute to about 30 minutes, this allowed us to compare the levels of blood-based biomarkers, such as glucose and lactate, across the range of hook times, to see if longer fights resulted in elevated stress. Interestingly – we did not! Over the short hook times, which are characteristic of the fishery, blue sharks were resilient to the capture process, and showed no change in blood-based biomarkers.”
As the team completed the statistical modeling, they noticed something peculiar: smaller blue sharks were associated with higher levels of blood glucose, which could potentially be the result of spending large amounts of energy while fighting on the line. “However, it is very difficult to attain baseline values for blood-based biomarkers, and we know that the diet and habitat of blue sharks shifts throughout their lifespan, so we can’t rule out the possibility that the observed variation in blood glucose is just a result of younger sharks consuming different prey that may be richer in carbohydrates or other molecules that may lead to higher baseline levels in juvenile sharks,” said Shea.
He hopes that this latest study, published in the Fisheries Research journal, encourages anglers to minimize their hook times. “We show here that blue sharks are robust to rod-and-reel capture in the fishery over relatively short hook times, so anglers should aim to get them to the boat quickly to be sure they swim off healthy,” said the scientist, who thinks it is important for the public to understand that under the right conditions – species, fishery, gear, etc. – sustainable catch-and-release shark fisheries can exist. “In that same vein – when releasing blue sharks, fishers should do their best to get the hook out! While some anglers have expressed concern about keeping the shark on the line during the added time needed to safely remove the hook, our research suggests that the physiological stress from this handling is minimal.”
As Shea points out, shark conservationists know that participation in shark fishing can be associated with positive attitudes toward shark conservation, so this new researcj and its possible implications may represent a potential opportunity for community members to engage with sharks and, in turn, become advocates for conservation themselves. “As such, angler education is critical - previous work has actually shown that anglers informed on conservation measures are more likely to adopt fishing practices that minimize harm to sharks. Finally, recreational capture of sharks is increasing, both through bycatch and targeted shark fishing, so studies such as this one are important to help inform the development of effective fisheries management plans,” he concludes.