Catch And Release Shark Fishing May Have Physiological And Behavioral Impacts.

“Do you enjoy fishing?” the older gentleman asked me, as I watched him cast his line into the frothy ocean waves. I had been fishing a handful of times in my life, but I couldn’t really say I enjoyed the three hours of waiting for something to bite. Nothing ever did.

I gave a non-commital answer and we both turned to look towards the blue horizon. I almost wondered aloud what he thought he would catch so close to shore when the rod bent towards the ocean, clear line racing out into the turquoise waters. Something was caught. Something big.

In these waters, it was probably a shark. Often caught by recreational anglers and scientists for pleasure or research, the practice of “catch and release” fishing is common for fishers with a strong conservation commitment. But as it increases in popularity and may seem like a humane practice, a new study finds it may still do tremendous harm to these ocean predators… and begs the question if we know enough about its impacts on threatened species.

The only way to answer this question was to go back to the anglers themselves, figured Lucy Harding, a PhD candidate from Trinity College Dublin’s School of Natural Sciences. Working directly with shark angling operators to assess how catch-and-release fishing might be impacting shark behaviour and physiology, the team set out to three different locations and got to work. Collaborators in the Bahamas, off the coast of Cape Cod in USA, and in Irish waters around Co. Cork caught tiger sharks (Galeocerdo cuvier) by drum lines and blue sharks (Prionace glauca) by rod-and-reel angling. A total of 19 sharks were caught: 4 tiger sharks and 15 blue sharks. The team then inserted thermometers into each of the animal’s muscle to monitor temperature changes once released, and also attached biologging devices to their fins to record both body temperature and water temperature.

Tiger sharks are common in tropical and sub-tropical waters throughout the world, known to swim between shallow coastal habitats and deep open-ocean environments. Famous for eating just about anything they can get their mouths around, they’re a favorite for fishers due to the fight they put up on the end of the line. Blue sharks also have a wide distribution in all temperate and tropical oceans, and are one of most common and abundant species in oceanic fisheries; in fact, they’re one of the most heavily exploited species of sharks in the world, both in commercial and recreational fisheries. The subject of discussion by international and regional fisheries management organizations, it makes sense as to why this shark and the hardier tigers were chosen for this study.

Some species of sharks, like the “lamnid sharks” (e.g. white or salmon sharks), have a unique system called a ‘counter current heat exchange,’ keeping their body warmer than the outside waters. But most are “cold-blooded” (poikilothermic) animals, meaning their body temperature is the same as that of the surrounding water. Since neither blue or tiger sharks have this rare heat exchange system, the idea would be that their temperature measurements should match the water temperature.

But it didn’t.

“The temperature measurements we took show that catching sharks on a line resulted in rapid spikes in their body temperature, with one blue shark showing an increase of 2.7°C in just a few minutes – which, in physiological terms, is a huge elevation,” explained Harding in a press release. “By combining biologging, temperature measurements immediately after capture and blood biochemistry, we show that capture significantly and rapidly (up to 0.8°C min⁻¹) elevates the body temperature of sharks.”

Acute body temperature elevations have been known to cause a number of physiological, ecological, and behavioral consequences for sharks; in terms of physiology, rapid elevations of body temperature can speed up digestion rates, increase the expression of heat shock proteins (a family of proteins expressed in response to a wide range of biotic and abiotic stressors), and and affect biological rates (e.g. like metabolism, potentially contributing to a deficit in the sharks’ daily energy budget). Elevations of body temperature can also result in behavioral changes in the sharks, causing distributional changes to behaviourally thermoregulate or expand their horizontal range if topographically limited (e.g. the tiger sharks in the Bahamas are depth limited and therefore may travel further offshore to seek out colder, deep waters). This sudden and abnormal increase in temperature is a shock to the shark’s system, and some individuals took up to 40 minutes to cool down after they had been released back into the water.

“Results like these illustrate the physical exertion sharks undergo during catch-and-release fishing. We don’t yet know whether the rapid, fishing-induced heating has a damaging effect on shark physiology but it’s something that is important to study in more detail in the future,” says Dr. Nick Payne, Assistant Professor in Trinity’s School of Natural Sciences, and co-author of the study. The authors hope their data might help with future fisheries-based management: “With fast temperature elevations […] it might be important for future research to explore any physiological impacts that this heating might cause, to better manage catch-and-release programs. Welfare outcomes of different handling protocols have been examined in other species and exploring implications of factors such as removing sharks from the water (versus leaving them submerged) or catching them near the upper limit of their thermal niche could be helpful for this group of animals.”

Adds Payne: “Results from these studies could be used to design best handling practices for shark angling going forward; if we can adopt the least stressful fishing methods then it’s a win for the sharks and also for future generations of anglers.”