We so often hear that massive overfishing has lead to serious declines in shark populations around the world, that it can be overlooked that there are many fishers out there who do not target sharks. In fact, many fishers will even go out of their way to release sharks alive if they can. Whilst this might be done with the best of intentions, sadly, many sharks suffer such serious stress during the process that they can die after they are released anyway. But what is it that actually causes the shark so much stress? And can this be avoided, so they can be safely released?
Sharks are Not Always Targeted by Fisheries!
Whilst there are many shark fisheries around the world directly targeting sharks for their fins, meat and liver oils, there are also many incidences where sharks caught by fishers are released alive. There are many reasons why fishers might choose to throw back a shark... For example, many sharks are released when they are caught as "bycatch" in commercial fisheries targeting other species, as they are less valuable. They may also be caught in areas where it is illegal to land sharks, such as within shark sanctuaries or during seasonal shark fisheries closures, so they must be thrown back by law. Sharks may also be released when fishers have already met their quota, or because they are personally involved in conservation initiatives for endangered species (Gallagher et al, 2014; Hutchinson et al, 2021).
Sharks Can Die from Stress Even When They are Released
However, lovely as the sentiment of releasing a shark might be, sadly, sharks can suffer many problems after catch and release. Entanglement in fishing gear and/or being hauled on deck and/or handled by humans all cause a shark signiifcant stress (Gallagher et al, 2014).
The effects of this stress can be sublethal; like physiological disturbances, minor injuries, or changes in their behaviour, which might affect how well the sharks can feed and breed. On the other hand, these effects can also be lethal, either immediately (known as "at vessel mortality") or some time later (known as "post-release mortality"). The full effects can be quite delayed, with some sharks dying from stress long after release, or succumbing to infection from injuries, or being picked off by predators in their weakened state (Gallagher et al, 2014; Hutchinson et al, 2021).
Some Fishing Gears are More Damaging to Sharks than Others
Not all fishing gear is the same! There are many different types of vessels and many different methods for fishing seafood, depending what you are trying to catch and some of these gears cause more damage to sharks caught as bycatch compared to others (Ebert et al, 2021).
Gillnets are generally the most destructive to sharks caught as bycatch, as the gear is designed to ensnare their gills and be difficult to remove, so can cause serious injuries. However, hook damage from catch in a longline fishery can also cause significant damage; affecting the sharks ability to eat and breathe. Purse seine fishing and trawling have relatively low rates of bycatch in general, but can entangle sharks, especially if a "fish aggregating device" (FAD) is used. This can cause stress and/or injury (Ebert et al, 2021).
Studies of the gummy shark (Mustelus antarcticus) have shown that up to 70% of these animals that are caught in gillnets will die, compared to only 8% which are caught on longlines (Ebert et al, 2021).
Certain Sharks are More Vulnerable to Stress than Others
Not all sharks respond to stress in the same way; some are more hardy, others more vulnerable. For example, sharks which live near to the ocean floor (known as "demersal") have thick skin, which is less likely to be damaged by fishing gears. They also breathe by a process known as "buccal pump ventilation"; using the muscles in their mouths to pump oxygenated water over their gills and so do not need to be moving forwards to breathe. Therefore, they are less susceptible to drowning when immobilised in fishing gear. These types of sharks are known to survive both capture in gear and handling on deck relatively well compared to the faster swimming species which breathe by "ram ventilation" (requiring forward motion to keep oxygenated water running over the gills (Ellis et al, 2017).
But even amongst ram ventilating sharks there are real differences in stress responses. For example, a study conducted with Mediterranean fisheries showed that 84.4% of blue sharks (Prionace glauca) were considered to be in a good condition after catch and release on longlines. A much smaller proportion of shortfin makos (Isurus oxyrinchus) and common thresher sharks (Alopias vulpinus) held up so well; only 43.8% of thresher sharks were considered to be in a good enough condition that they were likely to survive (Ellis et al, 2017).
Another study comparing the stress responses of different shark species, concluded that great hammerheads (Sphyrna mokarron) are especially at risk of post-release mortaility. Comparatively, tiger sharks (Galeocerdo cuvier) seemed to suffer less physiological disturbance after stress (Gallagher et al, 2014).
Even within one species, differences between individuals can also make one shark more susceptible to stress than another. For example, there is evidence that the sexes respond differently to the stress of catch and release, and that size of the shark can affect their likelihood of survival (Gallagher et al, 2014, Ellis et al, 2017).
Blood samples taken from struggling blacktip sharks (Carcharhinus limbatus) have shown that smaller animals have much higher levels of chemicals which are indicative of physiological stress, compared to larger sharks, when they fight on fishing gear for extended periods of time (Gallagher et al, 2014).
Physiological Changes can Cause a Shark to Die from Stress
When a shark is under stress, a slew of chemicals reactions can be detected in their blood: glucose and carbon dioxide levels rise as respiration increases, stress hormone levels rise, and blood acidity and body osmolarity changes, as the shark fights to escape (Gallagher et al, 2014).
Stress hormones (like corticosteroids and adrenaline) rise significantly in the blood during capture and release of shortfin makos, common threshers and blue sharks. You have probably felt a rush of adrenaline before when you were anxious or excited. These hormones lead to the production of energy, which stimulates shark to fight harder. However, they can lead to serious strain and exhaustion (Hassanein, 2010; Ellis et al, 2017).
There is also a large build-up of lactate in sharks' blood after they have been stressed during capture and release. Lactate is produced when muscles become fatigued, as they switch to aerobic respiration after prolonged exertion. Something similar has probably happened to you at some point - it is lactate build-up that causes the stitch in your side after strenuous exercise. Lactate levels rise significantly when blue sharks and blacktip sharks are hooked, and the longer the shark is left fighting, the higher the lactate levels rise. Basically the shark becomes completely exhausted from trying to escape (Hassanein, 2010; Gallagher et al, 2014; Hutchinson et al, 2021).
Dusky sharks (Carcharhinus obscurus) also have a significant rise in "plasma metabolites" magnesium (Mg2), potassium (K+) and calcium (Ca+) during capture and handling. This is because their heightened lactate levels, cause electrolytes to leak out of muscle cells. This is known as "osmotic stress" (Hassanein, 2010).
The problem is that these physiological responses can lead to the build-up of metabolites and shifts in homeostasis, and it can take a long time for the shark's body to restabilise after the stress of capture. The physiological strain can remain long after the shark has been freed, sometimes even causing them to die many days later (Hassanein, 2010; Ellis et al, 2017; Hutchinson et al, 2021).
How Can We Reduce Shark Post-Release Mortality?
Thankfully, all of these studies have given us the knowledge needed to reduce the mortality of sharks caught in fisheries:
Removing sharks from fishing gear as soon as possible, it a sure-fire way to reduce their stress and increase their likelihood of survival after release,
Changing the mesh size of fishing nets and making longlines from weaker material the shark can easily break, would allow sharks to escape if they became entangled,
Circle hooks, as opposed to J hooks, cause less significant injuries to a shark's mouth and gut. Switching hook types on longlines would reduce the likelihood of post-release mortaility by infection, starvation or predation,
Including an excluder grid on trawls, or shark repellant devices (like noxious chemicals, magnets and/or electrical fields) on other gears, and making fishing nets in brighter, more visibile colours, could help to reduce the amount of shark bycatch in fisheries,
Banning fishing gears which are notorious for shark post-release mortality and switching to more sustainable methods would be ideal. In Belize, gillnets have been completely illegalised for this very reason!
Bycatch in fisheries is one of the greatest threats to endangered species of sharks today. If we can design fishing methods with more care and train fishers how to safely release sharks, we could avoid many unnecessary and wasteful deaths, and maybe even save some species from extinction!
References
Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.
Ellis JR, Mccully Phillips SR & Poisson F (2017). Review of capture and post-release mortality of elasmobranchs. Journal of Fish Biology, 90:3, 653–722. Access online.
Gallagher AJ, Serafy JE, Cooke SJ & Hammerschlag N (2014). Physiological stress response, reflex impairment, and survival of five sympatric shark species following experimental capture and release. Marine Ecology Progress Series, 496: 207–218. Access online.
Hassanein L (2010). The physiological and physical response to capture stress in sharks. The Plymouth Student Scientist, 4:1, 413-422. Access online.
Hutchinson M, Siders Z, Stahl J & Bigelow K (2021). Quantitative estimates of post-release survival rates of sharks captured in Pacific tuna longline fisheries reveal handling and discard practices that improve survivorship. USA National Oceanic and Atmospheric Administration PIFSC Data Report, DR-21-001. Access online.
By Sophie A. Maycock for SharkSpeak
