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Guardian of the Fishery

Updated: Jan 4

Sharks, rays and skates (known collectively as "Elasmobranchs") are amongst some of the most threatened animals in the world today. Globally 1/3 of all elasmobranch species are threatened with extinction and oceanic shark populations have declined by 71% on average over the last century. Many people assume that finning is the biggest driver of these declines... and whilst it is true that finning has been an enormous factor, many people don't realise that the incidental catch of sharks in commercial fisheries targeting a completely different species is actually also one of the greatest threats to sharks and rays! So why does "bycatch" happen? And how can we stop it from happening!?

Scientists are developing new technology that can help reduce shark bycatch in commercial fisheries (Image Credit: Ben Phillips / Pexels)

Bycatch is as Serious as Finning?!

A lot of people assume that finning is the biggest threat to sharks... and whilst it is true that finning has contributed to shark declines, it is not the only type of shark fishing that is problematic! Bycatch - where an animal is caught in fishing gear that is deployed to target a completely different species - has also been a massive driver in the decline of sharks and rays globally. The problem is that even if sharks are thrown back after they are caught, many die later-on thanks to stress or injuries (this is known as "post-release mortality"). Being crammed into nets and dragged through the water, thrashing to escape hooks or being poorly handled by fishers means that "at-vessel mortality" is also very common for sharks and rays (Molina & Cooke, 2012).

"Bycatch is the most frequent threat for sharks, accounting for 66.9% of shark species reported by the IUCN that are facing conservation threats".

- Molina & Cooke, 2012

Many people assume finning is the greatest threat to endangered sharks and rays, but bycatch is also an enormous threat all over the world (Image Credit: Andreas Altenburger / Shutterstock)

Reducing and Improving Bycatch

As it is such a serious issue, there have been many efforts to attempt to reduce shark and ray bycatch and also to reduce bycatch mortality in commercial fisheries. The problem is that there are many different types of fishing gears targetting different commercially valuable seafoods and each unique species of shark varies in how resilient they are to the stress of being caught, so there is no 'one-size-fits-all' solution (Molina & Cooke, 2012; Doherty et al, 2022). To learn more you can check out If You Love Me, Let Me Go and A Fish Out of Water.

Bycatch has been a huge driver of endangered sawfishes declines because their rostrum is easily entangled in fishing gear (image Credit: Ryhor M Zasinets / Shutterstock)

Changes in mesh shape and net sizes have been shown to help to somewhat reduce shark and ray bycatch in trawl fisheries. Changing hook size and shape can be helpful in long-line fisheries. Reducing soak times of the gear can also ensure that incidentally caught sharks can be released more quickly, with fewer injuries. Fisher handling of trapped sharks can also help to mitigate the impacts of bycatch, as proper handling techniques can reduce at-vessel- and post-release-mortality of sharks and rays. In some places limits are also placed on where and/or when fishing can occur to avoid seasons when sharks may be most vulnerable to being accidentally caught (Gilman, 2011; Molina & Cooke, 2012).

Endangered great hammerheads are especially vulnerable to mortality as bycatch, as they cannot recover from the stress (Image Credit: Martin Voeller / Shutterstock)

Shock Therapy

Scientists are now wondering if we may be able to develop more advanced technologies that can help to reduce shark and ray bycatch in fisheries... As sharks have a very acute electroreception sense, engineers have been trying to develop devices that use electrical currents and/or magnetic fields to repel sharks away from fishing gear. The beauty of this approach is that sharks and rays alone possess these electroreceptive organs (named "Ampulae of Lorenzini"), so these devices should not repel the fish or custaceas that the fishery is actually trying to catch (Gilman, 2011).

The inclusive of certain alloys and metals (known as "rare-earth metals"), such as lanthanide in fishing gear has shown some promise, as has the implementation of magnets onboard. However, these are yet to be perfected. Sometimes sharks become "habituated" to these methods, so they are only effective for a short time. Sometimes the metals degrade rapidly and become ineffective. In other cases, the techology is very expensive or difficult to impliment on the boat (Gilman, 2011).

The Ampullae of Lorenzini (seen as black spots) allow sharks to detect electric current and magnetic fields (© Sophie Maycock)

A New Hope?

A new technology called SharkGuard has been developed that could help to reduce shark and ray bycatch in commercial fisheries! SharkGuard produces a powerful, short-range electric pulse that can overstimulate the Ampullae of Lorenzini and cause sharks to flee (Doherty et al, 2022).

To test how effective this technology is, scientists equipped hooks in a long-line tuna fishery with the SharkGuard and then compared the how many sharks and rays were caught compared to naked "control" hooks. They measured shark catches using "Catch per Unit Effort" (CPUE), which is a rate of catches - counting how many animals were caught over a specific period of time, say one hour or one day.

Their results were startling and very exciting! They found that hooks that were equipped with SharkGuard had significantly lower catches of blue sharks (Prionace glauca) and pelagic stingrays (Pteroplatytrygon violacea) compared to normal hooks. As big as a 91.3% reduction for blue sharks and 71.3% for the stingrays in fact! (Doherty et al, 2022).

The technology is not yet perfect. There is some concern that the electrical fields may have slightly reduced tuna catches as well, which would obviously make fishers reluctant to use them in their work. It has also been challenging to build a battery that can hold enough charge to power the device over long time frames, but if we could compel fishers to see the benefits of the device, there is enormous hope that SharkGuard could have a huge impact on reducing shark and ray bycatch around the world (Doherty et al, 2022)

"Broad-scale deployment of a device such as SharkGuard could meaningfully reduce the number of sharks caught in longline fisheries".

- Doherty et al, 2022

Pelagic stingrays are commonly caught as bycatch in tuna fisheries (Image Credit: D Ross Robertson / WikimediaCommons)


Doherty PD, Enever R, Omeyer LCM, Tivenan L, Course G, Pasco G, Thomas D, Sullivan B, Kibel B, Kibel P & Godley BJ (2022). Efficacy of a novel shark bycatch mitigation device in a tuna longline fishery. Current Biology 32, R1245–R1261. Access online.

Gilman EL (2011). Bycatch governance and best practice mitigation technology in global tuna fisheries. Marine Policy, 35:590–609. Access online.

Molina JM & Cooke SJ (2012). Trends in shark bycatch research: current status and research needs. Reviews in Fish Biology and Fisheries, 22:719–737. Access online.

O’Connel C, Abel D, Stroud E & Rice P (2011). Analysis of permanent magnes as elasmobranch bycatch reduction devices in hook-and-line and longline trials. Fisheries Bulletin, 109:394–401. Access online.

By Sophie A Maycock for SharkSpeak.

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