A Fish Out of Water
One of the greatest threats to sharks today is fisheries. The Food and Agriculture Organsiation (FAO) estimates that in the region of 700,000 tonnes of sharks and rays are fished from our oceans globally each and every year! But it is important that we do not jump to conclusions and assume that all fisheries are bad for sharks... In fact, every fishery is quite different. Some shark fisheries are even sustainable! So which types of fisheries are most harmful to sharks? And how can we start to improve things?
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. Where "targeted shark fisheries" use gear specifically designed to catch sharks, "catch-all fisheries" use a combination of many different types of gears at once, to target a wide-range of commercially valuable species, including sharks. Other fisheries do not seek to catch sharks at all (Ebert et al, 2021).
That doesn't mean these fisheries don't catch sharks by mistake though! In fact, one of the greatest drivers of population declines of sharks, is their "bycatch" in fisheries targeting other commercially valuable species. Sometimes sharks caught as bycatch are landed with the rest of the catch. In other instances, they are removed from fishing gear and thrown back. Yet, whilst some some sharks do survive this process, many die some time later from stress or injuries (this is known as "post-release mortality") (Ebert et al, 2021). To learn more you can check out If You love Me, Let Me Go!
So which types of fishing gear affect sharks and which are potentially less damaging?
"Longlining" or "long-line fishing" is specifically used to target sharks, as well as marlin and swordfish. It consists of a central line with many shorter bait lines projecting off it at regular intervals. This is a very efficient method for shark fishing, responsible for a huge proportion of the sharks caught by targeted fisheries (Ebert et al, 2021).
And yet, compared to some other gears, long-lining is actually a relativly less destructive method of fishing, as it is fairly easy to unhook and release any bycatch animals. For instance, a study of Mediterranean longline fisheries showed that, overall, only 5% of sharks caught on longlines were dead and could not be released safely (Ellis et al, 2017, Ebert et al, 2021).
However, the type of hook used can have a significant impact on shark survival. Circle hooks can be removed more easily, where J-hooks can be swallowed and become deeply embedded into internal organs (known as "gut hooked"), resulting in significant injuries. This causes high levels of "at-vessel mortality" in sharks caught as bycatch (Cosandey-Godin et al, 2021).
Long-line fisheries also have high shark bycatch mortality if they have long "soak times"; meaning the lines are left in the water for long periods and/or not checked regularly. If a shark is left to thrash on a hook for a long time, it can easily suffer post-release mortality as a result of stress or injury (Ellis et al, 2017, Ebert et al, 2021).
Purse seine fishing is also (relatively) not too harmful to sharks. This method involves using an enclosed net, which surrounds large groups of schooling fish, like mackerel, anchovies and tuna, on all sides. Purse seines generally have little bycatch, but they can still be fatal to the sharks that they do catch. For instance, studies show that 81 - 89% of silky sharks (Carcharhinus falciformis) caught as bycatch in purse seines will die, with the smaller individuals having higher mortality rates (Ellis et al, 2017, Ebert et al, 2021).
Furthermore, modern purse seines employ "fish aggregating devices" (FADs) - raft-like devices, with a complex structure, designed to encourage fish into densely packed groups. Whale sharks (Rhincodon typus) are drawn towards high density fish schools, so are often caught in purse seines utilising a FAD (Ellis et al, 2017, Ebert et al, 2021).
Trawling involves towing an enclosed fishing net behind one or more boats. "Bottom trawls" (aka benthic or demersal trawls) drag the gear along the sea bed, whereas "mid-water trawls" pull it through the water column. The different heights are used to target animals (Filmalter et al, 2013, Ebert et al, 2021).
Trawling represents a significant threat to sharks, as they are commonly caught as bycatch. Bottom trawls bycatch many rays and small sharks, and mid-water trawls can catch larger sharks too (Ellis et al, 2017).
The process of being dragged behind a boat in a net and hauled onboard is so stressful, that even if they are released from trawls, many sharks die. For example, sharks, skates and rays caught as bycatch in prawn trawls suffer high rates of at-vessel mortality. Interestingly, male sharks seem to be more at risk of this stress-related mortality; with as many as 66% male sharks dying, compared to only 23% of females caught as bycatch in trawls (Stobutzki et al, 2002).
By far the worst gear-type for sharks are gillnets. Gillnet fishing involves dropping a rectangular net into the water, so it is suspended vertically. The mesh is designed to catch in the gills of fish, making it impossible for them to struggle free, with different mesh sizes used to target different species. Many sharks and rays (and many other animals for that matter) are caught as bycatch in gill net fisheries and there is a very high mortality rate (Ellis et al, 2017, Ebert et al, 2021).
This is because gillnets cause serious damage to the gills and can render sharks unable to breathe or feed after release (Ebert et al, 2021).
For example, studies have shown that the overall mortality of Atlantic sharpnose sharks (Rhizoprionodon terraenovae), blacknose sharks (C. acronotus), blacktips, (C. limbatus) and bonnetheads (Sphyrna tiburo) caught in gillnets, are as high as 78·6%! (Ellis et al, 2017).
Capture in gillnets can also lead to post-release mortality. This is especially true if sharks are left in the net for a long period of time. For instance, studies of spiny dogfish (Squalus acanthias) caught in gillnets for 19 - 24 hours have found that, suffer immediate mortality of only 17.5%, but overall mortality can shoot up to 55% over the next 48 hours. The production of stress hormones and the strain from injuries caused by gillnets, can have prolonged physiological effects, which may kill sharks long after they are released (Ellis et al, 2017).
Sharks and rays are fished at an absolutley unsustinable level today! And with over a quarter of all sharks and rays now considered at risk from extinction, it is of critical importance that we reduce the mortality of sharks in fisheries. Especially, needless mortality when they are caught as bycatch! We must enforce strict quotas, which limit the global landings of threatened species and we must also demand that fishing gears which are associated with high bycatch mortality are abolished! We already know ways to modify fishing gears to reduce bycatch or make it possible for non-target species to escape, and these solutions are often very simple and inexpensive! If we can cure Covid, fly to the moon and build hadron colliders, there is no excuse not to use our ever-advancing technology to design better, less destructive fishing gears!
Some shark fisheries have already been declared as sustainable! To learn more, you can check out Can Shark Fishing Be Sustainable?
Cosandey-Godin A, Carlson JK & Burgener V (2012). The effect of circle hooks on shark catchability and at-vessel mortality rates in longlines fisheries. Bulletin of Marine Science -Miami, 88:3. DOI: 10.5343/bms.2011.1054.
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.
FAO (2020). International Plan of Action for Conservation and Management of Sharks. Access online.
Filmalter JD, Capello M, Deneubourg J-L, Cowley PD & Dagorn L (2013). Looking behind the curtain: quantifying massive shark mortality in fish aggregating devices. Frontiers in Ecology and the Environment, 11:6, 291-296.
Stobutzki IC, Miller MJ, Heales DS & Brewer DT (2002). Sustainability of elasmobranchs caught as bycatch in a tropical prawn (shrimp) trawl fishery. Fisheries Bulletin, 100, 800–821. Access online.