Sharks around the world are in grave danger; they are apex predators, controlling fish populations and maintaining healthy oceans. Ocean health, and therefore the health of the planet, are reliant on shark populations. The dangers that sharks face every single day have led to drastic decreases in their numbers. Sharks are generally large and long-lived, contributing to their evolutionary success, but they don’t produce many offspring which is now causing them problems when faced with human effects. This is why shark protection, and mitigation against human damage is so incredibly important.
A sometimes overlooked issue for sharks is bycatch, when non-target species are caught. Sharks can become tangled in gear, or are attracted to bait. Catching non-target species is disadvantageous for the fishermen, as well as for the sharks; it is a waste of time, money, and resources to catch a species that you are going to throw back into the sea, when you could be catching more economically viable species.
In the Gulf of California, Mexico is an important Pacific Sierra fishery. Pacific Sierra is used for the popular Mexican dish ceviche, raw fish, marinated in citrus juices for a tasty appetiser. This fish is caught with gill nets, large sheets of net that catch the fish’s gills as they attempt to swim through. However the Pacific Sierra fishermen of the Gulf of California have a huge Hammerhead shark bycatch problem. The famously shaped head easily becomes tangled in the net, catching the shark, and causing its death.
The Pacific Sierra fisheries are close to the coast, which also happens to be a busy Hammerhead nursery area. Hammerhead shark females come to birth their pups in the safe, shallow waters. However these waters are no longer as safe as they once were due to the fishing intensity, and the shark’s vulnerability to the gear. While the very young sharks may be safe due to their proximity to shore the problem arises when pregnant females or juveniles move into or out of the area. They effectively have to ‘swim the gauntlet’, avoiding fishing nets, to make it out to the open ocean.
The capture of juveniles and pregnant females is the worst scenario for shark bycatch. Sharks are being caught that are either too young to have reproduced yet, or haven’t even been born yet! Effectively wiping out an entire generation of Hammerhead sharks, causing serious consequences for the ocean. Sharks tend to eat large predatory fish, which in turn eat smaller predatory fish, like the Pacific Sierra. Therefore the consequences of Hammerhead bycatch loop back and impact the fishermen, as their catches of Pacific Sierra are depleted if their predators are no longer controlled by the sharks.
However the situation can still be changed, there is hope for Hammerhead sharks! If the nursery areas can be identified restricted areas can be created, where fishing cannot take place. With a large enough buffer zone around the nursery Hammerhead juveniles and females can go to and from the nursery areas safely, and the oceans can become healthy again!
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Coiraton C.; Amezcua F.; Salgado-Ugarte I.H. 2017. Estructura de longitudes de las capturas del tiburón martillo común (Sphyrna lewini) en el Pacífico mexicano. Ciencia Pesquera. 25(1): 253-259
Schindler D.E.; Essington T.E.; Kitchell J.F.; Boggs C.; Hilborn R. 2002. Sharks and tuna: Fisheries impacts on predators with contrasting life histories. Ecological Applications 12(3):735-748