The Blight of Bycatch

Representing 40.4% of global marine catches with a multitude of direct and indirect ecological effects, it is unmistakable that around the world current governmental actions to address bycatch are insufficient.


Bycatch is the unintentional capture of marine animals (fish, turtles, mammals, and seabirds) whilst fishing for a particular target or size of species. When people hear ‘bycatch’ they often think of non-target species being caught up in the nets of fisheries who were aiming to catch a different species. For example, if you attended our recent webinar on the Vaquita porpoise (Phocoena sinus) you might have heard that this species is the world’s smallest and most endangered cetacean, which is often caught in gillnets targeting Totoaba fish (Totoaba macdonaldi) in the Gulf of California. However, the term bycatch also refers to the removal of undersized or immature organisms of the target species. If these young organisms are removed from the ocean before they have reached sexual maturity and have had young of their own, this can lead to population crashes.


Source: Porpoise.org (2021)

So, whilst the Vaquita is, unfortunately, an excellent example of bycatch directly threatening the species with extinction, bycatch can have significant impacts on the entire ecosystem throughout the food chain which is often overlooked. The bycatch of large-bodied herbivores or even predators high up in the food chain can lead to greater proportions of low trophic level species. This is referred to as ‘Trophic Downgrading’.


A clear demonstration of trophic downgrading is the bycatch of crabs in Brazil. Analysing the gillnets from artisanal fisheries found 4 lobster species and 10 crab species that were unintentionally caught by this indiscriminate fishing method. As a direct result of their removal, algae in the area increased by 75%. Without lobsters and crabs to graze on the algae, the algae population boomed and began to outcompete the corals. These crabs were also responsible for removing parasites from coral cleaning fish, removing sediment that could choke up the corals, and eating dead tissues of other benthic organisms. As they were continuously caught in gillnets as bycatch, disease and parasites spread rapidly and nitrate concentrations increased as a result of unconsumed decaying matter.


Source: Giraldes, B and Sampaio, C (2015)

Bycatch needs to be considered just as dangerous as overfishing. It does not only affect the species inadvertently caught but has a cascade of impacts throughout its food web. It is not simply a case of saying ‘return bycatch to the sea’ as most of the time the organisms are either dead or dying. They do not really stand a chance at life after being freed as they incur high levels of stress which reduces their ability to heal any injuries or fight infections obtained as a result of capture.


Despite major improvements in global consciousness and the initial developments of pole and line catching, stricter policies and increased governance is required to drive forward the development of better mitigation strategies. It is up to the consumer to drive demand for fish, which will nearly always contribute to bycatch. If people come together to demand better, stricter controls on the sourcing of fish or boycott the industry altogether then real change may finally occur.





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