How ‘sustainable’ seafood is contributing to the climate crisis

Supposedly sustainable fishing methods may have worse impact on carbon emissions than so-called unsustainable methods, according to a new study.

Researchers at the University of California, Santa Cruz and the National Oceanic and Atmospheric Administration found methods that reduce ‘bycatch’ could emit more greenhouses gases than trawling.

Bycatch, the fish and other mammals unintentionally caught when trying to catch a target fish, such as tuna, was one of the focal points of Netflix’s newest sustainability documentary Seaspiracy.

A key claim of Seaspiracy was that industrial fishing was unsustainable due to bycatch. However, the new study suggests the issue of seafood sustainability is more complex.

The researchers pointed out that although bycatch is an often-scrutinised topic, the relative climate impacts of methods hailed as more sustainable are being overlooked.

The study compared trawling to more selective tuna fishing methods – which results in less bycatch – by looking at greenhouse gas emissions and efficiency.

Selective equipment targeted at tuna, such as trolling lines which reel fish in one by one, are associated with higher greenhouse gas emissions, the researchers reported in the journal Elementa.

This was because vessels had to travel greater distances or spend longer at sea to catch their required amount of fish, meaning they used more fuel.

In comparison, bycatch-intensive methods such as purse seine trawling nets are very effective at catching fish and only require shorter journeys, so less fuel is used.

The study showed skipjack tuna had up to 12 times more estimated climate forcing – a measure of greenhouse gas emissions – when pursued with trolling gear than purse seine gear.

Skipjack caught with purse seine nets had an estimated carbon footprint almost as low as plant-based protein sources such as tofu, while the skipjack caught by trolling had a carbon footprint similar to that of beef.

Not all sustainable fishing results in higher carbon emissions. Albacore tuna caught on trolling had negligible bycatch and a low estimated climate impact, the study found.

The researchers pointed out the “explosion of popular sentiment surrounding sustainability and seafood” shows a disconnect between two metrics for sustainability: certification and carbon footprint.

Seafood rating systems which offer brands certification of minimal bycatch, such as dolphin-friendly tuna, claim to offer one type of sustainability.

But they do not necessarily take into account the greenhouse gas emissions of their fishing practices.

The researchers concluded a central goal of more sustainable seafood practices is necessary to minimise environmental damage.