On 14 June, the University of Washington released substantial analysis casting further doubt on the Sala et al. 2021 study that was published in the journal Nature, which was used to claim bottom trawling released as much carbon as air travel.

The critique of the Sala study’s questionable modelling and peer-review process is a must-read for anyone interested in ocean ecology, climate change, primary industries, and the efficacy of marine protected areas (MPAs). And because the Sala study came with the proposal to increase the number of MPAs, let’s hope policy analysts and decision-makers in this field have bookmarked the counter studies and response papers:

  • Epstein et al. 2022, which found contradictory results to the modelling in Sala; and
  • Hiddink et al. 2023, which concluded the Sala models over-estimated CO2 release by 100-1000 times.

The Sala study’s modelling indicated fishing nets that contact the seafloor disturb and re-mineralised significant amounts of stored carbon. However, this appears is not the case. The new analysis explains that as organic carbon in the ocean – e.g., from an organism that has died – starts to sink, it either decomposes or is eaten. Most of it is mineralised into carbon dioxide and dissolved into seawater to be reused by living creatures.

Only a small portion of organic carbon even reaches the seafloor, where most of it is also consumed. The small amount of carbon that gets buried in the seafloor’s sediment is what acts as “carbon storage”.

Fishing nets only contact the very top layer of sediment, if at all. The University of Washington points out carbon stirred up by bottom trawling is likely to be the same carbon that would re-enter the water column anyway when broken down by the activity of microbes or other organisms. The vast majority never reaches the atmosphere as it’s recycled within the seafloor ecosystem.

Closer to home, seafood’s low carbon footprint is supported by a recent study commissioned from Crown Research Institute AgResearch by the Seafood New Zealand Deepwater Council. The study measured the average carbon footprint of seafood harvested on 21 deepwater vessels between 2021 and 2022.

Results indicate that with an average footprint of 1.19kg of carbon emissions per kilogram of whole fish, wild-caught seafood such as hoki and orange roughy have one of the smallest averages globally.

It also has the lowest carbon footprint per 100g of protein when compared to New Zealand beef, dairy beef, sheep meat, milk, farmed oysters and mussels.

Although the study did not take into account any CO2 that may be released from the seabed by trawling, those impacts look increasingly minimal, and nothing like the alarmist headlines that were initially generated.