Tuna carry evidence of the human causes of global heating

by Claire Heath | 17 February 2020 | News

School of tuna. Photo: Fabien Forget, ISSF.

A group of 22 scientists has discovered that tuna carry inside them clues to how climate heating caused by humans is changing ocean water and types and quantities of plankton – and therefore the tuna we eat.

At the core of their research is one of the elements at the heart of a heating planet: carbon.

By tracing two of the most abundant forms of carbon, the isotopes carbon-12 and carbon-13, they were able to show that a significant amount of global heating is caused by human activities rather than natural processes.

They also found that several other factors also influence the amount of different carbon isotopes in tuna. 

Scientists find evidence of changes in the food chain

One of the most important happens at the start of the food chain, with a group of plankton known as phytoplankton, which use sunlight and carbon to make the energy they need. 

The scientists showed that the abundance of different kinds of phytoplankton has changed in the past 15 years, directly as a result of the burning of fossil fuels. They also showed that the proportion of carbon-12 and carbon-13 available changes the kinds and abundance of phytoplankton. These changes don’t stop here, but alter the kinds and abundance of animals, including tuna, all the way up the food chain.

Numbers of some phytoplankton are shrinking, and this too is affecting the abundance and location of tuna. 

The change in the balance of phytoplankton is made worse by another effect of climate change: ocean stratification. Surface and deep waters of oceans now mix less, and that fewer nutrients are stirred up and made available for plankton to consume. 

The research also showed changes in how quickly phytoplankton grow. 

An example of phytoplankton. Photo US National Oceanic and Atmospheric Administration MESA Project.
Phytoplankton, the foundation of the oceanic food chain. Photo NOAA MESA Project.

The scientists traced two forms of carbon

The research involved scientists from several fields. Among them was Valérie Allain of the Pacific Community (SPC). 

The scientists took 4,500 samples of muscle from albacore, bigeye and yellowfin tuna over 15 years, from 2000 to 2016, from the Pacific, Indian and Atlantic Oceans. They found that changes were most pronounced in the Pacific Ocean.

They traced two forms of carbon, carbon-12 and carbon-13. This is possible because each isotope each has a different weight and also behaves slightly differently. 

Carbon is found naturally in living things, and in the air, land and water. It is also present in coal and oil, and when these burn, carbon-12 is released into the atmosphere. 

More than 90% of atmospheric carbon is absorbed by the oceans. From there, it enters the food chain, being taken up by plankton and passed on to each predator up the chain, until it ends up in tuna, along with other forms of carbon such as carbon-13.

Reporting on their findings in the most recent issue of SPC’s Fisheries Newsletter, Valérie Allain and another researcher, Anne Lorrain, said that the data will be “of inestimable value” in projecting the effects of climate change on the health and quantity of seafood, and in validating modelling. This is because they collected so much data over such a long time and a very large geographical area.

Their research makes much more certain that humans do affect the environments and inhabitants of the open oceans.