Farmed shellfish and seaweed have the best credentials for sustainable food production at sea. They contribute little to carbon dioxide, sulfur and nitrogen emissions and put little pressure on natural resources on land. Small-scale fishing also scores well in these areas, as does the farming of certain fish species. But the differences between the industries are large and many improvements are possible. That is what researchers from all over the world state in a study to emissions from seafood production. The research is part of a series of five articles that this week appears in the magazine Nature.
Food from the sea is seen as holding great promise for feeding all the mouths of the earth. There is an increasing demand for fish, shellfish and seaweed – both farmed and harvested from the wild. But this also raises concerns about its sustainability. Apart from issues such as overfishing, the disturbance of the bottom by beam trawling and pollution around breeding tanks, there are also other forms of environmental pressure: the emission of carbon dioxide, sulfur and nitrogen (for example by the ships that collect the food) and the consequences on land, for example, from the cultivation of soy as fish feed. This substance balance has now been mapped out on a large scale for the first time.
Nurseries and Fleets
The researchers made models in which they calculated almost three quarters of the world food production at sea, based on data from 1,690 fish farms and more than 1,000 fishing fleets. For the first time, researchers can now compare the environmental impact of different forms of marine food.
“It is very good that these environmental influences have been portrayed in this way,” responds Marnix Poelman, project leader of the Blue Growth program at Wageningen Marine Research, who was not involved in this Nature research. “Model calculations have already been made before, including by ourselves. But it has now been done on a much larger scale, and the authors also provide clear perspectives for improvements.”
Isn’t it a pity that overfishing, seabed disturbance and pollution around fish farms are not covered in this study? “The total picture is important,” says Poelman. “If you take food from the oceans on an increasingly large scale, which we are doing now, you have to limit the environmental influences as much as possible. Certainly, you have to work on overfishing and pollution, but improvements are also needed in the area of that carbon and nutrient balance.”
Soy as fish food
The researchers included all kinds of aspects of the carbon and nutrient balances. For example, shellfish emit carbon dioxide when they grow. On the other hand, they also sequester carbon in their biomass. Fishing vessels and fish farm machinery emit carbon dioxide, sulfur and nitrogen. The fish from those fish farms eat food that sometimes comes from the sea, such as fish meal, but sometimes from land, such as soy. Their droppings fertilize the ocean. Seaweed, on the other hand, absorbs sulfur and nitrogen from the water in order to grow. How that balance works out differs per sector and per location, the researchers emphasize.
Flatfish and shrimp
On a net basis, seaweed and farmed shellfish appear to come out the most favourable. Fishing for flatfish and for lobsters and shrimps score the worst. Salmon and carp do relatively well among farmed fish, and tilapia relatively poorly – but still as good as chicken.
“It is important that these authors propose concrete improvements,” notes Poelman, “for example, in the field of fish feed. By selective breeding you can ensure that farmed fish convert fish feed into biomass more efficiently. And in fisheries, we can focus on systems that are more animal-friendly and emit fewer greenhouse gases.”
The researchers emphatically do not conclude that it is a bad idea to get more food from the sea than we do now. Poelman: “This study mainly shows where the differences lie between the forms of food from the sea, and where we can make the most profit in terms of sustainability.”