Seafood farms could step up to meet demand

Aquaculture already produces nearly half of the seafood we consume (or more, if you include seaweed), and we’ll have to increase that if we are to avoid decimating wild fish stocks. Under a fishing ban, aquaculture could be our only source of seafood, meaning that, initially, we’d be eating a lot of Atlantic salmon – by far the most farmed fish across Europe. “[Wild] fisheries allow you a diversity of products that aquaculture would probably take many years to get to,” says Dr Sofia Franco at the Scottish Association for Marine Science. But she hopes to see a wider range of farmed seafood on the menu in future, as expertise in different species and farming systems develops. Until now, production has been largely in farms open to the sea, rivers, or lochs. Newer, land-based systems, such as tanks with recirculating water, could reduce pollution and damage to aquatic environments compared to the older systems.

But could you supply all the world’s fish suppers without using a drop of actual seawater? Dr Rebecca Gentry, a marine scientist at Florida State University, suggests we wouldn’t need to. Theoretically, aquaculture in the sea could produce the equivalent of the world’s fishing catch in less than 1 per cent of the ocean surface, her 2017 paper shows. “It’s an interesting thought experiment,” she says. “If we close all wild fisheries, look at this huge amount of ocean area that we’re no longer having an impact on.” She doesn’t want to paint “too sunny a picture” of aquaculture, though, noting that any largescale food production fundamentally changes the environment.

Temporary bans on fishing of certain species are already used worldwide to maintain fish stocks and protect the environment. Some last a few weeks or months annually. These seasonal bans are designed to protect fish during their breeding seasons, for example, or to protect the sea bottom from damage, as with shrimp trawling bans. Others last most of the year, or longer, as in the current moratorium on fishing in the Arctic, which could last 16 years. A total global fishing ban would increase stocks, while helping to rebalance upset ecosystems. Eating less lobster thermidor, for example, would help keep seaweed forests in good health, as the crustaceans prey on sea urchins that destroy kelp – a type of seaweed.

However, there are no guarantees of a full recovery in our oceans. According to Purcell, some species are already so badly affected by overfishing that they might never recover. In Papua New Guinea, the edible sea cucumbers that he studies – popular in Asian cookery – have been so voraciously harvested that their populations are down to one-hundredth of their prefishing levels. “Once they get down to less than one animal per hectare, it’s very hard for the mates to find each other, particularly for these species that aren’t moving very fast,” Purcell says. “They have to crawl around on the seafloor to find each other.” Meanwhile, north of Australia, some shellfish populations exploited by Indonesian fishers have declined to the point where so few are now reproducing that rebuilding their populations looks impossible.