Red mullet are moving into Scandinavian waters in significant numbers. Courtesy British Chefs.
There is plenty of hoohah over the fact that the oceans are warming and melting the ice caps, threatening our shoreline towns and cities. But what about the folks that live in the sea? How is the increase in sea temperature affecting them? It seems that, like humans, many species are moving away from threatened zones. Yes, it’s true - fish are emigrating. By James Teo.
Sydney, Australia. July 2021. “What we're seeing in our oceans is that everything's starting to look the same,” says marine researcher at the University of New South Wales, Associate Prof. Adriana Vergés. “Before, we had tropical systems that were very different to temperate, or colder water systems. But now with … tropical fish coming south, there are less stark differences between the tropical and temperate regions.” The northerners are moving down south to cooler waters.
“With climate change, things are changing so fast.” - UNSW’s Adriana Vergés. Courtesy UNSW.
And in the northern hemisphere, the same is happening but in reverse. Species only previously associated with warm equatorial waters are being found in more northerly, previously colder seas.
Indeed, the coast of northeast America, and particularly the Gulf of Maine north of Cape Cod has warmed faster than many other marine regions. This, say the scientists is global warming exacerbated by natural oceanic currents such as the Atlantic Multidecadal Oscillation. These together saw an increase in sea surface temperatures of more than 2 deg C in the decade up to 2013.
This is bad new for fish populations. Fish are cold-blooded animals, and so are massively sensitive to changes in ambient temperature. Most also (in order to keep from getting chilly and slow) live in habitats close to their upper temperature limit, meaning that even a slight increase in temperature makes them overheat, affecting their ability to feed and breed.
Rising sea temperatures also impact the food they normally eat – in some places the handy meadows of sea grass and kelp have simply withered and died due to excessive temperatures, leaving local fish with no option but to uproot and head for the poles where cooler water keeps their food chain lively.
“Some “(tropical) fishes might be found across more of our coastline while others will become less abundant. Our study has shown that the loss of kelp … has negative effects on temperate fish but not tropical species” says Vergés.
Faroe Islands fishing fleets were caught by surprise at dramatic changes in whiting fish stocks. Courtesy Tridge.
And this upsets not just the fish, but the fishermen too. In the late 90s, around the North Atlantic Faroe Islands, numbers of Blue whiting suddenly surged by a massive amount. Within a few years, it became the third largest marine fishery in the world. But then the warming seas changed things again – the fish moved on to cooler water. This left a huge fishing fleet and only very few fish.
"Research … showed that this (sudden) high-productivity was associated with dramatic changes in the North Atlantic climate,” says Denmark-based researcher Mark Payne from the Technical University of Denmark.
Payne says that unusual species, like Red Mullet and hake, are also migrating to Scandinavian waters although they were previously only found much farther south. “They are moving because (in the sea) they can't find respite from unfavourable temperatures, unlike land mammals,” he explains. “On land, if it gets too hot, you can go and shelter under a tree or maybe burrow, whereas the ocean tends to be much more uniform. There is no shelter," he says.
Tasmania’s kelp forests are suffering from climate change related die back and possible over grazing. Courtesy UTAS.
This migration has a knock on effect too – competition for fish food is on the rise. In parallel with the arrival of warm-water species from the Bay of Biscay and the Mediterranean such as tuna, hake and anchovy, local fisherfolk have noticed a distinct decline in previous fish species, such as Baltic cod.
Not only that, the fish are getting smaller as competition rises. One report indicated that young cod are generally smaller than in previous years, as they are using more energy for reproduction than on growing. And in northern Europe, says Prof. Steve Simpson, a marine researcher at the Exeter University, sardines have replaced herring, cod and haddock are heading north, and “flat fish are really in trouble."
"Unless (flat fish like flounder) can change their habitat and diet in the next 20 or 30 years, or adapt to two degrees or more warming - which is a big ask - then they will decline," he warns.
The knock-on effects on multiply. Herring have all but vanished from the Gulf of Maine – resulting in starving baby puffins, which cannot swallow the new inhabitants, butterfish, that have moved in to their place.
“With climate change, things are changing so fast that you cannot preserve what used to be there,” says UNSW’s Vergés. “You really need to (try to) understand what's coming up in the next 10 years.” She says that new rules and expectations will be demanded to help maintain healthy ecosystems that have been invaded by fleeing migrants from overheating waters.
“Our studies can help us understand what’s coming in the future, which in turn can help us manage our marine ecosystems in a more dynamic way,” she says.