Uncharted Waters

Sardines are on the move. Anchovies and mackerel, too. And it’s not for some hijinks-filled, forage-fish version of Road Trip.

After looking at nearly 50 years of data from 57,870 survey trawls, where scientists scoop up the tiny fish with massive nets, researchers say several forage fish species of the North Sea are disappearing from their old swimming grounds and showing up in new waters. I’ll give you one guess why they’re headed north. That’s right, climate change.

The study, published in Global Change Biology last month, isn’t the first to show that climate change is spurring migrations, of course. We’ve seen pikas and polar bears surrendering territory and pink sea slugs conquer new shores in search for more suitable homes.

But when something happens to forage fish, it sends ripples throughout the entire ocean. Sardines may not have the tastiest reputation among humans (c’mon, Tom Hanks, just eat it already), but they are crucial fixtures in marine food webs.

In the open ocean, forage fish act as middlemen between microscopic zooplankton and massive predators like whales. Sardines roam the high seas in schools 10 million strong, devouring tiny copepods by the billion. In turn, the silver army is eaten by just about everyone else—from bigger fish to seals to dolphins to humpbacks. Even seabirds get in on the forage frenzy. And when the avian, mammalian, and piscine predators all chow down at once, it makes for one of the most jaw-dropping displays in nature.

So what happens when these swirling, silver fish balls start playing a game of musical habitats?

Ignasi Montero-Serra, the study’s lead author and researcher at the University of Bristol in the United Kingdom, says that’s a tough question to answer.

For better or worse, how the redistribution of forage fish will affect the ecosystem (let alone the fishing industry) remains to be seen. Montero-Serra says the research only proves that large changes are occurring as a result of global warming. Whether the repercussions are good, bad, or ugly, there’s no denying that they’re different from what came before.

With measurements taken from the Global Ocean Surface Temperature databank, Montero-Serra and his colleagues looked at water temperatures in the North Sea from the last half-century or so. They found that the sea has been warming by an average of 0.02 degrees Celsius per year, with some areas showing increases of up to 1.3 degrees.

During this time, the North Sea’s fish community also underwent a quantifiable shift. Between 1972 and 2012, Atlantic herring and European sprats declined significantly, while the numbers of Atlantic mackerel and Atlantic horse mackerel, who usually prefer warmer seas, started climbing. At the same time, European pilchards (a.k.a. sardines) and European anchovies began swimming up from the south.

Scientists don’t know for sure why these shifts occurred, but higher temperatures have been shown in other studies to alter the life cycle of these fish in many curious ways. Montero-Serra says warmer waters can increase the duration of spawning periods and speed juvenile growth rates while decreasing survival rates over winter. They may also mess with mating locations.

And where the big sardine ball goes, predators might follow. In 2012, bluefin tuna were caught in the high latitudes east of Greenland, well outside the species normal range. Another study from 2007 found a “rapid northwards range expansion” for the Balearic shearwater. The study covered in last week's seagull story also illustrates the ways a change in forage fish populations can affect their predators for generations.

“These systems are highly dynamic and complex,” says Montero-Serra. “In my opinion, we are still very far from anticipating accurately how they will respond to further warming.”

In other words, it’s not as simple as a particular fish enjoying a particular temperature and then migrating to find that sweet spot. Every species responds differently based on its unique characteristics, and a fish’s biology is tailor-made to work a certain way at certain temperatures.

One tweak in behavior may lead to another, which may lead to another, which may lead to a tiny silver fish (and a few million of its friends) swimming toward strange new seas.