On land and in the seas, animals and plants are being driven from their usual habitats by climate change. It's easier for us landlubbers to see on land than in the sea, yet a growing body of evidence indicates that fish are also on the move.
Ashore, animals are climbing to higher elevations or moving north or south to cooler climes, depending on the hemisphere. In the sea, some fish are diving deeper, and in the northern hemisphere some fish are fleeing toward the North Pole.
Different species are moving at different rates. We know that. What we don't know is if rapidly relocating to cooler climes is actually helpful to the fish.
A paper from 2023 found that fish everywhere live in the temperatures that match their thermal tolerances. In other words, they live where it's comfortable. This could ostensibly support the widespread perception that "fast adapting" fish who flee north to escape warming are "climate change winners." But are they?
If moving rapidly to a new area is adaptive, then those fish populations would be expected to do as well or better. But that had never been empirically checked.
Now it has been checked – and the news is bad, according to a paper published in Nature Ecology and Evolution by Shahar Chaikin and Jonathan Belmaker of Tel Aviv University, with an international team of colleagues.
The team focused on 146 fish species belonging to 68 families dwelling in mostly temperate and subpolar zones in the Atlantic, Pacific and Indian oceans. What they found is a "slightly negative" relationship between population trends and species range shift velocity.
In the cases of some species, the trend was not slightly negative, it was horrible.
For instance, the whiting pout (Trisopterus luscus) was found to have shifted range very fast and its population size was found to have declined by almost 93 percent every decade.
The greater pipefish (Syngnathus acus) also shifted swiftly and its population size declined by 75.5 percent every decade, the team estimates.
The team also estimates that the populations of the fastest poleward movers declined by around 50 percent every decade, very roughly speaking.
Bottom line: rapid range change seems to correlate with decline in fish population sizes.
That unfortunate association emerged in most ocean regions: the northeastern Atlantic and Pacific, and Oceania. The faster the population shifted poleward, the faster it declined, the scientists revealed.
They also checked a few tropical species, but Chaikin qualifies that data from the tropics are extremely underrepresented in global datasets: "This is a major knowledge gap that may hopefully shrink over time."
To be clear, the negative association between the speed of range shift and population trends wasn't caused by the extreme outlying values, Belmaker explained by phone from his car. Their study is a statistical analysis of the big picture. At such scales, even a slight negative trend may reveal substantial deterioration – that the trend among swift-shifting fish is, apparently, collapse.
Fish we like to eat
Take the European seabass (Dicentrarchus labrax). Actually, "sea bass" is a generic name for hundreds of species in the family Serranidae. Grouper is a sea bass, and so is the cony and the belted sandfish. Some sea basses are the size of your pinkie and some are the size of a minibus.
Their common attributes include carnivory, a long dorsal fin and relatively neutral flavor that have made them extremely popular among the seafood set. But we may have to curb our enthusiasm for the European variant. It's among the fish doing poorly after rapidly shifting to new waters, the team found.
Note that each fish to its own: the birdbeak dogfish (Deania calceus) isn't going north; it's swimming south toward the equator.
How was this study done? The team combined data from two sources: a database that tracks fish population size over time, and a database that compiles range shift velocities among marine fishes. Their goal: To test whether the shift seems advantageous or portended population collapse over time.
"The first dataset, Bioshifts, collects range shift velocities from published papers, explains co-author Chaikin. "The second database is called BioTIME and it gathers species abundance time series worldwide. It includes datasets that document the abundance of species at a specific geographical point and a specific time frame."
Over what time frame? That varies, but some datasets have abundance time series that span more than 90 years, Chaikin answers. Briefly, the examination spans 34 years on average, and the longest time series length was 97 years. "We estimated population trends by regressing abundance against years and extracting the slopes of the regression lines. However, a simple regression may underestimate the trends, and hence we employed more sophisticated models."
Asked how many fish populations declined among the 146 species they list, Belmaker explains that the trends they describe are the big picture: the patterns that emerge from the data. "We also tracked specific species, but our confidence in any specific species is low relative to the big statistical picture that emerges when we combine all the data," he says.
So: Most of the fish species seem to be staying put, with a tendency to shift northward. In other words, more of the species they checked are slowly repositioning north than are not. Population trends among the slowly shifting fish and fish that didn't move are roughly stable, the team found. They are not clear winners or losers of climate change, so far.
But the fast movers such as the sea bass are in trouble.
"Apparently, it is difficult for these populations to adapt to their new surroundings," Belmaker observes.
Why? There could be any number of reasons. But there was one possibly telltale oddity in the data. The problem is especially acute for the populations in the northernmost, colder part of the species' ranges.
That might seem counterintuitive. Wouldn't a fish suffering from warming do worse in the southernmost, warmer part of their range and better up north, that being the point of shifting?
Northern exposure
Put backwards, hoofing it northward quickly seems to do fish no favors – and this was most evident for the ones at the coolest part of the range, of all places.
"It might have been assumed that populations closer to the cooler polar margins of the species range would be less affected by climate change," Chaikin says. "We found that the opposite is true: fast poleward range shifts of populations from higher latitudes resulted in a more rapid decline in abundance compared to equatorial populations of the same species."
Crazy stuff. Why is there decline looking mostly toward the pole?
"We don't know," Belmaker responds. But one of the features of the destabilized climate is violent temperature fluctuations – on land, mark you. "Global warming" of 1.5 degrees Celsius versus the preindustrial era, which is reportedly the point we have already reached, is an average that doesn't reflect the extremes.
Recent years have featured freakish heat waves at both poles, featuring temperatures as much as 40 degrees Celsius (71 degrees Fahrenheit) above normal. The Arctic was warmer than Tel Aviv in the winter of 2020. And sometimes the weather gets much colder than it once was.
Point being, the fluctuations are greater, the sea surface is already failing to behave as expected (it's growing warmer, faster than we thought) and possibly, the poor fish that escape discomfort in their usual range and flee north find they swam from the frying pan into the fire.
"It's too cold for them in cold years. They get hammered," Belmaker suggests: they could suffer mortally during arctic cold spells, even if there are warmer years.
Note that in 2023 the team published a paper showing that fish along Israel's Mediterranean coast that flee the warming waters for cooler depths are showing population decline.
"We were surprised by the decline of poleward populations, as generally these may represent relatively cooler regions and hence were predicted to remain relatively stable," Chaikin says, answering the question of whether the results were surprising. "We were even more surprised to see equatorward populations of rapid shifters remaining stable. We predicted that these might show large declines as these may approach their upper thermal limits and hence extirpate. A lot of research still remains to be done about the mechanisms behind the patterns we describe."
So what have we? We have a failure to find the expected positive association between poleward range shift velocity and population trends – at the very coldest portion of the range, which debunks the assumption that rapid range shifts protects against local population declines, the team concludes. The poor fish can't win, can they? The fast movers are, surprisingly, climatic losers. If there is any conclusion from all this, it's that we need to get a handle on carbon emissions and, the team urges, give the badly adapting fish priority in conservation policy.
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