Picture Jeff Schmucker
Climate change may be responsible for the decreasing size of the oceans fish stocks. This is on top of migrating fish into cooler waters which has been tip toeing behind us for awhile now.
The surface temperatures in the Northeast Shelf Large Marine Ecosystem are rapidly increasing. But changes in local fish populations could be caused by any number of things.
But now, for the first time, a widespread study concludes that it’s not a local problem.
“Global ocean warming already affects the fisheries around the world by changing the species compositions of the catch,” explained William Cheung, a fisheries scientist at the University of British Columbia and the lead author of the study. He analyzed fish catches globally from 1970 to 2006. What Cheung found is a clear trend — not just here, but all over the world.
“We are seeing changes in composition of fish catch, with increasing catch of warmer-water species and less proportions of cooler water species,” Cheung said.
Cheung said the trend of more tropical and sub-tropical species is especially acute in the more temperate American waters off of Alaska and New England."
In addition,
Blue Fin Tuna are now being caught in the Arctic ocean, particularly off of the Greenland coast. And that fact is horrific news.
Mackerel have been increasing their reach since the mid-2000s, according to MacKenzie, moving from the European continental shelf out toward the Faroe Islands and on to Iceland.
The oily fish is a preferred prey for tuna, which usually only search in waters where the minimum surface temperature is above 11 degrees Celsius, said MacKenzie. That the tuna were brought in with a load of mackerel in 2012 suggests there was a school of tuna hunting the smaller fish, he said.
Finding bluefin tuna off Greenland is more evidence that climate change is shuffling the species swimming about the world’s oceans. Fish generally found in warmer waters are being spotted in regions formerly filled by cold-tolerant species or are expanding their range. Mackerel have moved into the waters south of Iceland, and anchovy now swim the North Sea.
According to a study published in
Global Change Biology, decreasing body size of fish is proposed as a universal response to the oceans increasing temperatures.
Here, we show that over a 40-year period six of eight commercial fish species in the North Sea examined underwent concomitant reductions in asymptotic body size with the synchronous component of the total variability coinciding with a 1–2 °C increase in water temperature. Smaller body sizes decreased the yield-per-recruit of these stocks by an average of 23%. Although it is not possible to ascribe these phenotypic changes unequivocally to temperature, four aspects support this interpretation: (i) the synchronous trend was detected across species varying in their life history and life style; (ii) the decrease coincided with the period of increasing temperature; (iii) the direction of the phenotypic change is consistent with physiological knowledge; and (iv) no cross-species synchrony was detected in other species-specific factors potentially impacting growth. Our findings support a recent model-derived prediction that fish size will shrink in response to climate-induced changes in temperature and oxygen. The smaller body sizes being projected for the future are already detectable in the North Sea.
An additional study published in
Nature Climate Change adds to the growing agreement among Marine Biologists.
Here, we employ a model to examine the integrated biological responses of over 600 species of marine fishes due to changes in distribution, abundance and body size. The model has an explicit representation of ecophysiology, dispersal, distribution, and population dynamics3. We show that assemblage-averaged maximum body weight is expected to shrink by 14–24% globally from 2000 to 2050 under a high-emission scenario. About half of this shrinkage is due to change in distribution and abundance, the remainder to changes in physiology. The tropical and intermediate latitudinal areas will be heavily impacted, with an average reduction of more than 20%. Our results provide a new dimension to understanding the integrated impacts of climate change on marine ecosystems.
Reporting on the warming oceans impact on the worlds most favored fish species, Clare Leschin-Hoar, "Widely consumed North Sea species, including haddock, whiting, herring, and others, have shrunk in size by as much as 29 percent over nearly 40 years, as water temperatures have increased between one and two degrees Celsius, researchers from the University of Aberdeen in Aberdeen, Scotland, revealed"
She notes that over fishing is a component of decreasing fish size, but the elephant in the room is increasing ocean temperatures.
Intense fishing pressure or low food supply may also contribute to smaller fish, but lead author Dr. Alan Baudron says concurrent size declines in different schools of fish points to climate change as a smoking gun.
“What is interesting is this was detected across a range of fish species eating different diets, living at different depths, and experiencing different levels of fishing mortality,” said Baudron. “The synchronicity suggests that the one common factor they all experienced—increasing water temperatures—could have been at least partly responsible for the observed reductions in length.”
Baudron and his team analyzed age and length data—collected by the International Councils for the Exploration of the Sea—from commercial fish caught between 1970 and 2008.
Not all species experienced a decline in size. Cod sizes were not affected, and female sole only shrank by 1 percent. Exactly why is unclear.
The question arises is how very subtle changes over a few decades impact the size of fish populations?
Baudron says the reason is that most fish grow more rapidly during their early life when temperatures are cooler. As oceans warm, young fish, known as juveniles, become mature at a smaller length; they don’t grow as large as they would have in colder water.
David Conover, professor of Marine Science in the School of Marine and Atmospheric Sciences agrees higher temperatures are the most likely the cause.
"It's well-known from theory and experiments that animals in warmer environments do generally tend to grow to lower maximum sizes, so the data match model predictions," he says. But that data only shows correlation, not causation. "Other factors like fishery-induced evolution could also be contributing."
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Indeed, warmer water does not seem to be the sole culprit. A study published in 2012 in Nature Climate Change also found that climate change has contributed to less oxygen in the oceans. The result? Fish with smaller body sizes.
“We were surprised to see such a large decrease in fish size,” said the study’s lead author, William Cheung, of the University of British Columbia, in a statement. “Marine fish are generally known to respond to climate change through changing distribution and seasonality. But the unexpectedly big effect that climate change could have on body size suggests that we may be missing a big piece of the puzzle of understanding climate change effects in the ocean.”