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 in a study published in the April issue of Global Change Biology.
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.
“Disentangling the effects of these factors can be extremely difficult,” Baudron said.
So just how could a subtle change in temperature over several decades impact the size of a vast variety of species?
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."
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.”