University of Maine ecology professor Brian McGill helped discover that average fish body sizes are shrinking as part of an international project with 16 other researchers.

The team, led by York University macroecologist Ines Martins, analyzed body size trends for several plants and animals around the world from 1960 to 2020 using data from the BioTIME database. While size trends varied among organisms, the group experienced a dominant pattern of decline among marine fish species.

They also discovered that in some ecosystems, the number of individuals of larger fish species decreases, and they are replaced by individuals of smaller species. At the same time, the total combined biomass of all fish in these places remained fairly constant as a result of them being small-sized fish.

In the Gulf of Maine, for example, larger fish such as spiny cod and cod have shrunk in recent decades, as the average individual has become smaller. Their species is also seeing a decline in the number of individuals, and the numbers of smaller mackerel species are increasing. The net effect is a reduction in the average body size of a randomly selected fish in the Gulf of Maine.

This study is the first to show that this pattern occurs widely in fish around the world. The study also identifies body size as a key factor in why some fish species are replaced by others in different ecosystems, which will help scientists predict how the latter will respond. That’s why large stickleback cod being replaced by mackerel in the Gulf of Maine may face a similar fate to large cod.

The finding that total biomass remains roughly constant across many systems, even as the sizes and numbers of certain species change are also new findings.

Beyond fish, the overall pattern is less consistent. Some species are increasing in size and number and replacing smaller species, such as shrubs and trees in the Arctic replacing grasses and forbs, and the opposite is occurring in some parts of the desert southwest of the United States. However, in all cases, the researchers found that a significant portion of the change is actually driven by species of one size replacing species of another size, and that there is a general pattern of compensation such that the total amount of biomass remains fairly constant.

The team published their findings in the journal Sciences. Determining the reasons behind these trends will require further research.

“It’s really fascinating, as a scientist, to learn how some aspects of nature change so radically while other aspects of nature show so much stability,” McGill says. “But economically, this has significant practical implications in a fisheries-dependent state like Maine. Large fish are favorite fishing targets, but large fish species are becoming smaller and are being replaced by smaller species. We now know that this is not happening in Not just the Gulf of Maine, but all over the world.

Read the full story on York University website.