Global heating and the curious case of shrinking fish

This story was originally published by The Guardian and appears here as part of the Climate Desk collaboration.

A global analysis of thousands of animal and plant sizes has found that species are shrinking, an effect most clearly found by researchers in changes to the body size of fish, which are getting smaller.

Species such as the thorny skate, a North Atlantic fish that can grow up to a metre in length, have become smaller, while smaller-bodied species such as mackerel are growing in abundance, according to the researchers, changing the composition and functioning of ecosystems.

While shrinking was most commonly observed in fish, it was also recorded in some plants and invertebrate species. Other species were found to be increasing in body size, such as plants in the Arctic.

Scientists are still researching the causes of such changes in body sizes, some of which exceed 10 per cent, but they have suggested it may be the consequence of global heating and overconsumption — defining features of the Anthropocene, the proposed term for our geological epoch that describes humanity’s impact on the Earth.

The report supports other findings about shrinking fish sizes in the North Sea and smaller Pacific salmon in Alaska, which some researchers believe is linked to the climate crisis.

The study, published in the journal Science, was carried out by an international team of scientists from 17 universities, compiling data on 4,292 mammals, invertebrates, plants, fish, amphibians and reptiles, including a range of seabed species.

The lead researcher, Dr. Inês Martins from York University, said: “The core finding is that the body size is not just predominantly shrinking but that organisms are becoming smaller through a combination of species replacement and changes within species populations.

“Within some species, individuals are becoming smaller and smaller. And larger species are being replaced by smaller ones when they disappear.

“These trends were most evident among fish, where we saw clear evidence of shrinking body size. For other organisms, we have less data available and we don’t really see any changes from the average [but] it is unquestionable that we are observing quite big changes in biodiversity and the type of biodiversity we find in different places,” she said.

Small numbers of large organisms were being replaced by many smaller ones, keeping the biomass constant, according to the paper, supporting the idea that ecosystems compensate for changes.

A senior author of the paper, Prof. Maria Dornelas of the University of St Andrews said: “We think this suggests that, when large organisms disappear, other ones try to take up their place and use up the resources that become available.

“Recognizing and exploring this complexity is imperative if we want to understand the mechanisms involved in how body size is changing through time,” she said.

“It’s clear the widespread species replacement we see around the world is having measurable consequences. Organisms becoming smaller has important effects as the size of animals mediates their contribution to how ecosystems function, and how humans benefit from them — bigger fish can usually feed more people than smaller fish,” she said.

Dr. Franziska Schrodt, a co-author from Nottingham University, said: “Unfortunately, we currently lack data on many organisms other than fish to draw clear conclusions — future research will benefit from a greater investment in these kinds of measurements, particularly when exploring food webs and other species interactions.”