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Are fish shrinking?

image: boy fishing.

Scientists discover a global phenomenon, but the reasons behind it aren’t clear cut.

Research Need                        

Global demand for seafood continues to grow. Increased wild-capture fishing efforts have resulted in the decline of numerous fish populations worldwide. We’re catching more fish than some fish populations can sustain.

But scientists want to know whether sheer numbers of fish are the only problem, or whether other factors are at play in the ability of a fish species to maintain its population. 

What did they study?

An international research team looked at the body sizes of multiple species of marine fish both through measurements in the field and average estimates from published databases. They were searching for evidence of global and widespread shrinking.

In total, the scientists considered 5,025 time-series from 45 studies around the globe. These time-series ranged from 5 to 56 years, spanning from 1960 to 2020, and covered 1,971 species. They then determined changes in average individual body sizes over time, as well as changes in total number of different fish species in the same habitat areas.

What did they find?

Nearly three-fourths of the marine fish populations in the study have experienced a decrease in the average size of individual fish. The research team also found that, on average, the variety of fish species, as well as the abundance of fish in a given habitat area, slightly increased through time.

However, the total weight of the fish species in this study has not decreased.

Anything else?

While the research team concluded many marine fish species are becoming smaller, they could not conclusively determine a specific reason, citing three possibilities: the selective capture of large-bodied fish (often for their higher market prices and meat yield), the warming climate, and/or the reduced availability of resources like food and suitable habitats.

So what?

The researchers theorize that when larger fishes disappear, smaller ones take their place and use the resources that become available. The total quantity or weight of fish in each area therefore is quite stable, as decreases in body size trade off with increases in overall numbers. 

But size changes are not without impact. Often the larger–body size fish are top predators, and their loss disrupts predator-prey dynamics. Bigger fish also produce a larger number of eggs. These patterns suggest that body size, numbers, and total weight of fish species living together in an area are linked, and that change in one has implications for change in the others within an ecosystem.


Martins, I.S., et al. (2023) Widespread shifts in body size within populations and assemblages. Science381(6662): 1067-1071. DOI: 10.1126/science.adg6006.


This work was supported by funding from a Marie Sklodowska-Curie Actions Individual Fellowship, the European Union Horizon 2020, the German Research Foundation, the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior-Coordination for the Improvement of Higher Education, the Leverhulme Trust, the European Union, a Schmidt Science Fellowship, a Fisheries Society of the British Isles PhD Studentship, the Knut och Alice Wallenberg Foundation, the U.S. Department of Agriculture, and the U.S. National Science Foundation.


Lead photo courtesy of NC State Photos.

The text from Hook, Line & Science is available to reprint and republish at no cost, but only in its entirety and with this attribution: Hook, Line & Science, courtesy of Scott Baker and Sara Mirabilio, North Carolina Sea Grant. 

image: Hook, Line & Science logo.