Global fish communities are undergoing significant shifts in size and feeding relationships, even in areas where the total number of fish species remains stable, according to a study published this week in Science Advances. Researchers analyzed nearly 15,000 marine and freshwater fish communities, finding a consistent trend toward smaller-bodied fish and altered food web dynamics.
The study, led by scientists at the German Centre for Integrative Biodiversity Research (iDiv), the Martin Luther University Halle-Wittenberg (MLU), and the Friedrich Schiller University Jena, combined data spanning up to 70 years with detailed information on fish body size, diet, and trophic position. While overall species richness did not show a consistent trend, the composition of fish communities changed markedly over time.
“We often say ‘considerable fish eat small fish,’ and in nature it’s true—it’s an ecological rule,” said Dr. Juan Carvajal-Quintero, first author of the study and an alumnus of iDiv, currently an Assistant Professor at Dalhousie University. “Fish predators are usually larger than their prey, and this size difference determines who can eat whom. When the size of predators or prey changes, feeding relationships shift, reshaping food webs and how ecosystems function.”
The research revealed that food webs are becoming more densely connected, with species exhibiting a broader range of prey. This indicates a rise in generalist feeders – fish less specialized in their diets. The proportion of large top predators, such as sharks, goliath groupers, muskellunge, and marble trout, has declined, while mid-level predators and primary consumers have become more prevalent.
These changes are occurring across both marine and freshwater ecosystems, suggesting a widespread phenomenon. Researchers emphasize that simply counting the number of species present does not provide a complete picture of ecosystem health or change. Shifts in species traits, like body size and feeding habits, can significantly alter ecosystem structure even without a loss of species richness.
The study’s findings build on previous research highlighting the vulnerability of aquatic ecosystems. NOAA’s resource collections detail the interconnectedness of aquatic food webs, emphasizing the importance of understanding predator-prey relationships for maintaining sustainable fish populations and healthy environments.
