Toxic tiddlers? Mechanisms driving within species mercury levels uncovered in anchovies

Capture fisheries have been highlighted as vital to ending world hunger because they provide essential nutrients (not least high-value protein, with over 50% of animal protein in several Asian and African countries coming from aquatic animals) to billions of people (FAO, 2024). Despite the numerous health benefits obtained from consuming fish, one risk threatens to undermine them all: heavy metal contamination (Bosch et al., 2016). A fear of this has percolated into societal trends, such as the aversion to eating fish when pregnant, to prevent risks to foetuses from methylmercury, despite the benefits to foetus neurocognitive development provided by a diet rich in fish (Bramante et al., 2018). Of course, not all fish are equal when it comes to their mercury potency. Those fish that exist at higher trophic levels, and that are longer lived, are higher risk because of bioaccumulation, which include sharks, tuna and rays (Xu et al., 2025). However, this is not to say that smaller, shorter-lived, species are always low risk, as other factors are at play. For example, a study in Brazil looking at the mercury concentration of eight seafood species served in restaurants showed that, after tuna, the tiny black anchovy contributed most to mercury intake (Morgano et al., 2014). Even within a species, mercury levels can show marked variation. In Japanese anchovies, mercury levels increase with trophic position (Chen et al., 2024), although the mechanisms behind this variation were not fully understood. That was until this issue, where Chen et al. (2025) explore these very mechanisms, using fatty acid analysis to understand whether shifts in the anchovy's diet is to blame for increasing mercury concentrations in larger individuals. Many fatty acids are obtained from diet and are transferred from prey to predator, allowing them to act as biomarkers, linking consumers to their food source. Using the fatty acid profiles of the Japanese anchovies caught in the northwestern Pacific Ocean, as well as their mercury levels, Chen et al. (2025) confirmed previous results that larger fish have higher mercury levels. But, more importantly, this work showed that niche overlap was lowest between larger and smaller sized fish. The difference in niche overlap between size classes indicated that after a body length of 120 mm is reached, a dietary shift occurs, potentially to a more contaminated food source. Not only this, but Chen et al. (2025) also showed that mercury levels were positively correlated with fatty acids characteristic of microzooplankton (such as copepods), zoobenthos and dinoflagellates. Collectively, these results indicate that a dietary shift (favouring higher-trophic zooplankton) occurs in larger anchovies, which looks to facilitate their growth, but at the expense of increasing their mercury levels. These insights help us to understand the mechanisms behind within species mercury variation in a widely consumed seafood species that could have important health consequences. So, next time you are looking for that umami hit, do not pass on picking the tiddlers of the bunch.