Scientists are investigating why anchovies in the Bay of Biscay are shrinking. This trend is visible year after year. The AZTI technology center, which specializes in marine and food research, reported how the average size of anchovies in Bizkaia Bay has declined over the period from 1990 to 2021.
A study published in Global Change Biology confirms a clear reduction in both height and average weight of adult European anchovy (Engraulis encrasicolus). The data show not only smaller specimens but also a slimmer overall shape among recent samples.
“The decrease in weight is slightly more pronounced than the drop in overall length; the reduction rates have accelerated by up to 25 percent per decade in the last twenty years,” notes Fernando Taboada, a principal author of the AZTI study.
The size trend varies with age, showing that younger fish tend to be more affected than mature individuals. The observed changes point to a continual decline in body size as anglers observe a shrinking cohort of anchovies each year.
Reasons for decrease
What drives this pattern? While ocean warming plays a role, researchers emphasize a complex mix of environmental factors. Fishing pressure also contributes, but the study highlights a broader environmental signal that is influencing growth trajectories.
“The reduction in size correlates with the abundance of anchovies, yet the link to population density weakens at later life stages. Temperature appears to be the main driver of the size decline,” explains Guillem Chust, a scientific author of the study.
“In general, juvenile fish in warmer waters grow faster than those in cooler waters. Higher temperatures can slow later development, which leads to a smaller adult body size, a pattern explained by the temperature size rule (TSR).”
The researchers tested two ecogeographic hypotheses to explain how fish size shifts with temperature. The first suggests that closely related fish species tend to be larger in colder waters. The second focuses on populations of the same species, showing that many anchovy populations across Europe tend to be larger in colder seas.
AZTI cautions that the study relies on observations and has limitations. Still, there is additional evidence suggesting the size reduction may reflect a broader response to climate change and shifts in the Bizkaia Bay ecosystem.
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