Bay of Biscay Anchovies getting smaller, research points to environmental factors

Scientists are studying why anchovies in the Bay of Biscay are becoming smaller yearly. The AZTI technology center, which focuses on sea and food research, has shown how much the size of these anchovies has decreased from 1990 to 2021.

A study published in the Global Change Biology journal confirms that adult European anchovies' size and average weight have significantly decreased. The research suggests that the fish are not only smaller but also thinner.

Fernando Taboada, the main author of the study, points out that the decrease in weight is a bit more than the decrease in length. In the last twenty years, the reduction rate has sped up to 25% per decade.

Surveys reveal anchovy size trends

Researchers monitor the Bay of Biscay anchovy population through spring and fall surveys, recording body size data. Fall surveys (Juvena) focus on young anchovy using acoustic methods, while spring surveys (Bioman) assess spawning biomass through acoustic surveys and the daily egg production method. Data from 1990-2021 reveal trends in anchovy size at age.

The study includes statistical analyses and models to assess ecological patterns, trends, and potential drivers of changes in anchovy size. Computer codes and data for replication are available.

The study explores the impact of environmental factors, resource availability, and conspecific biomass on anchovy size in the Bay of Biscay.

The study examined the Bay of Biscay anchovy population's size at different ages using data from Juvena and Bioman surveys. Results showed a decline in anchovy length and weight, especially in adults, with a notable change point in the early 2000s.

The decline in juvenile size was around 10% per decade. Environmental factors like temperature, spawning biomass, and chlorophyll a concentration influenced size variations. For adults, temperature and biomass were significant factors affecting size, with temperature having a more pronounced impact on adult age classes. The study suggests a complex interplay of environmental and ecological factors in anchovy size changes.

Causes of reductions

Now, what is the reason for this situation? Although warming of the oceans partly associates with these changes, the researchers participating in the study suggest a complex interaction of environmental factors, but they rule out the exertion of a significant influence by fishing pressure.

Guillem Chust, another scientific author of the study, adds, "We have observed that the size of the specimens decreases with the abundance of anchovy. However, this relationship becomes less clear in the more advanced stages, where we have identified temperature as the primary cause for this decrease in size."

"Experts explain that, in general, juvenile fish living in warmer waters grow faster than their conspecifics in colder waters. However, these higher temperature conditions slow down the juvenile fish's subsequent development, resulting in a smaller adult body size. This is known as the temperature-size rule (TSR)."

Study matches growth model, anchovy weight

The study added that as expected, a growth model was able to match the average weight of anchovies at different ages. This model gave estimates for the growth rate (k) and the maximum weight (M∞) for average conditions in the Bay of Biscay.

To predict how the size of anchovies changes with age and temperature, the study tested predictions based on the activation energy from an analysis of geographical rules. These predictions were adjusted using a length-weight scaling factor.

The study found that the predictions fell short when compared to the actual changes in size with temperature based on survey data. To match the predictions with the estimates, the study used a raw estimate based on the observed relationship between anchovy weight and temperature.

The adjusted value was at the lower end of the estimates for one of the geographical rules. The adjusted predictions agreed well with the estimates and showed the observed increase in the rate of size decrease with age.

The study also found a clear effect of the early growth rate on temperature. This effect helped to bring the growth paths of young anchovies closer together. The study showed this with purple lines in the top row of plots, which represent the predicted changes when the early growth rate remains constant. Solid and dashed lines in the growth trajectory plots show the growth paths of young anchovies.

Larger in colder waters

Researchers have confirmed two ecogeographic hypotheses that explain the change in fish size when the water temperature fluctuates. The first hypothesis states that generally larger fish are found in colder waters when comparing closely related species of fish, for example, species of the genus Engraulis.

The second hypothesis concentrates on different populations of the same species. Observations by the team show that individuals from different European anchovy populations generally tend to be larger in colder seas.

However, as AZTI points out, “it is crucial to be cautious, since the study is based on observations and has its limitations. "Additional evidence points, however, to the fact that the decrease in the size of the anchovy could be an indicator of the response to changes in climate and the functioning of the ecosystem of the Bay of Biscay."

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