Small fish cause great agitation

The underestimated effect of cumulative muscle strength: According to research, fish appear to be more of a contributor to mixing the water layers than previously thought. Scientists have found that schools of anchovies spawning on the Spanish coast cause turbulence in the water similar to storms. As a result, fish distribute nutrients and oxygen, and thus likely make a significant contribution to the formation of the marine ecosystem, the researchers explain.

Warm, light water forms the upper layers, while cooler and therefore denser water forms the lower layers: According to this basic principle, the oceans form structures. However, it is important for life that these layer systems are at least to some extent disrupted: mixing in the oceans ensures the transport of heat, oxygen, and nutrients between the different layers. This exchange is the basis of life for many living things in various marine ecosystems. Physical processes play the most important role: tides, flow systems, wind and weather cause the water to move and turbulence which leads to mixing.

What influence do living things have?

Previously, it was not clear to what extent the kinetic energy of creatures such as fish could also contribute. A rather small effect was assumed and it was found difficult to prove the relevant processes. So-called biomixing was therefore not initially of interest to the research team led by Fernández Castro from the University of Southampton. “Our discovery came as a surprise in terms of our measurements: we really wanted to investigate how turbulence affects marine life. But in the end, we were able to document that living things can cause water to move, which in turn affects marine life, ”said Castro.

Scientists conducted research in a bay on the northwest coast of the Iberian Peninsula. They lowered the measuring device from the boat into the water, which for two weeks with high sensitivity registered the fluctuations in the speed of the current and the temperature every half an hour. Scientists wanted to investigate the extent to which turbulence such as wind and weather affects the layering in the bay.

In the footsteps of turbulence “Stormy”

They were surprised to discover that each night in the bay there is turbulence and mixing of the layering system, much like a storm. However, these movements of the water were also visible in perfectly calm weather. A look at the information provided by the vessel’s sonar provided a clue as to the cause: acoustic data indicated the shoals of fish that appeared in the study area each night. Later studies then found that these were shoals of anchovy (Engraulis encrascicolus): The scientists’ research coincided with the spawning season of the small fish in the shoal – when they create shoals and provide offspring. According to the researchers, the reason for the significant turbulence in the bay is the insane spawning behavior of anchovies.

Scientists say that in the open ocean, such bio-mixing is likely to be of little importance for mixing as the different layers of water tend to be very thick there. However, research suggests that this may be different in some ecologically important coastal areas. “We’ve shown that in coastal areas where the layers change over much shorter distances, the fish can mix them up,” says Castro.

The vertical displacement generated by the shoals of fish can therefore have a significant redistribution of temperature, nutrients and other important water parameters such as oxygen content. These factors are fundamental to the functioning of the ecosystem, and the fish themselves depend on them, too, the researchers explain. According to them, the results therefore document the ability of living things to influence and redesign their physical environment. In conclusion, Castro and his colleagues write: “Our results show that biologically driven turbulence can be a powerful confusing factor and prompt a re-examination of its effects on productive ocean regions.”

Source: University of Southampton, Article: Nature Geoscience, doi: 10.1038 / s41561-022-00916-3

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