Anchovies stirring up the sea may be influencing ocean mixing

It isn’t clear how marine life contributes to the roil of the ocean, but researchers working off the coast of Spain have found turbulent mixing that can only be explained by anchovies gathering to breed

By Jason Arunn Murugesu

Shoals of breeding anchovies may be stirring the ocean off the coast of Spain. The finding suggests that fish could play a bigger role in vertical ocean mixing than previously believed.

Bieito Fernández Castro at the University of Southampton in the UK and his colleagues ocean turbulence off the Galician coast, in north-west Spain, for two weeks in July 2018.

The researchers initially planned to study the effects of vertical ocean mixing on marine life in various parts of the ocean, but quickly changed tack when they saw a major unexplained rise in ocean turbulence in their first few nights at the same spot.

“We were very surprised,” says Fernández Castro. Ocean mixing is largely thought to be controlled by wind and tidal energy, but neither factor could explain this sudden night-time rise, he says.

Ocean mixing is important because they allow nutrients from the depths to mix with water closer to the surface, providing marine life with sustenance.

The researchers moored their vessel at the same spot for two weeks and monitored turbulence 24 hours a day. The team did this by using an instrument that measures the velocity and temperature of the ocean more than 1000 times every second.

The team found that ocean turbulence increased every night by a factor of between 10 and 100. Using an echo sounder, which can identify schools of fish by transmitting and recording the responses to sound pulses emitted in the water, the researchers discovered that anchovies were gathering near the boat every night. Investigating further and finding anchovy eggs led the team to conclude that the fish were gathering there to reproduce.

“The possibility that biology contributes to ocean mixing is quite controversial,” says Fernández Castro. “But we were able to measure and quantify it in this case. What we don’t know yet is how important it is for overall ocean circulation.”

“There has been a long-running controversy over how effective biogenic turbulence is in driving mixing in the ocean,” says Thomas Rippeth at Bangor University in the UK. This finding suggests that such mixing is more potent than previously believed in specific regions of the ocean, he says.

Journal reference: Nature Geoscience, DOI: 10.1038/s41561-022-00916-3

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