Anchovy sex is a force of nature

It’s not the size of the fish that counts, it’s the motion of the ocean.
The big clue that fish sex might be responsible for the turbulence scientists were detecting in a Spanish bay came when they placed algae nets in the water and pulled up a bounty of anchovy eggs. Further research shows that during breeding season these eggs are being deposited by the thousands night after night. DepositPhotos

This article was originally featured on Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at

Bieito Fernández Castro wasn’t expecting to find a turbulent hotbed of anchovy sex.

Commissioned by the Spanish government to investigate the conditions behind algae blooms, which kill mussels, Castro and his team were studying a peaceful spot in a bay in northwestern Spain. In the absence of strong winds or waves, toxic algae blooms occur more frequently here compared with surrounding areas, to the detriment of the resident mussels—and mussel farmers. But after two weeks of monitoring the apparently tranquil water with sensors that measure small shifts in temperature and velocity, Castro and his colleagues found that the bay’s calm surface belied what was happening below.

“Every night and without any apparent reason, we were seeing very, very high levels of turbulence,” says Castro, a physical oceanographer at England’s University of Southampton. Castro and his colleagues eventually traced the source of all this mixing: the frothing free-for-all of an anchovy orgy.

Most animals mate, but few do so with such frequency, and with so many bodies packed so closely together, as anchovies. As Castro and his colleagues’ data shows, these fornicating fish churn the water as much as a major storm.

Anchovies are among the ocean’s more amorous residents. The fish move in large aggregations of millions or more, and a female anchovy can release between 20,000 and 30,000 eggs each year, which males promptly fertilize like aquatic crop dusters.

All that “frantic activity,” as Castro calls it, causes quite a stir. And it’s something other sea dwellers might actually benefit from.

Turbulence is crucial for mixing heat and nutrients throughout the ocean. Previous research largely shows that the turbulence animals cause living their lives isn’t enough to substantially mix the layers of the ocean’s water column. But Castro’s study—which was published in 2022 and won a 2023 Ig Nobel Prize for humorous, thought-provoking scientific achievement—shows that within ocean layers, anchovy spawning causes significant, if subtle, swings in temperature. This finding suggests that in shallower water, the ruckus produced by plentiful piscine participants procreating all at once might be more powerful and more important for ocean mixing than previously thought.

In general, winds, tides, and forceful currents are the main things that stir and mix the ocean. Kirstin Schulz, a physical oceanographer at the University of Texas at Austin who studies small-scale mixing and wasn’t involved in Castro’s research, says scientists don’t typically consider animal movement a major cause of mixing in the ocean as a whole. However, Schulz says researchers have a lot to learn about how tiny turbulent motions mix ocean layers of different densities, salinities, and chemical makeups in shallow bays and other waterbodies. “This study shows that it definitely happens and can be of importance in a more local setting,” she says.

It’s even possible, Castro says, that his study actually underrepresents the effects of anchovy sex. Local fishermen told him the anchovy aggregation he studied was much smaller than similar swarms spotted farther offshore. In places like La Jolla, California, researchers have seen anchovy aggregations of between 10 million and one billion fish—schools so vast and dense they look like an oil spill cutting through clear water. Other schooling species, such as sardines and herring, swim in groups of similar sizes. But scientists have very little data on whether these species produce similarly titillating turbulence.

Curtis Deutsch, an oceanographer at Princeton University in New Jersey who studies oxygen and nutrient cycling and wasn’t involved in the study, says that to get the full picture of the extent to which fish and other marine life might be mixing the sea, scientists will need to study their effects on the deep ocean as well as surface waters. Water in the deep ocean is generally calmer than that at the surface, as it’s not stirred by wind or waves. Down there, Deutsch says, biological activity would be disproportionately important for ocean mixing. Unfortunately, he adds, that’s where “a lot of schooling behavior goes undetected.”

While much more research is needed to fully understand ocean mixing and the role marine animals might play in the process, Castro’s accidental anchovy discovery shows there’s more to the sultry lives of sea creatures than we surface dwellers might think.

This article first appeared in Hakai Magazine and is republished here with permission.