What do we know about the sleeping habits of fish?

When and where species sleep can determine how vulnerable they are to overfishing.

Research Need

Many reef fish populations are declining due to disruptive fishing practices. However, in order to create more sustainable practices, it is essential to understand fish behavior during the day and at night.

Parrotfishes are a large species group that are important to reef ecology as a source of food (for both humans and other fish), and they also help control algae populations that can smother corals. Even their waste is beneficial, because it can help distribute sediment throughout reefs.

Parrotfishes, however, have become victims of overfishing — particularly nighttime spearfishing.

One reason parrotfishes are especially vulnerable at night is their unique sleeping habits. Parrotfishes sleep 13 hours a day. Rather than choosing sheltered locations to sleep, though, most parrotfish species choose to sleep in exposed substrates (base layers or surfaces), which makes them easy nighttime fishing targets.

What did they study?

The Marine Extractive Reserve of Arraial do Cabo, located near Rio de Janeiro, Brazil, provided the sites for the study. The reserve, although technically only open to local anglers, faces poor regulation, and disruptive fishing often occurs day and night.

The researchers first assessed the most abundant species in the area and then sought and tracked sleeping fish from each species. They recorded types of substrate, whether each substrate was a living organism, the water depth, and the sites’ geological features.

What did they find?

Water depth of the sites varied most among species, while all species of parrotfish chose sediment as their preferred substrate.

Geological features also varied, with one species markedly preferring crevices and isolated areas, while the others were more likely to choose open areas, like slopes and interfaces.

Water depth was the most influential variable in where the fish slept. These results are consistent with those of previous parrotfish habitat studies that have been conducted around the world, such as in Japan and the Mediterranean region.

Anything else?

All the species avoided areas where certain species of corals were present, which is likely because they can carry parasites and diseases that could infect the resting fishes.

So what?

Thus, studies like this one can help inform best fishing practices and protect vulnerable species. Because parrotfishes have a wide habitat range throughout the tropics, studying them can help authorities determine guidelines for the size of marine protected areas.

Doing so will not only improve fish health and preserve ecologically crucial reef habitats, but it will also improve the economic benefits of the fishery.

Reading

Lucena, M., Mendes, T. C., Cordeiro, C. A. M. M., Barbosa, M. C., Batista, J., Eggertsen, L., Hackradt, C. W., & Ferreira, C. E. L. (2024). When the Light Goes Out: Distribution and Sleeping Habitat Use of Parrotfishes at Night. Fishes, 9(10), 370. https://doi.org/10.3390/fishes9100370

Photo: Banana parrotfish (Scarus zelindae). Credit: Igor Cristino Silva Cruz/ CC-AS-4.0 international license.

The text from Hook, Line & Science is available to reprint and republish at no cost, but only in its entirety and with this attribution: Hook, Line & Science, courtesy of Scott Baker and Sara Mirabilio, North Carolina Sea Grant.