Researchers illuminate a role for moonlight in fisheries management

A University-led study shows how the lunar cycle influences the behaviour of larval fish predators and prey and as a result growth rates.

A school of fish swimming at night

The phases of the Moon could be harnessed to improve the management and conservation of fisheries around the world.

Research by Te Herenga Waka—Victoria University of Wellington’s Professor of Ecology Jeffrey Shima and colleagues shows the brightness of the moonlight plays a major role in the growth of larval fish just below the sea surface.

By factoring the reliable 29.5-day patterns of the lunar cycle into existing models, fisheries management could be made more sustainable.

Professor Shima, who is in the School of Biological Sciences, says incorporating nocturnal illumination into fisheries models, which already include environmental variables like temperature and primary productivity, would be relatively simple.

The researchers’ newly published paper in Proceedings of the Royal Society B“Lunar rhythms in growth of larval fish”, studied the daily growth rates of the sixbar wrasse (Thalassoma hardwicke) around the island of Mo’orea in French Polynesia.

They found the “best” nights for the sixbars were near the last-quarter moon, when larval fish grew about 0.012mm a day more than average.

On the “worst” nights, near the first-quarter moon, they grew about 0.014mm a day less than average.

“We think the difference is due to the relationship between the sixbars’ prey and predators. Zooplankton—potential prey—respond quickly to the arrival of darkness, while micronekton, which hunt larval fishes, may take much longer to reach surface waters from the depths.

“So prey availability for sixbars in surface waters may be hindered by early nocturnal brightness, while the arrival of predators may be held up by late nocturnal brightness.

“As a result, growth may be worst at the first-quarter moon because prey are suppressed but predators are not, while it may be best at the last quarter because predators are suppressed and prey are not,” says Professor Shima.

Predicting marine population dynamics remains largely unreliable, even after more than 100 years of research into what causes variations in the growth and survival of larval fish.

“Considering some of the important biology that happens at night may improve our forecasting ability and lead to better management recommendations.

“Given we struggle to make such predictions in today’s conditions, it becomes even more complicated when we introduce future climate scenarios to the mix.”

Professor Shima says the findings are also relevant to fisheries in temperate zones.

“These vertical migrations of prey and predator have a global distribution and appear to be affected by moonlight in the same way everywhere.

“In an earlier study, we evaluated the effects of the Moon on the growth of larval temperate fish and found a similar effect, although the effect is stronger and more nuanced in our latest paper, most likely because the waters in the tropics are comparatively clear.

“Our findings also hint that other factors which cause night-time illumination of the sea may disrupt marine ecosystems, such as reflection of artificial lights from cities, suspended sediments, and significant changes in cloud cover due to climate change.”

Professor Shima’s collaborators were Professor Craig Osenberg from the University of Georgia, Associate Professor Erik Noonburg from Florida Atlantic University, and Professor Suzanne Alonzo from the University of California, Santa Cruz, all in the United States, and Professor Stephen Swearer from the University of Melbourne in Australia.

They have written an article about their research for The Conversation. It is available for free republication under Creative Commons.

Their research was supported by a grant from the Marsden Fund, which is administered by the Royal Society Te Apārangi on behalf of the Ministry of Business, Innovation and Employment.