Under the moonlight: a little light and shade helps larval fish to grow at night

Jeffrey Shima, Author provided

Jeffrey Shima, Te Herenga Waka — Victoria University of Wellington; Craig W. Osenberg, University of Georgia; Stephen Swearer, The University of Melbourne, and Suzanne Alonzo, University of California, Santa Cruz

At night on any one of hundreds of coral reefs across the tropical Pacific, larval fish just below the sea surface are gambling on their chances of survival.

Our latest research shows the brightness of the Moon could play a major role in that struggle for survival by affecting the availability of prey and keeping predators away.

Understanding how that works could help in fisheries management, specifically the prediction of changes to harvested fish stocks that allow us to anticipate how many adult fish can be taken without destabilising the fishery.

Many fish populations experience boom-and-bust cycles largely because parents routinely produce millions of offspring that have very low, but fluctuating, survival rates.

The large number of larval fish that are produced means any environmental conditions — for example, increased nutrients — that improve survival odds even only marginally can lead to a big influx in the number of surviving offspring.

Adult sixbar wrasse in courtship.
Author?, Author provided

When the Sun goes down

In the past we failed to take into account the influences the night may have on fish development.

In our research we found the daily growth rates of the larvae of sixbar wrasse (Thalassoma hardwicke) around the island of Mo’orea, in French Polynesia, are strongly linked to phases of the Moon.

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Their growth appears to be maximised when the first half of the night is dark and the second half of the night is bright.

Cloudy nights obscure the Moon, and thus allowed us to check our models by contrasting growth on cloudy versus clear nights, which confirmed the effect of moonlight on growth of these fish.

Phases of the Moon

We found that on the best nights of the lunar month for sixbars, around the last Quarter Moon when the Moon rises around midnight, larval fish grew about 0.012mm a day more than average.

But on the worst nights, around the first Quarter Moon when the Moon is overhead at sunset and sets around midnight, they grew about 0.014mm a day less than average.

Phases of the Moon from the Southern Hemisphere.
Wikimedia, CC BY-SA

For a typical larval sixbar of 37.5 days old, that means its growth is 24% more on the best night than on the worst one. This is important, as growth is inextricably linked to survival and ultimately fisheries productivity.

We think the Moon affects larval growth in this way because of how it changes the movements of deeper-dwelling animals, those that migrate into shallow water each night to hunt for food under the cover of darkness.

Zooplankton — potential prey for larval sixbars — respond quickly to the arrival of darkness, and move into the surface water to supplement the diets of sixbars.

Micronekton, such as lanternfishes, which hunt larval fishes, may take much longer to reach surface waters and seek out their prey, due to their migration from much deeper depths.

Four graphs showing the larval fish (in yellow) and the amount of predator (red shading area) and prey (brown shading area) rising to the surface during each phase of he Moon.
Proceedings of the Royal Society B, Author provided

As a consequence, prey availability for sixbars in surface waters may be hindered by early nocturnal brightness while the arrival of predators may be impeded by late nocturnal brightness.

Thus, larval fish grow best when their predators are absent but their prey are abundant — around the last Quarter Moon.

In contrast, around the first Quarter Moon, prey are suppressed but predators are not, leading to the slowest growth.

During the New Moon, when the surface waters remain dark throughout the night, influxes of both prey and predators may be high, with the latter preventing the larval fish from enjoying the increased numbers of prey.

On the other hand, during the Full Moon, when surface waters are well-lit, the movement of prey and predators may be suppressed, reducing the risk to the fish but also eliminating their food.

Impact on fishing

More research is needed to quantify these lunar effects on other marine populations. But our findings to date are good news for those working to strengthen fisheries management, given that phases of the Moon are predictable and cloud cover that can modify moonlight is being measured by satellites.

Observing the sixbar wrasse spawning.
Author?, Author provided

This makes the incorporation of moonlight into existing fisheries management models relatively simple.

We think this will have implications around the world, not just in the tropics. This is because the nightly upward movements of deep-water animals is ubiquitous — it is the largest mass migration of biomass on the planet, and it happens everywhere.

The suppressive effect of moonlight on this movement of potential predators and prey is also a global phenomenon.

We evaluated effects of the Moon on growth of larval temperate fish in an earlier study and found a similar effect (moonlight enhanced growth).

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The effect is stronger and more nuanced in our latest study, most likely because the waters in the tropics are comparatively clear.

Our findings also hint that other factors which affect night-time illumination of the sea may disrupt marine ecosystems. This includes the reflection of artificial lights from coastal cities, suspended sediments in the water column, and changes in cloud cover due to climate change.

In the future, we may be able to harness this extra information to help forecast fish population change to better guide the management and conservation of fisheries around the world.

Jeffrey Shima, Professor of Ecology, Te Herenga Waka — Victoria University of Wellington; Craig W. Osenberg, Professor of Ecology, University of Georgia; Stephen Swearer, Professor of Marine biology, The University of Melbourne, and Suzanne Alonzo, Professor of Ecology & Evolutionary Biology, University of California, Santa Cruz

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Red sky at night, shepherd’s delight: the science of beautiful sunsets

If you live in a place where the weather moves west to east, then an old proverb could help you predict the weather.
TimOve/flickr

Adam Morgan, Australian Bureau of Meteorology

“A red sky at night is a shepherd’s delight! A red sky in the morning is a shepherd’s warning.”

Perhaps this saying came to mind if you caught a spectacular sunrise or sunset recently.

Since biblical times and probably before, proverbs and folklore such as this developed as a way for societies to understand and foretell prevailing weather conditions.

The “red sky” proverb has endured across cultures for centuries, and modern science can explain why this is so.

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What causes a red sky at sunrise and sunset?

The Sun is low on the horizon at sunrise and sunset. At these times of the day, sunlight has had to travel through more of the atmosphere to reach us. When light hits the atmosphere it is scattered, particularly when dust, smoke and other particles are in the air.

This scattering affects the blue part of the light spectrum the most. So by the time the sunlight reaches our eyes there is generally more of the red and yellow parts of the spectrum remaining.

Dust and smoke particles commonly build up in the atmosphere beneath high-pressure systems, which are generally associated with dry and settled weather.

If you’ve ever been to Darwin in the Northern Territory during the dry season (the period between May and September), you’ll know glorious red and orange sunsets are an almost daily occurrence.

This makes sense – the sky across the Top End at this time of year is often full of dust particles whipped up off the land by dry southeasterly winds, as well as smoke from bushfires burning through the landscape.

What can red sky tell us about the weather?

In areas of the world where weather systems move routinely from the west to the east, including across southern areas of Australia, the “red sky” proverb often holds true.

A red sky sunrise suggests that an area of high pressure and fine weather, with its trapped dust and other particles, has moved out towards to the east. This allows for an area of lower pressure and deteriorating weather – perhaps a cold front and band of rain – to move in from the west during the day.

On the other hand, a red sky sunset tells us the worst of the weather has now eased, with higher pressure and improving weather approaching from the west for the following day.

Across northern Australia and other areas of the tropics, the “red sky” proverb is an unreliable method to predict the weather. In these regions, weather patterns are often very localised, moving in no particular direction at all, and larger tropical weather systems usually move from east to west.

Red skies and cloud

What often makes red sky sunrises and sunsets even more spectacular is the position of the Sun in the sky, relative to cloud.

When the Sun is low on the horizon, rays of light shine back up onto the underside of cloud high in the sky, reflecting back those bright orange and red colours that make it look as if the sky has turned to fire.

With a red sky sunrise, the eastern sky is more likely to be cloud-free with finer weather, allowing the Sun to shine upon the higher cloud moving in with the deteriorating weather from the west.

With a red sky sunset, it’s the western sky more likely to be clear, with the Sun’s rays shining up onto cloud further east.

So the next time you spot a spectacular sunrise or sunset, keep the “red sky” proverb in mind and you’ll become a pro at forecasting the weather in no time!

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Adam Morgan, Senior Meteorologist, Australian Bureau of Meteorology

This article was originally published on The Conversation. Read the original article.

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6 Tips for Night Hiking

Girly Camping®

The first time I went night hiking I’ll be honest with you- I was scared! I didn’t want bugs to get me, a bear to see me as a meal, or encounter a crazy woodsman! I was really nervous and didn’t know what to expect! But, like everything else I do, I feel in love and now, a third of when we hike is at night! (Check out my first night hiking experience- The Night Hiker ). But I could have used some tips when I first started so here are some tips for hiking at night:

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