Wednesday, September 28, 2022

Do you want to avoid a blue bottle stitch? Here’s how to predict on which beach they will land

If you’re among the one in six Australians experiencing the bitter pain of a marine sting like a blue bottle, you’ll know how quickly they can end a fun day at the beach.

We can not stop the summer winds that these creatures bring to our coast, but we can choose the safest places to swim.

Our recent research provides the first evidence of what blue bottles transport to Australian beaches.

We have found that the direction a beach faces, relative to wind direction, largely determines how many blue bottles are pushed to shore. We hope these findings will help beachgoers safely plan where to take their next dive.


We can avoid blue bottles by understanding more about their drive. Photo: Jim Tiller / Associated Press

Delicate sea drivers

The blue bottle is a jellyfish that occurs mostly along Australia’s east coast.

Most blue bottle stings occur while swimming, and are the main reason why people seek help from surfing lifeguards.

Blue bottles are not a single animal. They are a floating colony of individual organisms, each of which is differently responsible for the reproduction, capture or digestion of food and capture the wind.

The blue bottle’s long, trailing tentacles are designed to stab prey and creatures through which they feel threatened, including humans.

Blue bottles do not swim, but float on the sea surface. Their inflated blue bladder is sensitive to aerodynamic forces and acts as a sail.

Currents drive a blue bottle’s long tentacles below the sea’s surface and wind drives the sail above it.

The blue bottle as a sailboat

A blue bottle’s body, including the tentacles, is not in line with its sail.

Some sails point to the left side of the body, and others to the right side. This peculiarity is thought to help the population to survive.

If all blue bottle sails point in the same direction, an entire group can pick up a prevailing wind and be blown to shore. But when half the group has sails pointing the other way, some individuals are blown in a different – and hopefully less dangerous – direction.

Our previous research attempted to shed light on blue bottle propulsion by examining physical equations that determine how sailboats respond to winds and currents.

That research found that wind power can cause right-leaning blue bottles to float about 50⁰ to the left of the wind coach, while left-leaning individuals float about 50⁰ to the right.

coastline and swimmers

The direction a beach faces largely determines its blue bottle numbers. Photo: Sam Mooy / AAP

Choose your swimming spot wisely

Our latest research has examined how winds and other environmental factors affect blue bottle stranding.

We analyzed daily blue bottle numbers and stitches at three Sydney beaches – Maroubra, Clovelly and Coogee – over four years. The project was led by Master Student Natacha Bourg.

Blue bottle numbers were highest during the summer, with a peak a few weeks before maximum sea temperatures.

It was previously thought that cold temperatures hinder blue bottle movements. But we have recorded blue bottles on beaches in winter and spring, suggesting that other factors are at play.

Our research found that wind direction was the main factor driving blue bottles ashore. On Australia’s east coast, both northeasterly and southerly winds bring blue bottles to the beach.

We also found that the shape of the coastline, and its orientation relative to prevailing winds, influenced the rate of blue-bottle arrivals.

Maroubra faces east and is the longest and most wind-exposed of the three beaches. We found that a summer northeasterly wind at Maroubra led to a 24 percent chance of blue bottles the next day.

But at nearby Clovelly Beach, the odds were just 4 percent. Clovelly front south and sits relatively sheltered at the end of a narrow bay. After southerly winds, however, the chance of blue bottles there increased to 12 percent.

Coogee looks south and is smaller than Maroubra. A small rocky outcrop limits exposure to the sea and therefore exposure to blue bottles.

In general, blue bottles were most likely found at Maroubra, followed by Coogee and then Clovelly. It reflects their different beach lengths and orientation with respect to prevailing winds.

coastline, sunbathers and swimmers

Maroubra had a 24 percent chance of blue bottles after a summer northeast. Photo: Sam Mooy / AAP

Plan your day at the beach

These conclusions can be applied beyond the beaches we have studied. By checking beach orientation with wind direction, we can make an educated guess as to whether the chances of encountering blue bottles at any beach are great.

We know blue bottles are pushed about 50⁰ left or right of the wind direction. Such a quick drawing in your head or on the sand can tell you which nearby beach is likely to be the safest.

But there are exceptions to this rule. Strong ocean currents, for example, can affect blue bottle drift, especially when winds are weaker.

Tearing and the circulation of water in surfing zones are also linked to blue bottle beach.

And blue bottles can lengthen and contract their sails and pitching tackles which can change the direction of their drift.

Therefore, take many precautions against blue bottles and other hazards before entering the water. Surf Life Saving Australia encourages all beachgoers to:

  • stop and look at your surroundings
  • look for cracks, big waves, rocks and other dangers
  • plan to stay safe, including swimming at a patrolled place
  • visit

lifeguard patrolling beach

Plan to swim at a patrolled beach. Photo: Richard Wainwright / AAP

Learn more

Further research is needed to better understand blue bottles, including how climate change, and the subsequent warming oceans, will affect their drift.

Civil Science offers a powerful opportunity to learn about blue bottle distribution, size and arrival at our beaches.

Next time you see blue bottles at the beach, take photos and upload them to this project in the iNaturalist app.

In this way, you can help researchers discover more secrets of these beautiful sea creatures – which will hopefully lead to less painful blue-bottle encounters.

The conversation

Amandine Schaeffer, Senior Lecturer, UNSW Sydney and Jasmin C Lawes, Deputy Assistant, UNSW Sydney

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

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