What To Do When Aquatic Critters Attack During a Swim (2024)

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When I signed up for a 2.5-mile cross-channel swim race from Nevis to St. Kitts, I expected to make friends in the water. Perhaps I should have been more specific. As I swam between these two glorious Caribbean islands, I was distracted briefly from the gorgeous scenery by a zap on my skin. Then another, and another. I was surrounded. Oh, no, I realized. The sea lice are vacationing here, too.

It wasn’t until I talked with the experts that I learned what zapped me wasn’t actually sea lice – those are a problem for fish, not humans. Instead, it was jellyfish larvae, which, as I learned firsthand, are as capable of stinging as their grown-up counterparts. I also learned jellyfish encounters are increasingly common in ocean swimming and triathlon events, as these brainless, spineless lumps of goo only seem to be thriving in the face of climate change.

During another swim – this time, in the North Channel between Scotland and Northern Ireland – I made a different friend. Her name was Cecilia, and she was a seal. Unlike the jellyfish larvae, Cecilia swam alongside me at a respectable distance for over an hour; I only became aware of her presence when she charged up from the deep toward me at full speed and scared the bejesus out of me. Though she was just being curious and playful, it wouldn’t have been difficult for her to take a nibble, too.

Whether you regularly swim in open water or only dive in when a race calls for it, chances are you’ve wondered what’s lurking beneath the surface. After all, when we dive into a body of open water, we’re really diving into a habitat for a menagerie of aquatic creatures. Though most of them are friendly, that isn’t always the case. Jellyfish aren’t the only critters who may demand a toll from athletes swimming through their neighborhood – there are lamprey, sea lions, and more. So what’s a swimmer to do when they find themselves in an aquatic attack? We asked the experts how to deal.

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What To Do When Aquatic Critters Attack During a Swim (1)
Potentially problematic species Sharks, stingrays, and lamprey
Where they live Open bodies of fresh or salt water
How they could hurt you Bites and stings


Naturally, during any training session or race that involves heading into the ocean, the first fear that crops up for many people – no thanks toSteven Spielberg and Jaws – is sharks. But this fear is exaggerated and largely unfounded.Jon Dodd, executive director of the Atlantic Shark Institute in Wakefield, Rhode Island, puts the fear in context:

“There are about 70 shark attacks worldwide every year, and about five of those attacks result in a person dying. But if you compare that to the hundreds of millions of people that swim every year, it’s an amazingly small chance that you’re gonna get bitten by a shark.”

Dodd adds that when a shark bites a person, it’s “really a mistake. That’s why it’s a bite-and-release, because the shark bites and realizes it’s not prey.” Small comfort if you’ve lost a limb in the interaction, but the point is, we’re not on the menu and despite what Hollywood would have you think, sharks don’t stalk humans.

If you see a shark while swimming, the best thing to do is stay as calm as you can and slowly back away from the animal. Avoid splashing, thrashing, screaming, or otherwise provoking the shark – simply keep swimming toward shore.


Stingrays, which are related tosharks, are also not interested in you, but can turn painfully defensive when disturbed in the shallows where they tend to settle. Rays have a long tail with a venomous barb that can produce an excruciating sting. If you encounter a stingray, it’s unlikely you’ll suffer the same fate asSteve Irwin, but it’s possible to step on them and meet the business end of the barb. This is why it’s always wise to shuffle your feet when entering water where they’re known to congregate, as this warns the rays something is coming, causing them to swim away.


Saltwater definitely has its share of critters, but freshwater isn’t a guarantee of safety either.Declan McCabe, professor of biology at Saint Michael’s College in Colchester, Vermont, says thelampreythat inhabit nearby Lake Champlain have been known to interact with swimmers sometimes.

“It’s more of an Oh, wow, something just poked me type of thing rather than a serious threat,” McCabe says. But being inspected by a lamprey can certainly fuel future nightmares.

What to do when fish (sharks, stingrays) attack

If a shark has bitten you, seek emergency medical care ASAP. Lamprey can usually be brushed off without incident, but if it has pierced the skin, seek medical care. Same goes for a stingray – carefully remove the barb, stanch any bleeding, and seek medical attention.

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Aquatic invertebrates

What To Do When Aquatic Critters Attack During a Swim (2)
Potentially problematic species Jellyfish, crab larvae, parasites, and amoebas
Where they live Open bodies of fresh or salt water
How they could hurt you Stings or infection

While sharks make all the headlines, it’s jellyfish, crab larvae, and other stinging or biting critters that you really need to watch out for. They’re everywhere, and are completely indiscriminate when stinging. Jellyfish, in particular, have no capacity for critical thought, so while it might feel like it’s personal, they’re literally just drifting along, trying to survive, when they end up accidentally spoiling your swim.


Fresh water can harbor invisible threats such as the parasites that cause swimmers’ itch, also called duck itch. Avoid stagnant or mucky bodies of water, swim into deeper waters (the parasites prefer shallow shores) and always rinse off and towel dry after swimming to remove any parasites that may be on your skin. Most cases of swimmers’ itch can be relieved by applying baking soda paste or over-the-counter anti-itch lotion to the rash.


A rare but far more frightening and deadly organism affecting open-water swimmers is Naegleria fowleri, found in some warm, freshwater locations. This amoeba gains entry to the host’s body via the nostrils and travels to the brain where it causes an infection that’s nearly always fatal. The best way to avoid exposure is to wear a nose plug and skip swimming in warm, freshwater locations where it’s known to be present.


If you can see where to swim to avoid patches of them, that’s obviously the best way to avoid contact with jellyfish. However, you’ll likely experience a jellyfish sting at some point during your ocean training and racing career anyway. Depending on the species, it’s not usually a major issue, but some types, such as the box jellyfish, can be fatal.

In all cases, know what type of jellyfish tend to inhabit the water you’re entering and seek local advice forhow to cope with stings. And no, that won’t include peeing on the sting; the slightly acidic nature of urine can actually cause any stinging cells that may still be stuck to the skin to fire, causing even more pain.

What to do after a jellyfish encounter

After a jellyfish sting, rinse the affected area with salt water to remove any stuck tentacles or stinging cells and seek medical attention if you develop a rash, pain worsens, or you start to experience an allergic reaction.

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Marine mammals

What To Do When Aquatic Critters Attack During a Swim (3)
Potentially problematic species Seals, sea lions, and otters
Where they live Open bodies of salt water (some otters live in freshwater)
How they could hurt you Bites

Not to be outdone, some sea mammals like to get in on the action when humans stumble through their aquamarine spaces.Seals, sea lions, and otters have all been known to occasionally interact with swimmers.

Sometimes, these interactions get a little too close – swimmers in San Francisco Bay have sometimes reported bites from overly-territorial sea lions.Otters have also been spotted chomping on triathletes, so all of these aquatic mammals bear a mindful eye – they have teeth they can use and they may attract sharks, orcas, and other predators you probably don’t want to meet, either. Always give these mammals a wide berth.

These marine mammals are known to carry a variety of bacteria, some of which can be harmful to humans when transmitted via a bite wound. The resulting infection can cause necrosis or even death, so don’t try to pet these animals, even if they do look friendly.

What to do after an encounter with marine mammals

Get away from the animal immediately. If bitten or scratched, seek emergency medical care.

What To Do When Aquatic Critters Attack During a Swim (2024)


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