Sargassum in St. Barts: where to swim today, beach by beach

Today’s beach-by-beach status is shown on the map and list above, updated every morning. Green dots mark the clear beaches, orange or red dots the ongoing landings. Conditions can change within 48 hours depending on the wind, so check again on the day of your outing.

Sargassum is a floating brown seaweed (genus Sargassum) that drifts in huge rafts across the sea. The phenomenon has surged since 2011: pushed by tropical Atlantic currents, it crosses the ocean from a vast belt between Africa and Brazil — the great sargassum belt — before washing ashore.

Out at sea it shelters useful marine life; the problem is the massive landings: piling up on the shore, the seaweed rots and becomes a health and economic headache. Natural processes, then, but on an unprecedented recent scale.

Landings mainly hit the north- and east-facing (windward) beaches: St-Jean, Lorient, Anse des Cayes, Grand Cul-de-Sac, Petit Cul-de-Sac and Anse Maréchal.

But "affected" doesn’t mean "off-limits": outside a heavy landing, these beaches are often still swimmable. The leeward south/west beaches are simply the calmest more often. The day’s per-beach score is the only real judge.

Often on the south and west (leeward) side: Gouverneur, Grande Saline, Flamands, Colombier and Shell Beach in Gustavia are generally spared, sheltered from the north-easterly Atlantic swells.

That said, you don’t have to chase "zero sargassum": a beach with light or moderate seaweed is perfectly swimmable. Confirm on the map on the day.

Landings mostly occur from March to October, often peaking between April and August — but the season varies a lot from year to year. Some years are nearly spared, others break records. A landing can last from a few days to several weeks, and a clean beach one morning can get a landing 48 hours later: which is why a daily update beats a seasonal average.

As it decomposes, sargassum releases hydrogen sulfide (H₂S) and ammonia, a rotten-egg smell. This mainly concerns heavy, prolonged landings and sensitive people (asthma, infants, pregnant women) — a beach with light to moderate seaweed is generally no problem for a swim. Knowing the day’s risk is the best protection, which is why a smell index accompanies every beach.

Partly. Forecasts rest first on satellite monitoring of the offshore rafts: imagery spots the mats tens of kilometres from shore, giving a few days’ warning depending on currents and trade winds. Longer term, monthly bulletins (NOAA, University of South Florida) estimate how much seaweed is in the Atlantic. FanGass combines these with the official weather bulletins to anticipate, beach by beach — no forecast being 100 % reliable, hence the daily update.

Each beach gets a 0-to-5 score and a color. Green (0-1): clear, go for it. Yellow (2): some seaweed, perfectly swimmable. Orange (3): a moderate, visible amount — but still swimmable; plenty of people go in, your call depending on the day’s smell. Red (4): heavy landing, it stinks, skip it. Black (5): evacuated zone. In short, only red and black beaches are really to avoid; below that, a swim is fine. A smell index (none / light / strong) completes the score.

No need to cancel a holiday in St. Barts: there’s almost always a swimmable beach, and most stay swimmable even with light to moderate sargassum. The south/west beaches (Gouverneur, Saline, Flamands) are often the calmest on heavy days; but there’s no need to write off St-Jean or the whole north side. Best move: check the day’s per-beach score.

Every day, FanGass reads the official monitoring bulletins (Météo-France Antilles, NOAA AOML, the University of South Florida monthly outlook, regional bulletins), cross-checks them against satellite imagery of the offshore rafts, then distills it all into a clear per-beach score. On-the-ground user reports adjust the scores live when several agree. The service is free, offered by the Katchak agency.