Large jellyfish on Arcachon beaches: What’s behind the current spike

Arcachon – 30.06.2026: Along the beaches of the Bassin d’Arcachon, lifeguards and visitors have been reporting an increased number of large jellyfish for several days, some nearly a meter in diameter. The unusual sightings are unsettling for bathers, but marine biologists emphasize that the observed species generally only cause mild to moderate skin irritation. Life-threatening situations are rare according to current knowledge.

According to experts, these are mostly representatives of the rhizostomes, including the Atlantic-distributed lung jellyfish (Rhizostoma pulmo). These species do not have long, freely floating tentacles, but rather fleshy, fringed appendages whose stinging cells are comparatively weak. Sightings are accumulating especially in shallow coastal waters and shore zones left exposed at low tide, where animals are driven ashore by wind and swell.

Researchers cite several overlapping factors as causes: seasonally strong and changing currents that carry animals from the open sea into bays; an early warming of the upper water layers that favors the growth of jellyfish larvae and their food — plankton; as well as natural population cycles. In years with favorable environmental conditions, jellyfish populations can increase sharply in localized areas. Such episodes are known from various sections of the French Atlantic coast and recur without necessarily indicating a new threat.

The prefecture and municipal services are monitoring the situation. There are no widespread bathing bans, but beach patrols advise caution. Notices at access points inform visitors to report sightings and not to touch stranded animals. Some municipalities document finds to better understand the drivers of currents and seasonal patterns and, if necessary, to adapt visitor guidance — for example the locations of supervised swimming areas.

For beachgoers: avoid contact where possible, rinse affected skin with seawater, do not rub, and seek medical advice for stronger reactions. Home remedies like vinegar are not generally recommended for Atlantic species; when in doubt consult the lifeguards. People with known allergies should be especially careful and should not leave children unattended in shallow water.

Marine biology networks and citizen projects are currently collecting reports from Arcachon and neighboring sections of the Gironde. The ongoing data collection should help to better classify frequency, species composition and movement patterns. Experts point out that long-term developments — such as warmer summers, altered food webs or fluctuating predator populations — can influence the intensity of such phases. Statements about trends, however, require multi-year observations, which is why regional recording continues during the season.