Sargassum seaweed has inundated Coral Bay on St. John for more than a week, causing toxic conditions in the inner portions of the harbor. In recent days, dying fish and other forms of marine life have washed up along the shoreline, adding to the already unpleasant odor created by the rotting sargassum as it piles up on the shore.
“I had to wade through dead fish to get to the boat,” said Colin Hanson, co-owner of FlyAway Charters VI based in Coral Bay. As he looked out over the harbor, Hanson said the dead fish looked like “white popcorn” scattered among the brown sargassum plume.
The recent influx of sargassum comes as no surprise. On May 4, Source reporter Jesse Daley wrote:
“Sargassum levels across the eastern Caribbean and Western Atlantic reached record highs in April 2025, with continued growth and coastal impacts expected into May. The increase raises the likelihood of beaching events across the region, including the U.S. Virgin Islands.”
“The record amount of sargassum detected was included in a sargassum outlook report released on April 30 from the University of South Florida Optical Oceanography Lab. The OOL at USF, which uses satellite imagery to monitor the presence of sargassum across the tropical Atlantic, Caribbean Sea, and Gulf of Mexico via satellite, has recently measured the gigantic increase in the amount of seaweed,” Daley reported.
A visitor who flew down from the States this week told Hanson he looked out the airplane window to see a carpet of sargassum that extended “as far as the eye could see.”
Sargassum plumes typically arrive in the Caribbean at this time of year. “When it first comes in, it has no smell at all. It washes up on the beach and turns crispy and dry,” Hanson said.
However, when large amounts wash in, “Decomposing sargassum releases hydrogen sulfide gas and ammonia, which can cause respiratory, skin, and neurocognitive symptoms in both local residents and tourists,” according to the National Library of Medicine.
As the sargassum decomposes, oxygen is depleted from the water, which further threatens marine life. Hanson said he’s seen eels, shrimp, barracudas, squid and fish of all kinds wash up in Coral Bay Harbor. “The fish are gasping for air. Only the tarpons and sharks don’t seem to care.”
“It is so sad to see little founders, eels, and juvenile fish you rarely get to see wash up ashore,” said Ann Gracie, who lives aboard a boat in Coral Bay Harbor. “The frigate birds and the laughing gulls are diving down and picking up the fresh ones. Someone should be doing a fish survey to see what’s being lost.”
The Source reached out to officials with the Virgin Islands Department of Planning and Natural Resources to ask about health risks to humans and marine life, as well as mitigation measures and funding to implement them.
Emails and messages left with Coastal Zone Management, Fish and Wildlife, and Environmental Protection were not returned, but DPNR published a blueprint for a comprehensive management plan in 2023 that answers many questions posed by the Source.
David Silverman, who served on the Coastal Zone Management Board for St. John for two years until 2020, has been tracking the recent influx of sargassum.
“The sargassum bloom throughout the tropical Atlantic and the Caribbean Sea in the spring of 2025 has vastly exceeded all prior observations and is estimated to be 200 percent higher than the volumes seen in prior years,” he told the Source.
Coral Bay is especially vulnerable to sargassum influx because of the southeast direction of prevailing wind and waves. “Wind driven mats of sargassum can be seen being driven into inner Coral Harbor, from which there is no exit or opportunity for dispersion,” Silverman said. “Over the night of May 14-15, 2025, a vast mat of sargassum accumulated on the northwest lee shore of Coral Harbor, far greater in extent than any accumulation ever before observed in this location.”
Silverman, who serves as president of Save Coral Bay, has long advocated against the construction of a marina planned by the Summers End Group on the northwest shore of Coral Harbor.
The predictable influx of sargassum on the lee shore — the precise location of the proposed marina – is a matter of great concern, he said.
“The presence of approximately one thousand concrete and steel pilings supporting the marina structures would have entrapped large volumes of sargassum, with the potential to clog input ports of vessels and foul the propellers of any yacht at the marina,” Silverman said. “The decomposition products of the sargassum, including hydrogen sulfide gas, would produce corrosive conditions and respiratory impacts to humans and marine life in the vicinity of the marina.”
Sargassum has caused headaches – literally and figuratively – at resorts, marinas, and other developments along the shores in the Virgin Islands and throughout the entire Caribbean.
Follow this link to learn more about the costs to businesses, mitigation measures, and reasons for the increase in sargassum events.
