North Sea Ecosystem Dramatically Changed: Watch Out for Weever Fish, Far Fewer Shrimp

The ecosystem of the Belgian North Sea coast has undergone a fundamental transformation over the past 30 years, with cold-water species in dramatic decline and warm-water species surging, according to new data from the citizen science project SeaWatch-B, coordinated by the Flanders Marine Institute (VLIZ). The findings come amid an ongoing marine heatwave that has lasted 144 days, with water temperatures reaching 3.1 degrees Celsius above normal.

A Sea Change in Numbers

The research, comparing data collected between 2014 and 2025 with a baseline survey from 1996-1997, reveals stark declines across multiple cold-water species. Grey shrimp populations have plummeted by 78 percent — from 147 to just 32 specimens per 100 square meters — and the decline is accelerating, with numbers halving between 2014 and 2025 alone. Shore crabs have fallen by 88 percent, gobies (small fish under 10 centimeters) by 78 percent, and herring and sprat by a staggering 91 percent.

“I was deeply impressed when I first saw the data,” said Jan Seys, marine biologist at VLIZ. “Nothing is what it seems anymore. The beach water fauna is impoverished, ‘warmed’, alienated, and slimed.”

The Rise of the Weever Fish

While cold-water species struggle, warm-water species are thriving. The lesser weever fish — a small, venomous fish that buries itself in the sand — is now 24 times more common than 30 years ago, with an average of one specimen per 10-by-30-meter strip of beach water. In July 2025, weever fish numbers were seven times higher than the same period the previous year, with densities 10 to 15 times higher than in the late 1990s, according to VLIZ Testerep magazine.

Seys himself no longer walks barefoot at the beach. “The number of weever fish has increased quite spectacularly,” he said. “I no longer walk barefoot.”

The weever fish’s venomous spines deliver an intensely painful sting when stepped on. Treatment involves immersing the affected limb in hot water (up to 45 degrees Celsius) for 20 to 90 minutes, as the venom breaks down with heat. Wearing water sandals is recommended for prevention.

Marine Heatwave and Ocean Warming

The changes are being driven by rapid warming of the North Sea, which has heated by 2 degrees Celsius over the past 60 years — twice the global average for oceans. As of late June 2026, the Belgian North Sea had experienced a 144-day marine heatwave, with absolute record temperatures recorded between June 25 and 27 — 3.1 degrees Celsius above the 1982-2011 baseline. Globally, sea surface temperatures reached a record 20.86 degrees Celsius on June 21, 2026, according to the Copernicus Climate Service, as VRT NWS reported.

“For the first time, we see scientists truly astonished by phenomena that even their most advanced computer models could not predict,” Seys said. “That worries all of us.”

Food Web Disruption

Beyond the decline of familiar species, the research documents the proliferation of gelatinous zooplankton, particularly the American comb jelly and the sea grape. These organisms consume large quantities of plankton but are not themselves edible, potentially creating a “short circuit” in the food web. A similar invasion of the American comb jelly caused an ecological and economic collapse in the Black Sea.

“American comb jellies can disrupt the food chain,” Seys warned. “We can already see that plankton is dying almost en masse in summer. Yet they form a crucial link in the ecosystem.”

Ocean acidification adds another layer of stress. As the ocean absorbs more carbon dioxide from the atmosphere, it becomes more acidic, threatening all organisms that need calcium to build skeletons or shells — roughly half of all marine life.

Phenological Shifts

The timing of species’ life cycles is also changing dramatically. Gobies now peak in October instead of July — three months later than 30 years ago. Sea grapes peak in April, one and a half months earlier than before. Seabass, once peaking in July, now shows peaks in February and September. These phenological shifts can create mismatches between predators and their prey, further destabilizing the ecosystem.

What This Means for Beachgoers and Fisheries

For the millions of people who visit the Belgian coast each summer, the most immediate change is the increased risk of weever fish stings. With densities now at roughly one fish per 70 square meters, the probability of an encounter while wading barefoot has become significant.

For the fishing industry, the 78 percent decline in grey shrimp carries serious economic consequences. Shrimp fishing is a traditional and culturally significant industry along the Belgian coast, including the UNESCO-recognized horse-drawn shrimp fishing in Oostduinkerke. The broader shifts in fish populations — with mackerel and herring migrating northward and warm-water species like squid and seabass moving in — are reshaping commercial fishing operations.

A Transformed Coastline

The SeaWatch-B project, which trains approximately 20 citizen scientists to conduct standardized monitoring four times per year, has provided an unprecedented window into how the coastal ecosystem is changing. The data paint a clear picture: the North Sea of today is fundamentally different from the North Sea of a generation ago.

“Nothing is what it seems anymore,” Seys concluded. The question that remains is whether these changes represent a new equilibrium — or whether the ecosystem has yet more transformations in store.