A Danish recreational diver has uncovered a marine disaster in the Little Belt Strait where dozens of dead fish, crabs, and starfish lie lifeless on the sea floor. Marine scientists warn that this is a sign of one of the worst oxygen depletion events Denmark has experienced in over two decades.
Mass Death on the Ocean Floor
A disturbing underwater scene has emerged near the Little Belt Strait in southern Denmark, just south of the coastal town of Fredericia. Ulrik Vendelbo, a recreational diver, captured shocking footage during a routine dive showing widespread death across the sea floor: dead fish, crabs, starfish, and marine snails all lying motionless in murky, sludge-filled water.
The dive took place on the morning of September 15, 2025, and lasted 63 minutes. The video has since circulated among marine scientists, who were shaken by the visuals and confirmed that such a mass die-off is highly abnormal for the area.
The site is typically rich in marine life and benefits from strong natural water flow, which usually protects it from environmental stressors. However, the dense swarms of decaying marine life point to a large-scale environmental disturbance – specifically, a severe lack of oxygen in the water known as hypoxia.
Oxygen Depletion Raises Red Flags
Experts from Denmark’s leading marine research institutions, including the Technical University of Denmark (DTU Aqua), have reviewed the footage. They concluded that this event is likely the result of an acute oxygen depletion episode affecting bottom-dwelling sea life. This sudden incident has left localized marine species especially vulnerable, as they cannot escape to oxygen-rich waters like more mobile fish.
Denmark is currently facing its most severe case of oxygen depletion in coastal waters in 22 years, according to national monitoring programs. The latest report by Aarhus University was released just over a week ago, documented widespread hypoxia across large sections of Danish waters in the Kattegat, Little Belt, and Baltic Sea.
Causes Rooted in Agriculture and Climate
Marine biologists point to an overload of nitrogen and phosphorus nutrients that primarily come from agricultural runoff, as the main contributors to algae blooms that lead to oxygen depletion when the algae die and decompose.
Up to 70% of the nitrogen that ends up in Denmark’s coastal waters is attributed to agriculture. When excess nutrients enrich the water, it creates conditions where massive algae growth flourishes. After these algae die, they sink to the ocean floor, where bacteria break them down in a process that consumes enormous amounts of oxygen and eventually suffocating marine life.
Climate factors have worsened the problem. Higher seawater temperatures and low wind conditions during the summer have reduced the natural mixing of water layers. Normally, surface winds help replenish deep-water oxygen supplies, but these conditions have been minimal this year.
Sudden Spread from Adjacent Regions
Though the Little Belt location typically escapes the worst of these hypoxia incidents, recent winds may have pushed oxygen-deprived water masses into this region from other hotspot zones, such as the inner Baltic. Scientists suspect this shift occurred quickly, catching marine life by surprise and leaving no time for escape.
According to estimates from Danish researchers, over 4,000 square kilometers of sea area, roughly the size of Rhode Island, are now suffering from critical oxygen depletion levels. This marks the largest affected area recorded since 2002, and conditions remain unstable.
Underwater Ecosystem at Risk
The Little Belt Strait is home to many unique and fragile species, with a high level of biodiversity among both plant and marine animal life. Local underwater forests and reef structures serve as important habitats for native fish like the bushy-headed sculpin. The death of such niche species could signal the collapse of localized ecosystems and wider disturbances along Denmark’s coastline.
Marine experts warn that if nutrient runoff is not drastically reduced, and if climate conditions continue to promote hypoxia, coastal waters across Denmark may become biological dead zones for longer periods of the year. Restoring balance to these marine regions would require consistent efforts in environmental policy, agricultural regulation, and restoration of natural coastal environments.
As visual evidence continues to circulate, the grim discovery made by one concerned diver may serve as a wake-up call for the broader public and policymakers alike.
