In some cases, scientists say climate change may be leading to more algal blooms and other events that starve fish of oxygen. Warming oceans and marine heat waves are driving sea creatures from their normal habitats. Human activities including coastal shipping are suspected in a spate of recent marine mammal deaths in the United States.
Here’s a look at some of the events that led to the deaths of swaths of aquatic creatures around the globe in the past year.
A killer red tide in Florida
Harmful algal blooms, called red tides, have sent scores of dead fish ashore in southeastern Florida in recent weeks. A similar red-tide event killed off thousands of fish in the San Francisco Bay Area last summer.
So-named because in large numbers the algae stain the water red, these toxic blooms occur when winds blowing across the ocean surface push that water away and bring deeper, nutrient-rich waters to the surface. This process, known as “upwelling,” creates the ideal conditions for algae to bloom.
The algae produce toxins that can kill fish and the seabirds that eat them, and sicken humans. They also block sunlight from reaching underwater plants, depriving the fish of oxygen.
The Environmental Protection Agency predicts that climate change will alter the “timing and intensity” of coastal upwelling, potentially leading to more algal blooms along the West Coast. Warmer waters caused by climate change could also spur other types of harmful algae, the EPA says. Toxic blue-green algae prefer warmer temperatures.
Storms and extreme weather events, which global scientists predict will worsen in the future, can also lead to algal blooms by causing nutrients to run off from the land into the water. Authorities suspect an algal bloom was responsible for dead sea animals, including sea urchins, starfish and crayfish, washing up on New Zealand’s east coast in February during a particularly stormy Southern Hemisphere summer.
It’s not just coastal environments that are at risk of harmful algal blooms, according to scientists. Droughts, which may become more frequent because of climate change, can cause marine algae to invade freshwater systems, which has happened a number of times in freshwater lakes in the United States over the past couple of decades.
Algal blooms can be caused by humans, too. A rare golden algae thought to have led to a mass fish die-off in the Oder River that runs along the border between Germany and Poland last summer may have been caused by industrial discharges into the river, according to German researchers.
Hundreds of little penguins wash up dead
When hundreds of little penguins washed up dead on New Zealand’s shores in June, local conservation officials said climate change was probably forcing the penguins to venture into deeper and colder waters to forage, making it harder for them to nest, breed and find food — and exposing them to deep-ocean predators.
On Australia’s southern island state of Tasmania, some sea creatures have nowhere to go to escape the rising heat because they can’t migrate farther south into the icy waters of the Southern Ocean, according to scientists.
Researchers found the population of common sea dragons (Phyllopteryx taeniolatus) had fallen by 57 percent over the past decade across the sites they were monitoring, Graham Edgar, a marine ecologist at the University of Tasmania, wrote in a recent article in the Conversation. Sea stars and sea urchins have also experienced “precipitous population declines” over the same period, he said.
According to a study last year, about a third of all marine animals could vanish within 300 years — as oceans absorb the excess heat created by humans, cooking them alive in their own habitats. Already by 2021, oceans contained more heat energy than at any point since record-keeping began six decades ago.
Whales and dolphins die after washing ashore
There has been a spate of whale and dolphin deaths in the Northeast this winter. More than a dozen humpback whales and several critically endangered North Atlantic right whales were stranded on or near beaches from North Carolina to New York between December and early March, according to the National Oceanic and Atmospheric Administration. Eight dolphins also washed ashore in New Jersey recently.
Officials in New Jersey said changes in ocean temperature and water chemistry — which they attribute to climate change — could be drawing the fish that whales feed upon closer to land, putting the whales at greater risk of colliding with shipping vessels. Post-mortem examinations have indicated that ship strikes are the probable cause of many of the whale deaths.
Conservation activists are questioning whether preconstruction activity on a slew of offshore wind turbines could be to blame, although government scientists say there is no evidence linking offshore wind activities to the whale deaths. The turbines are slated for construction in an area where an unusual number of humpback whales have died since 2016, and where scientists and federal officials are working to prevent the North Atlantic right whale, one of the world’s most endangered marine mammals, from going extinct.
Experts don’t always know why whales and other marine mammals wash up on land. Hundreds of pilot whales washed up on the remote shores of New Zealand’s Chatham Islands in two separate “mass stranding” events last year that occurred just days apart. About two weeks earlier, some 200 whales died on Tasmania.
Millions of dead fish clog up a river
Officials said a lack of oxygen sparked by rising temperatures and recent floods caused millions of fish to die in a river in southeastern Australia this month, although some locals blamed the government for water mismanagement.
Videos from the scene showed a thick carpet of silver fish carcasses on top of the water, and residents complained of the stench of rotting fish carcasses.
Joy Becker, an aquatic animal health expert at the University of Sydney, described it as a “blackwater event” where flooding caused a lot of organic material to enter the river system. That was followed by a period of “intense warm air temperatures” that led to a “very low oxygen event” in the river, she said. Fish need more oxygen to survive warmer temperatures, according to officials.
“Why do we have so much flooding and why do we have such extreme air temperatures?” Becker said. “What we’re seeing is a result of climate change.”
Sammy Westfall in Washington and Annabelle Timsit and Jennifer Hassan in London contributed to this report.