Shellfish on a beach near Batic sea/ Image by ASSY from Pixabay

Thousands of dead shellfish washed ashore on a beach in Saltburn-by-the-Sea, North Yorkshire on Tuesday. This is the same region that saw a mass crustacean die-off in 2021.

The latest incident has left authorities baffled as they continue to find the reasons behind it. The UK Environment Agency believes that high tides and stormy weather could have caused the mass die-off.

It added that the phenomenon is "normal at this time of year." Stuart Marshall, who owns beach huts on the promenade, told PA News agency that there were clams, oysters, and crabs along with the other species that were found dead.

The beach has black debris all over the shoreline, and locals believe that it could have been sea coal. However, there is no confirmation as to what this black debris really is.

Mass fish deaths have become quite a common occurrence over the last few years, not just in the UK but across other countries. Last year, the unprecedented heatwave witnessed by the UK caused significant damage to marine life.

Several incidents of mass fish die-offs were reported from various parts of the country last year. In June 2022, around 100 dead roach fish were found at Belper River Gardens in Derbyshire after temperatures went past 30ºC in the area.

In 2019, hundreds of thousands of mackerel and pilchards were found dead on a beach in Cornwall. The fish washed up on the beach at Marazion, near St. Michael's Mount, in the southwest of Cornwall. The authorities were unable to find a definitive cause in this case as well.

Around 2,500 endangered seals were found dead in the southern Russian province of Dagestan in December last year.The carcasses washed ashore on the coast of the republic of Dagestan. According to authorities, the seals likely died of natural causes.

Experts at the Federal Fisheries Agency did not find any evidence suggesting that the mass die-off may have been caused by pollutants. Caspian seals have been classified as endangered by the International Union for Conservation of Nature (IUCN) since 2008.

Recently, a similar incident was reported in Australia. Millions of dead fish were found in a river in the Australian Outback after the country saw record temperatures. The rotting fish clogged a portion of the country's second-longest river over the weekend.

The mass die-off occurred near the New South Wales town of Menindee on the Darling River. The fish are thought to have died due to a shortage of oxygen in water after the temperatures in the region reached 40°C (104 °F).

In August last year, Polish authorities recovered 100 tonnes of dead fish from the Oder River that flows through Poland and Germany. The "ecological catastrophe" baffled scientists in both countries, as the local agencies had been reporting seeing dead fish in the river since late July.

Mass mortality events have become a recurring phenomenon as the planet gets warmer and warmer. This could affect the ecosystem negatively in the long run.

According to a report by researchers at the University of Arkansas, incidents of mass die-off increase with the rise in temperature. Low levels of oxygen in bodies of water could also lead to mass die-offs in some cases. Since warm water holds less oxygen than cold water, it may lead to a reduction in water oxygen levels in the summer.

Algal blooms can also cause a shortage of oxygen in the water. Algal blooms are an overgrowth of algae in bodies of water. Some of them release dangerous toxins, making the water unfit for humans and marine life.

Algae produce oxygen through photosynthesis during the day, but at night they are forced to use available oxygen levels in water because an absence of sunlight implies they cannot produce oxygen.

Algae and other marine species use up most of the oxygen, eventually leading to its shortage. Mass fish deaths can also occur due to the presence of toxic compounds in water bodies.

A study by researchers from Washington State University found that a chemical called "6PPD-quinone" has been responsible for killing salmon in urban streams. The chemical is found in car tyre particles and is a ubiquitous pollutant. It is added to tyres to keep them from breaking down.

The study was carried out to find out the causes of the decline in the coho salmon population along the US Pacific Coast. The frequency of such events should be a cause for concern not just for the local population, but for the world at large. The extinction of even one species could have a severe impact on the entire food chain.