Scientists were puzzled to see dozens of dolphins suddenly perish in Florida in 2013. They have been working to solve the mystery since then, and now, 12 years later, they know what led to the mass deaths. According to reports, 77 bottlenose dolphins died in Florida’s Indian River Lagoon days after two-thirds of them started showing signs of starvation. A dense chain of events has been linked to the deaths, with human activity as the initial trigger.

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The "unusual mortality event" led to nearly one-tenth of the population losing their lives that year. Scientists started to dig into the probable cause of the deaths, which was likely linked to their diet.

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However, finding out the reason for the starvation was a complex process. To know which fish the dolphins were eating was not easy, especially when the observations were only being made from the surface. 

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Dolphin tissue was studied to understand reason for deaths

They turned to scientifically analysing the corpses of the dolphins to know what they had been eating. A method known as 'isotopic analysis' was used to study their tissue since every species that is consumed by a predator leaves a chemical imprint inside them. 

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The study of the traces that lasted over a decade has revealed that the dolphins were not consuming the right food for them. Their diet patterns had significantly changed from 2011. Before 2011, they had been feeding on ladyfish, which contained the right nutrients for their energy needs.

However, this gradually changed in the lead up to 2013 and they went from feeding on ladyfish to sea bream. The latter was not as healthy and nutritious for the dolphins, who slowly started to starve. Their bodies started losing fat, and they appeared leaner.

Algae bloom decreased ladyfish population

The entire diet change was linked to an algae that spread in the regions that catered to the ladyfish. Scientists say that the ladyfish population started declining because their seagrass habitat was taking a hit from the intense algae bloom.

An intense bloom of phytoplankton, a type of algae, lasted a long time and shaded the seagrass. It started shrinking, thus affecting not only the ladyfish but also the bottlenose dolphins, who didn't have enough food and had to change their ways to survive.

BBC Wildlife reported that humans were to blame for the growth of algae. As fertiliser and septic tank waste entered the waters, it triggered the algae bloom, which took over the ladyfish habitat.