Back in 2013, researchers monitoring bottlenose dolphins in Florida’s Indian River Lagoon noticed something concerning. Nearly two thirds of the dolphins were significantly underweight. In fact, they were starving.
Almost one tenth of the population died that year. This number was so high that the event was considered an 'unusual mortality event'. When such events occur, it’s vital to find out what happened so that we can try to stop it happening again.
But finding the cause of the starvation event was not straightforward. While scientists could glean some information about what fish species the dolphins were eating by observing them from the surface, this did not give a full picture of the dolphins’ diet – and why it was that they were starving.
To solve this puzzle, scientists have just done something called 'isotopic analysis' on the stranded dolphins’ tissue. Every species leaves a chemical trace in the tissue of whatever eats it, and isotopic analysis picks up on this trace. By using this method, scientists could see – over a decade later – which fish the dolphins had been feeding on.
Results showed that the dolphins’ diet shifted in the lead up to the 2013 starvation event. While they had been eating nutrient-rich ladyfish before 2011, they then shifted to eating less nutritious sea bream. The sea bream did not meet their energy needs.
Scientists linked this shift in diet to a lower abundance of ladyfish, whose population had suffered because their seagrass habitat was in decline.
But why was the seagrass suffering?
The culprit was phytoplankton, a type of algae. While not all phytoplankton blooms are detrimental to ecosystems, this bloom was intense and long-lasting. It shaded the seagrass, reducing its prevalence and causing knock-on effects to fish populations, and then dolphins.
The bloom was caused by nutrient-rich byproducts of human activity, such as fertiliser and septic tank waste, flowing into the lagoon.
“If we want systems like the Indian River Lagoon to support dolphins, we have to control delivery of nutrients resulting from our disposal of wastewater, use of fertiliser, and other activities,” Dr Charles Jacoby, an author of the study, tells BBC Wildlife.
Now the cause of the mass mortality event is known, the hope is that actions can be taken to lower the risk of this happening again.
The study was published in Frontiers in Marine Science.
Main image: Indian River Lagoon in Florida. Credit: Getty
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