Florida dolphins are starving to death because of harmful marine algae blooms, researchers have said.
In 2013, 8 percent of the bottlenose dolphins living in Florida’s Indian River Lagoon perished. Now, new investigations have revealed that the highly intelligent marine mammals may have starved because their hunting grounds were destroyed by a phytoplankton bloom produced by human activity.
“We linked mortality and malnutrition to a decreased intake of energy following a shift in dolphins’ diets,” Dr. Charles Jacoby, of the Florida Flood Hub for Applied Research and Innovation, explained in a statement.
“We linked the dietary shifts to changes in prey availability, and we connected changes in prey to system-wide reductions in the abundance of seagrass and drifting macroalgae. These reductions were driven by shading from an intense, extensive, and long-lasting bloom of phytoplankton,” he said.
Jacoby is the corresponding author of the research which was published Monday in the journal Frontiers in Marine Science.
The bloom was driven by the accumulation of fertilizer, sewage from septic tanks and other human by-products that are rich in nutrients, including nitrogen and phosphorus. Too many nutrients are in the water can cause phytoplankton to grow out of control and form harmful algae blooms that can have harmful effects on animals and people.
Harmful blooms can have multiple catalysts. A toxic algae bloom in Southern California is currently affecting sea lions and dolphins — but that bloom is produced by an upwelling of nutrient-laden water from the deep ocean that is producing a toxin that poisons marine mammals. Blooms driven by rising temperatures and nutrient pollution have also harmed Florida’s manatees, affecting their main food source.
For the bottlenose dolphins, researchers suspected that changes were caused by a 2011 phytoplankton bloom fueled by nutrient-rich by-products of human activity flowing into the lagoon. The bloom killed off key habitats for dolphins’ prey. Although, it was hard to prove what dolphins were eating, because surface observations don’t capture the full picture.
The dolphin population was “struggling,” with 64 percent of the 337 dolphins observed in 2013 deemed to be underweight; of those, 5 percent were emaciated and 77 died. Recent estimates show there are just over 1,000 dolphins swimming in the lagoon.
By analyzing muscle tissue taken from stranded dolphins between 1993 and 2013, scientists tracked dietary changes over time. They were able to determine what the dolphins were eating because of similar variants of the elements carbon and nitrogen in the dolphins’ muscle and their prey. Using their findings, the researchers found there was a shift in the dolphins’ diet between 2011 and 2013. They ate less fish associated with seagrass, known as ladyfish, and more sea bream.
Fisheries recorded changes in the availability of ladyfish that matched the falling abundance of seagrass and macroalgae habitat over the same period. Unfortunately, a shift to sea bream meant that dolphins would need to eat about 15 percent more prey to acquire the same amount of energy they normally would get.
Between 2000 and 2020, the authors said malnutrition caused 17 percent of all recorded deaths. In 2013, that percentage skyrocketed to 61 percent.
“In combination, the shift in diets and the widespread presence of malnourishment suggest that dolphins were struggling to catch enough prey of any type,” said Wendy Noke Durden of Hubbs-SeaWorld Research Institute, a co-author. “The loss of key structural habitats may have reduced overall foraging success by causing changes in the abundance and distribution of prey.”
