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One of Earth鈥檚 rarest whales is a picky eater.

A single fish species makes up roughly 70% of the diet, new research shows, unlocking yet another secret behind the elusive Gulf of Mexico whale that could help inform future conservation efforts.

The ultrarare whale鈥檚 entree of choice? The silver-rag driftfish, a tiny schooling fish that鈥檚 common in the gulf and high in calories.

Because the whales burn energy by diving more than 800 feet for their food, the deep-swimming driftfish have all the high-quality ingredients for the perfect meal, scientists say. Plenty of protein and lots of lipids.

鈥淚t鈥檚 a huge piece of information for us: If we want to protect these animals, we need to know what they eat,鈥� said Jeremy Kiszka, a marine scientist at Florida International University and lead author on published this week in Scientific Reports.

鈥淚f this prey is impacted by any human activity 鈥� or climate change 鈥� that might actually compromise the survival of this whale species in the Gulf of Mexico,鈥� Kiszka said in an interview.

鈥淣ow that we know what they eat, we need to better protect their habitat and their prey.鈥�

While this new clue is yet another tool to help inform policymakers鈥� conservation decisions, it also underscores the vulnerability of the Rice鈥檚 whale.

The animals are selective with their food, which could make them more vulnerable to fishery declines and habitat loss, Kiszka said.

鈥淓nvironmental changes in the region have the potential to influence prey species that would make them less available to Rice鈥檚 whales,鈥� the study reads.

Researchers believe there are only 50 Rice鈥檚 whales left, making it one of the world鈥檚 most endangered whale species.

Their only habitat is the northeastern Gulf of Mexico, and until recently, scientists believed they were a subspecies of the more common Bryde鈥檚 whale. But in January 2021, after genetic tests, federal ocean scientists announced the Rice鈥檚 whale was a species of its own.

The animals, which can grow up to 40 feet in length, like to feed during the day, according to Kiszka.

Usually when a researcher wants to study a whale鈥檚 diet, they can use washed-up carcasses to take a look in their stomachs.

But Rice鈥檚 whales are obviously hard to find, so this research team, which included scientists from the National Oceanic and Atmospheric Administration, had to get creative.

Over a five-year period, the team took noninvasive skin and blubber samples from 10 whales, or about 20% of the estimated population. They used a crossbow to collect their biopsy samples. Each whale they found was mature and seemed to be in good physical condition, with no signs of emaciation or struggling, according to the study.

Then they used a method called stable isotope analysis on those samples, which essentially traces the chemical makeup of the whale鈥檚 diet.

The team also collected local fish species and used mathematical models to reconstruct the whale鈥檚 diet by comparing the chemical composition of the fish to the whales.

As Kiszka explains it: 鈥淵ou are what you eat.鈥�

鈥淨uality probably matters the most for these animals. They鈥檙e looking for high-quality, high-fat content prey in their ecosystem.鈥�

Conservation efforts are needed

There鈥檚 evidence from across the world that when a prey declines, so does its marine mammal predator. In the study, scientists point to the decreasing abundance of the Mediterranean common dolphin. That species is struggling because of the decline of its prey, small pelagic fish in the Ionian Sea.

That鈥檚 a cautionary tale for what could happen to the Rice鈥檚 whale if conservation isn鈥檛 prioritized. Last year, scores of scientists to the Biden administration urging a moratorium on oil and gas exploration in the gulf, and also urged the reduction of large boat traffic.

鈥淭he apparent reliance on, and selectivity for, a prey species that constitutes the majority of the diet of Rice鈥檚 whales makes the protection of (the fish) an essential component of protection and conservation strategies for this whale species,鈥� the study states.

鈥淭his is especially important in light of the high level of industrial activity within the (Gulf of Mexico).鈥�

This is the first study highlighting what Rice鈥檚 whales eat in the gulf, and also the first to examine why they may be targeting their particular prey, according to the research.

The study began in 2017, before researchers knew the Rice鈥檚 whale was its own unique species. But that鈥檚 only increased the stakes of the research since the discovery, Kiszka said.

鈥淭his is a whale species that occurs right in our backyard 鈥� and nowhere else,鈥� he said. 鈥淭hey鈥檙e super critically endangered, and we need to protect them urgently.鈥�

This story was produced in partnership with the Florida Climate Reporting Network, a multi-newsroom initiative formed to cover the impacts of climate change in the state.