The exodus of sperm whales from Mexico's Gulf of California in response to jumbo squid population declines may be indicative of broader ecosystem instability, a new study has found. As apex marine predators, sperm whales are instrumental in controlling oceanic energy flow and can be a key barometer for marine health, according to the study, published on Tuesday in PeerJ Life and Environment.  Over the course of nine years, a group of scientists from several research institutes in Mexico harnessed extensive survey data and photo-identification tools to follow sperm whale populations near the eastern Midriff Islands in the gulf. Between 2009 and 2015, they observed that the number of whales in the region ranged from about 20 to 167 individuals, which would indicate a "super population" — an estimated total number of animals — of about 354. But from 2016 to 2018, the scientists noticed that sperm whale sightings ceased entirely. The departure of the whales, the researchers found, occurred following sharp declines in the presence of their main prey, the jumbo squid. "The departure of sperm whales from the Gulf of California serves as a sentinel signal, reflecting significant shifts in marine ecosystems," Lead author Héctor Pérez-Puig, of the Prescott College Kino Bay Center for Cultural and Ecological Studies, said in a statement. Pérez-Puig and his colleagues attributed the dwindling of both the sperm whale and the jumbo squid populations to environmental shifts, such as sustained ocean warming and El Niño-related weather extremes. These ecosystem dynamics have affected the jumbo squid in particular, and the smaller animals that replaced them have proved insufficient to sustain sperm whales, according to the study. Jumbo squid, per the study, were once staples in Mexico's fishing industry, but their populations suffered numerous fluctuations over the past few decades. Total Gulf of California catches, for example, went from more than 100,000 tons during a 1996-1997 La Niña pattern — characterized by calmer, milder weather — to 3,000 tons the next season, when the region was experiencing strong El Niño conditions. In 2018, jumbo squid catches reached an all-time low, clocking in at just 169 tons for the year. In light of their findings, the scientists stressed the importance of studying the impacts of climate change on marine species, with an emphasis on what they described as the "tropicalization" of the Gulf of California. "As the environment changes, so too does the delicate balance between predators and prey," Pérez-Puig said.