Bottlenose dolphins alter migration route off Costa Mar coast

For three centuries, the bottlenose dolphins that inhabit the waters off Costa Mar have followed a predictable seasonal pattern: they move north to the cooler, nutrient-rich waters around the Isla Blanca archipelago during the rainy season, then retreat southward as the dry season approaches, following the baitfish stocks that constitute their primary food source. That pattern appears to be breaking. Marine biologists at the Federal Oceanographic Institute's Costa Mar station have documented a significant shift in the timing and destination of the dolphins' dry-season migration, with profound implications for understanding how marine mammals are responding to environmental change.

The shift has been gradual but measurable. In the early 2010s, ninety-two percent of the identified dolphins departed the northern coast by October. Today, that figure has fallen to sixty-three percent, with the remainder staying through December and some individuals overwintering entirely. The dolphins that remain are concentrating in deeper water off the central coast, in areas where upwelling brings cold, nutrient-dense water from the deep. The change has been noticed by local fishers, tour operators, and the dolphins themselves, though the dolphins, of course, cannot explain their reasoning.

Dr. Humberto Vázquez, the Institute's chief marine mammal researcher, has been tracking the population since 2012 and has access to decades of earlier data compiled by his predecessors. He spoke to the bureau about what the dolphins might be telling us about the state of the coastal ocean.