Exploration of a dynamic thermal corridor: Experimental oceanography and migration of North Pacific loggerhead sea turtles.

 

Until recently, scientists had no idea how loggerhead sea turtles (Caretta caretta) migrate from their nesting beaches in Japan to nursery grounds in Baja California, Mexico. The Thermal Corridor Hypothesis (TCH, Briscoe et al. 2021) combined 16 years of satellite-tracked movement data to propose an intermittent thermal corridor that allows juvenile loggerheads to transition from the Central North Pacific (CNP) to the west coast of North America. The TCH proposes that this migratory corridor opens during anomalously warm conditions and closes during cool conditions, causing turtles to stay in the CNP. Here we report on the deployment of two experimental cohorts, one in 2023, during warm El Niño conditions, and one on 2024, during the onset of cooler La Niña conditions. In 2023, all loggerheads experienced warmer than average SST, due to a marine heatwave and an El Niño. The entire cohort moved north until September 2023, and then they moved south, with 7 of 23 turtles moving towards North America — 3 of which entered coastal waters of Southern California and Baja Mexico. These responses confirmed the TCH that under warm conditions, loggerheads can pass through the thermal corridor. In 2024, the experimental cohort was deployed with a projected onset of La Niña into the fall. Initially, nearly 2/3rds of the turtles headed east. It is possible that strong Ekman transport and/or warm SST anomalies, in response to especially strong westerlies, may have driven these eastward movements, leading to the loggerheads entering the California Current further north than is typical. Late fall movements tended to shift southward again, but the cold California current may prevent reaching the coastline. Distributional shifts due to changing ocean conditions will allow us to dynamically manage and protect this species. If a corridor were to open more frequently or in a changing ocean location, it could alter abundances and risks for these endangered turtles.