Interannual variation in the presence and absolute abundance of gray whales in the Bahía Magdalena complex, Mexico.

The Eastern North Pacific (ENP) gray whale population, an example of a large cetacean population that recovered from whaling depletion, migrate to subtropical wintering waters, and concentrate in strategic study areas, like the Magdalena Bay complex. Recent unusual mortality events (UME) and increasing threats have had impacts on the population, and the lack of updated information on the complex’s population makes it crucial to estimate essential parameters to understand its state and facilitate its conservation. Absolute abundance, survival rates, and factors making them vary were estimated to analyze the inter-annual variation in the absolute abundance of gray whales in the complex to understand the wingspan of such events and enhance management strategies. Using photo-identification gathered in the complex between 2016 and 2022 through the MARK program enabled the estimation of the ENP population’s trends, abundance, survival, and recapture probabilities depending on single individuals and breeding females. Results indicated decreasing survival from 2016 to 2022. While the UME rapidly lowered mother and calf annual abundance by 82% between 2017 and 2022, it lowered single individuals’ numbers by 25% between 2020 and 2021. This supports that the ENP population is declining, highlighting the need for continued monitoring for its survival and conservation.