Using Long-term Monitoring to Assess if Humpback Whales in the Gulf of Alaska are Really in Hot Water

Since the marine heat wave in the North Pacific (2014-2016), humpback whales from two feeding areas in the Gulf of Alaska, Southeast Alaska and Prince William Sound, have returned from their winter breeding grounds with fewer calves, skinny whales, and shifts in distribution compared to the previous decades. Declines in crude birth rate (number of calves per total number of whales) could be an indicator of a population not meeting its energetic needs due to oceanographic shifts in prey availability. Alternatively, a decline in reproduction is expected for a population approaching population equilibrium (carrying capacity) after decades of population growth since protection from commercial whaling in 1966. To help distinguish long-term population trends from climatic perturbations we asked the question: What crude birth rate would indicate a stable population? We simulated a humpback whale population trajectory using a range of population-specific parameters, derived from decades of long-term monitoring, for age at first parturition, age-specific survival and reproduction probability and calculated the crude birth rate when stable trajectories emerged. We found that observed crude birth rates following the marine heat wave were well below those predicted for population stability. The complexity of ecosystem variability under a changing climate highlights the importance of identifying accessible and responsive metrics of population health and creating long-term monitoring programs capable of reliably detecting population-level changes.