Sharp decline in humpback whale ( Megaptera novaeangliae ) survival and reproductive success in southeastern Alaska during and after the 2014–2016 Northeast Pacific marine heatwave

Understanding the ecosystem effects of ocean warming is increasingly important as marine heatwaves become more common and increase in severity. Here, we used Glacier Bay National Park long-term monitoring data (1985–2020) to investigate a sudden, sharp decline in humpback whale reproductive success and survival following the onset of the 2014–2016 Northeast Pacific marine heatwave (PMH). Oceanographic data confirm a persistent warm-water anomaly in 2015–2016 in Glacier Bay, months later than the PMH was documented in the North Pacific. We assessed changes in demographic parameters pre- and post-PMH using whale and calf counts and multi-state closed population capture–recapture models. Non-calf abundance decreased by 56% between 2013 and 2018, followed by increases in 2019–2020. The predicted proportion of females in the population declined in 2015–2017 (0.40–0.44). For 5 years during and after the heatwave (2015–2019) calf production was far lower than historic levels (0.041 calves per adult female, in contrast to 0.27 pre-PMH). Calf survival dropped tenfold beginning with calves born in 2013 (0.396–0.032) and midsummer calf losses occurred at an unprecedented rate starting in 2014. Non-calf survival declined from 0.982 pre-PMH to 0.899 post-PMH, lower than any value reported for this species. We surmise that documented changes to the forage fish and zooplankton prey base during and after the PMH were the main driver of reduced humpback whale survival and reproductive success. Humpback whale abundance and productivity in southeastern Alaska will likely take years to recover from the PMH, assuming a return to favorable feeding conditions. Our work highlights this population’s continued vulnerability as the climate warms into previously unobserved states.