Table 1_Shifts in marine bird abundance and species composition following the 2014−2016 Pacific marine heatwave.docx

The 2014−2016 Pacific marine heatwave (PMH) was an intense and prolonged environmental disturbance that significantly disrupted the marine food web, leading to widespread ecological impacts. The PMH contributed to major shifts in species distributions, mass mortalities, and reproductive failures among upper-trophic level species, including a massive die-off of common murres (Uria aalge) in the Gulf of Alaska (GOA). To assess the impact of the PMH on the winter marine bird community in Prince William Sound (PWS), a large embayment in the northern GOA, we analyzed changes in winter marine bird abundance and species composition in a series of bays before and after the PMH. The overall density of winter marine birds decreased and species composition significantly changed in PWS following the PMH. Specifically, common murres, cormorants, and loons decreased from pre-PMH survey densities, while marbled murrelet (Brachyramphus marmoratus) densities increased. The post-PMH increase in marbled murrelets, likely due to immigration, coincided with the rapid growth and spatial expansion in the PWS Pacific herring (Clupea pallasii) young-of-the-year population and with a smaller, 8-month marine heatwave across the northern GOA. We suggest the mass mortality and lack of recovery by the common murre population provided a competitive release enabling murrelets to exploit a growing forage fish population, and that murrelets may be more effective at shifting to warmer-water zooplankton during marine heatwave events. These results highlight the persistent upper-trophic level changes associated with the PMH and provide important insights into the ecological consequences of environmental disturbances. This is increasingly relevant given the predicted increase in frequency and intensity of marine heatwaves.