During the murre\u2019s annual breeding season, Renner explained, the researchers \u2014 equipped with binoculars or spotting scopes and a tally counter \u2014 spent long days, often in the cold and fog, observing the birds\u2019 populations in dense breeding colonies from afar, whether they watched while perched on an adjacent cliffside or wading in a small boat.<\/p>\n
\u201cDuring and immediately after the heatwave,\u201d Renner wrote in an email, \u201cmany murres didn\u2019t nest as they normally do. They would show up on the cliffs one day and another day nobody would be there.\u201d She added, \u201cIn 2015 and 2016, there were a number of colonies that had complete reproductive failure.\u201d<\/p>\n
\u201cImmediately after the heatwave, we knew there had been a tremendous loss, but we weren\u2019t sure how much was due to birds dying or alternatively whether they were just skipping breeding because conditions were tough,\u201d she added. \u201cWe had to wait several years for the birds to come back and resume breeding before we could be sure of the extent of the die-off.\u201d<\/p>\n The study authors wrote that the heatwave had an enormous effect on key habitat-forming species, like corals and kelps, which triggered cascading bottom-up harm toward the top of the food chain.<\/p>\n Common murres rely on fish as a food source, but the fish they normally prey on were largely absent due to the cascading effects of the heatwave, making the most likely cause of the die-off a mass-starvation event.<\/p>\n Most of the birds died during the winter months, Renner said, and although we don\u2019t know exactly what the murres eat in the winter, it\u2019s likely a combination of forage fish, juvenile pollock and krill.<\/p>\n \u201cWe know that many of the forage fish populations also collapsed at the time, and not just the numbers. They they were smaller at the same age, they [had] lower fat contents, their distributions changed,\u201d she said. \u201cAnother important part of it is that fish are cold blooded, and when the waters warm, their metabolism increases. And so a lot of the bigger fish that are competitors with seabirds had higher metabolisms and needed to eat more. So there\u2019s extra pressure on prey.\u201d<\/p>\n At the same time, the prey fish themselves, with their high metabolisms, struggled to find enough food, which reduced energy flow to predators. With increased competition and less total food, the murres\u2019 beaching rate \u2014 the rate at which their carcasses washed up on beaches \u2014 was \u201cone to three orders of magnitude above baseline for nine consecutive months,\u201d which all appear to be major contributing factors to what the authors believe to be the \u201clargest mortality event of any wildlife species reported during the modern era.\u201d<\/p>\n Despite having plenty of time, the murres\u2019 population has not begun to recover, but the cause is unclear, Renner said. One hypothesis is that with severely smaller colonies, it\u2019s harder to defend against predators, like apex predator eagles or seagulls that take their eggs.<\/p>\n Renner said that on top of long-term monitoring data that shows which species are vulnerable, conservation measures like removing predators introduced to their colonies could be a huge boon to their reproduction rates.<\/p>\n The study authors also warn that marine heatwaves are \u201cincreasing in frequency, duration, and magnitude in lockstep with global warming and at a pace that threatens the response rates of seabird communities.\u201d<\/p>\n \u201cWe suggest that the rapid and long-lasting decline of an abundant and widespread upper trophic predator, to less than half of its former population size in Alaska, may signal a new threshold of response to global warming,\u201d the authors concluded.<\/p>\n