ウミガラス約１００万羽が餓死、温暖化による熱波が原因か　研究

【ワシントンＡＦＰ＝時事】2015～16年に発生した熱波の影響で餌の確保が困難になり、北太平洋に生息する海鳥であるウミガラス約100万羽が餓死したとする研究結果が15日、発表された。鳥類ではこれまでで最大規模の大量死になるという。（写真は資料写真）
　米科学誌「プロスワン」に掲載された論文の共同執筆者で、米ワシントン大学の生態学者ジュリア・パリシュ氏はＡＦＰの取材に、こうした熱波の発生頻度の増加は「比較的新しい」現象で、気候変動に関連しているとみられると語った。
　１４年から１６年にかけ「ブロブ」と呼ばれる暖かい海水の巨大な塊が発生したが、これに強力なエルニーニョ現象が重なった結果、米カリフォルニア州からアラスカ州まで広がる地域に海洋熱波が生じた。
　エルニーニョは不規則に発生する世界規模の気象現象で、中・東部赤道太平洋での温かい海水の発生に関連している。
　１５年夏から１６年春にかけ、やせ衰えたウミガラス約６万２０００羽が死んだ、もしくは瀕死（ひんし）の状態で北米太平洋沿岸に打ち上げられた。この期間に米アラスカ州に打ち上げられた死骸の数は、通常の１０００倍に上っていた。
　今回の調査対象範囲の海岸に打ち上げられた死骸は全体のごく一部であり、研究チームは調査期間内に死んだ個体の総数は５０万～１２０万羽に上ると推定している。
　１９８９年にアラスカ沖で発生した石油タンカー「エクソン・バルディーズ」号の原油流出事故時に発見された海鳥の死骸は約３万羽、事故が原因で死んだ鳥の総数は３０万～６０万羽だと推定されていると、論文の筆頭執筆者で、米地質調査所アラスカ科学センターの調査生物学者、ジョン・ピアット氏は指摘した。
　ウミガラスの他に、海鳥のエトピリカやアメリカウミスズメ、アシカ、ヒゲクジラなど数種類の動物が大量死に見舞われている。
　だが、ウミガラスの大量死は総数、期間、地理的範囲などあらゆる計測値において、最大規模だったという。その影響範囲はカナダの面積に匹敵する。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2020/01/16-13:07）

Around a million North Pacific seabirds known as common murres died during a 2015-16 heatwave that disrupted their food supply, scientists said Wednesday, the biggest mass mortality event ever recorded among avian species.
Julia Parrish, an ecologist at the University of Washington and co-author of a paper on the subject published in the journal PLOS ONE told AFP that the increased frequency of such heatwaves appeared to be relatively new and linked to climate change.
From 2014 through 2016, an enormous mass of warm seawater nicknamed the blob was joined by a strong El Nino, with the two combining to create a marine heatwave stretching from California to Alaska.
El Nino is an irregularly occurring global climate phenomenon associated with warm waters developing in the central and east equatorial Pacific.
During the period from summer 2015 to spring 2016, around 62,000 emaciated murres washed ashore dead or dying along the North American Pacific coastline -- in Alaska alone the rate of die-off was a thousand times higher than normal.
Since only a tiny fraction of birds that die was ashore on areas that are accessible, they estimated the total amount of deaths was between half a million to 1.2 million.
By comparison, 30,000 bird carcasses were recovered after the Exxon Valdez oil tanker spill off the Alaskan coast in 1989, leading to overall death estimates of 300,000 to 600,000, said John Piatt, a research biologist at the US Geological Survey's Alaska Science Center and the paper's lead author.
- How it happened -
Parrish said the effects of the heatwave were two-fold. First, elevated temperatures reduced the quality and quantity of phytoplankton, which in turn reduced the quantity and quality of the fish eaten by common murres -- prey like herring, sardines and anchovies.
Second, as waters warmed, it increased the energy requirements of larger fish like salmon and Pacific cod, that have the same prey as the murres, foot-long birds with black and white bellies that are fast flyers and adept at diving at depths up to 200 meters below the water's surface to forage.
Piatt added that the murres' evolutionary Achilles heel was their need to consume half of their body mass every day.
Everything they do depends on that breast muscle. And when they can't eat three or four days, they burn up all that muscle, and can no longer fly or dive, he told AFP.
Compounding the problem, many of the birds that died were of breeding-age. Murre breeding colonies across the entire region failed to produce chicks for years during and after the heat wave event, the study found.
Several other species experienced mass die-offs during the same period, including tufted puffins, Cassin's auklets, sea lions and baleen whales.
But the common murre die-off was by far the largest recorded by all metrics -- overall number, duration and geographic extent -- the area affected was equivalent in size to Canada.
Taken together, the mass mortalities demonstrate that a warmer ocean world is a very different environment and a very different coastal ecosystem for many marine species, said Parrish, with sea birds, as highly visible members of that system, bellwethers of that change.