北極圏のクジラ、冬に南下せず　気候変動が回遊に影響か

【東京ＡＦＰ＝時事】不気味なすすり泣きや鳴り響くごう音、幽鬼のようなうめき声──海中マイクが拾った音には、言葉はなくとも物語があった。それは、クジラたちがその海域から動かなかったという証拠だ。（写真は資料写真）
　２０１８～１９年の冬にカナダ沖の北極圏のいてつく海で収集された録音から、ホッキョククジラの群れが例年と異なり、その年は南下回遊を行わなかったことが明らかになった。研究者らは、初めて確認された現象だとして、気候変動の影響で北極圏の生態系全体の動態（ダイナミクス）が変容する前兆の可能性があるとみている。
　カナダ沖の海域には約２万頭のホッキョククジラが生息し、距離６０００キロに及ぶ回遊パターンは通常なら予測がたやすい。
　クジラたちはベーリング海で越冬した後、北上してチュクチ海を抜けて東へ進路を取り、ボーフォート海からカナダ・アムンゼン湾の沖合で夏を過ごす。そして秋にはまたベーリング海へと南下していく。
　だが、２０１８～１９年の冬はカナダ沖で、例年ならとっくに姿を消しているはずのクジラの群れを見たとの目撃情報が相次いだ。そこで、研究チームがこの海域に複数設置してある水中マイクが記録した音を調査したところ、南の海で越冬するはずのホッキョククジラの特徴的な鳴き声を確認した。
　コンピューター解析では、これまでこの海域では記録されたことのない交尾に関連するとみられる鳴き声も確認できた。研究結果は２１日、英学術誌「ロイヤルソサエティー・オープンサイエンス」に掲載された。
　研究を主導したカナダ野生生物保護協会のスティーブン・インズリー氏は、海水温が群れの異例な行動に大きな影響を及ぼしたのではないかと考えている。
　インズリー氏によると、ホッキョククジラは零下０．５度～２度の海水温を好み、この温度帯以外の海域を避けることで知られる。
　「この海域全体が劇的に変化しつつあり、われわれはその始まりを見ているにすぎない。亜北極帯に生息する種の多くが北上を始めている」とインズリー氏は指摘。「これは、全面的な生態系の変化が起きているということだ。勝者と敗者が出てくるだろう」と述べた。
　研究チームはこの海域での録音を継続し、収集したデータを海水温の記録情報と比較して関連性の有無を明らかにしたいとしている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】

〔ＡＦＰ＝時事〕（2021/04/28-13:49）

Eerie wails, explosive trumpets and ghostly moans. The sounds from the underwater recorders had a story to tell, even without a single intelligible word: the whales had stayed put.
The recordings gathered during the 2018-2019 winter in the freezing Arctic waters off Canada proved that a population of bowhead whales had skipped their usual migration south.
Scientists believe this behaviour -- never previously detected -- could be driven by the effects of climate change, and be a potential harbinger of shifting dynamics across the region's ecosystem.
Ordinarily, the approximately 20,000 bowheads that make up the Bering-Chukchi-Beaufort (BCB) population around Canada have a fairly predictable migration pattern spanning 6,000 kilometres (3,700 miles).
They spend the winter in part of the Bering Sea, which lies between Russia and Alaska, and head north then east to the Beaufort Sea and Canada's Amundsen Gulf in the summer, before returning in the autumn.
But in winter 2018-2019, something different happened. Residents in the Canadian region reported seeing bowheads long after they would normally have disappeared south.
A team of scientists decided to comb through hours of audio recorded by underwater devices that are dotted around the region for regular data collection, listening for unusual sounds.
They found them: the distinctive calls of bowhead whales that should have been in their southern winter grounds but had stayed put.
Assisted by a trained computer programme, they even found recordings of bowheads singing, a behaviour believed to be associated with mating, which has never been recorded in the summer grounds before.
The whale noises appeared in between 0.5 to 3.0 percent of recording files collected between October to April at four summer spots.
The finding was highly unusual: recordings from some of the same and separate sites in the summer grounds in previous years picked up no whale sounds after October or December, depending on the location.
The evidence is clear that BCB bowheads overwintered in their summer foraging region in the eastern Beaufort Sea and Amundsen Gulf during the 2018-2019 winter and as far as we know, this is the first time it has been reported, says the study published Wednesday in the Royal Society Open Science journal.
- 'Ecosystem shift under way' -
Less clear however is why this happened, with the authors positing various theories mostly linked to climate change.
One possible factor could be shifting ice cover, with less ice than usual seen in the summer grounds during the 2018-2019 winter season.
But the record minimum ice concentration actually came in 2015-2016.
That suggests ice, and particularly timing and locations, is important but not the only factor, said Stephen Insley of the Wildlife Conservation Society Canada, who helped lead the study.
Another possible explanation is predator avoidance, with the bowheads steering clear of orca whales that are more frequently seen in some areas as warming seas lead to decreased ice cover.
Other phenomena linked to climate change could also be at play, like the increasingly erratic and early summer plankton bloom -- whales could be spending winter in their summer grounds to ensure they catch the key food source, the scientists suggest.
Insley suspects water temperature is playing a key role in the unusual behaviour, with bowheads known to avoid water outside a narrow range of around -0.5 to 2 degrees centigrade.
So could the behaviour be an adaptation to climate change, and if so, what does that mean for bowheads?
It may be positive now, but not down the road when water temperature is warmer, said Insley.
It's hard to say and that's why we're trying to keep an eye on it.
Bowheads certainly wouldn't be alone in reacting to a warming climate.
The whole region is undergoing dramatic change and we're just seeing the beginning of it. Many sub-Arctic species are moving north, Insley added.
It's a complete ecosystem shift under way and there will be winners and losers.
The team is continuing to record in the region and hopes to correlate its data with information about ocean temperatures to determine any link.
If the avoidance of warm ocean temperatures were the primary driver of this anomalous behaviour, it may be a significant warning sign for bowhead whales, the study cautions.