海藻を食べるヒツジ、温室効果ガス削減のカギとなるか　英国

【ＡＦＰ＝時事】英スコットランド沖オークニー諸島の小さな島に、冬の間の餌として海藻を食べるヒツジが数千匹生息している。このユニークな食性について科学者らは、地球温暖化の原因の一つである、メタンガスの排出削減に希望をもたらすものだと指摘している。（写真は資料写真）
　海藻を食べるヒツジがいるのは、人口約６０人のノースロナルドセー島。ターコイズブルーの海と岩だらけの浜に囲まれた、南北に５キロほど延びる小さな島だ。
　ノースロナルドセー島では１９世紀初め、ヒツジが畑や道路に入るのを防ぐために大きな石壁が設けられた。それ以降、ヒツジたちは内陸部に進入できなくなり、海岸沿いで生きることを強いられるようになった。島の限られた土地を作物の栽培と牛の放牧に最大限利用するという意図が農民たちにはあった。
　すると、予想もしなかったことが起きた──ヒツジたちは夏には草にありつけるが、冬の間は海藻以外に食べるものがなくなったのだ。
　海藻を食べる哺乳動物は他にもいる。だが科学者らによると、数か月間、海藻だけを食べる動物はノースロナルドセー島のヒツジ以外にいないという。
　海藻食はヒツジの複雑な消化系に影響を与え、放出されるメタンガスの削減に寄与しているとみられる。メタンガスは二酸化炭素（ＣＯ２）と比べ、大気中に熱を閉じ込める効果が約３０倍高い。
　地球環境が危機的な状況にある中、温室効果ガスの主要排出源の一つと指摘されている畜産業界にとって、ノースロナルドセー島のヒツジたちの食性は新たな可能性を開くカギになり得るとの見方も一部から出ている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2021/11/15-13:08）

On a tiny island in Scotland's far-flung Orkneys, thousands of sheep spend the winter munching on seaweed, a unique diet that scientists say offers hope for reducing planet-warming methane emissions.
Around 60 people share North Ronaldsay -- an island just over 3 miles (5 kilometres) long, ringed by rocky beaches and turquoise waters off the north coast of mainland Britain -- with the distinctive native sheep.
Boasting brown, beige or black wool, the animals are hemmed into its foreshore owing to a large system of stone walls -- called a sheep dyke -- built in the early 19th century to keep them away from fields and roads.
The island's crofters -- people who live and work on so-called croft agricultural land -- wanted to use every available space to grow crops and as pasture for cows.
The unintended result: in summer the sheep can nibble on grass, but by winter eating the plentiful seaweed is their only means of survival.
While some other mammals -- including Shetland ponies native to the neighbouring island chain, and red deer -- are known to snack on seaweed, scientists say that the North Ronaldsay sheep are unique worldwide for spending months eating only the marine plants.
- Methane reduction -
With the world facing a deepening climate emergency, they are increasingly seen by some as a case study that could lead to a breakthrough in methods for raising livestock, which is a major source of greenhouse gases.
Farm animals belch and fart methane gas which, though trivial sounding, is about 30 times more powerful than carbon dioxide at trapping heat in the atmosphere.
Given the vast scale of the global meat industry, the issue has become a major focus for climate scientists -- just as world leaders prepare to gather in the Scottish city Glasgow from Sunday for the crucial COP26 summit.
The seaweed diet of the Orkney sheep has an effect on their complex digestive system and appears to reduce the amount of methane produced.
There's different components in the seaweed that actually interfere with the process (of) how methane is made, said Gordon McDougall, a researcher at The James Hutton Institute in Dundee in eastern Scotland who has been examining the sheep's diet for two decades.
Researchers at The University of California, Davis, published results in March showing that a bit of seaweed in cattle feed could reduce methane emissions from beef cattle as much as 82 percent.
David Beattie, another James Hutton Institute scientist, stressed there is huge interest in such innovation.
There's a really big movement within the industry to try and cut out the carbon footprint that the industry as a whole has, he told AFP.
I see seaweed playing a part in that.
- Scale -
This would not necessarily mean cows and sheep switching to a diet entirely comprised of seaweed like the North Ronaldsay sheep, but it could supplement their usual feed.
Seaweed is not available in large enough quantities to feed so many animals, McDougall noted, and taking away too much from the sea could also damage the environment and ecosystems.
But the marine plants -- good sources of minerals, vitamins and omega-3 fatty acids -- could partly replace soy, which is heavily used in animal feed but transported for thousands of miles and linked to deforestation.
Researchers still need to determine the types and quantities of seaweed which could be best suited to adding to feed.
And then, can you scale that up to a level where you'd actually have an effect on the overall UK farming? said McDougall.
The plump North Ronaldsay sheep, who chow down strands of seaweed as if they were spaghetti, are set to keep providing a useful case study.