拝啓 トーマス・ブレーク・グラバー様
事故から４年半が過ぎてもなお、中禅寺湖では測定するほぼ全ての生き物からセシウムが検出されます。
あなたが追っていたマス類の汚染は、特に深刻です。
放射性物質の量を表す単位をベクレルといいますが、事故翌年に国が食品の安全確保のために決めた基準値「１キログラム当たり１００ベクレル」を超えるマスがいまだに見つかっています。なので、その年の４月からワカサギ以外の魚の持ち出しは禁じられたままです。栃木県では昨年６月、かつて東洋一の銅山があった足尾地区の渡良瀬川上流域でイワナへの出荷制限が解除され、中禅寺湖が最後に残りました。
生き物は体を維持する「代謝」という機能でセシウムを排出します。しかし湖や河川にいる「淡水魚」は代謝が遅く、セシウムを体に蓄積しやすいようです。
塩分が豊富な海にいる魚と比べて、淡水魚は、生きるために必要な塩分をなるべく体内にとどめようとします。
厄介なことにセシウムは、自然界に存在している塩類の一つ「カリウム」と性質が似ていて、体はそれらを区別できません。それゆえ、淡水魚はセシウムを排出しづらく、汚染濃度がなかなか下がらないそうです。
原発に近い福島県沖には、高濃度に汚染された膨大な量の水が流れ出したこともありました。しかし、今では魚からセシウムはめったに検出されません。太平洋に拡散して海水の汚染濃度が薄まっただけでなく、海水魚が塩類を積極的に排出することも一因とみられています。
中禅寺湖の魚は春から夏にセシウム濃度が下がり、秋から冬は横ばいとなる傾向があります。これは水温が高くなり、代謝速度が増したことによる影響とも言われます。
増養殖研究所の調査で、中禅寺湖の魚の汚染は、餌の汚染が主な原因と分かってきました。魚が体表やえらから取り込むセシウムは極めて少ないと考えられています。研究員の山本祥一郎（やまもと・しょういちろう）さん（４８）は「淡水魚の代謝は遅いが、環境の汚染がなければ体内のセシウム濃度は急激に下がるはず」とみています。
中禅寺湖では、大型マス類を頂上とする食物連鎖のピラミッドが確認されています。湖が閉鎖的な構造のため水や泥の汚染が長引き、プランクトンなどの微生物がセシウムを取り込んで、それらを食べる小型の魚の体内にも入ります。こうしたサイクルによって、事故後に生まれたマスたちも汚染が続いてしまっています。
Even though it’s already been 4 years after the accident, radioactive cesium is still detected from almost all kinds of living matters screened in the Lake Chuzenji.
Specifically trout, which you were in favor of, are seriously contaminated. A unit employed to measure radioactive material is called bequerell. After the accident, the Government has decided the standard limit of radioactive cesium in food under 100 bequerell per 1 kilogram of each. Even now, some of the trout in the lake are over that line. Apart from lake trout, all fishes have been prohibited to be brought away since the April of 2012. In Tochigi prefecture, the shipping restriction of the mountain trout in the upper of the River Watarase, located in Ashio area where there used to be the largest copper mine in the Eastern Asia, has been lifted. So the Lake Chuzenji is the only one left.
Living things excrete cesium by metabolism, a function to maintain the body. But in fresh water fish, the cycle of metabolism is slow that they tend to accumulate cesium in their bodies.
Different from sea fish, fresh water fish tend to keep salt necessary for their lives in their bodies. What is bothering is that the body cannot tell the difference between the cesium and the kalium, a kind of salt that exists in the natural world, because they are similar to each other in their property. Therefore, it’s hard for fresh water fish to excrete cesium, and their contamination levels hardly decrease.
In the coast of Fukushima, near the Fukushima Daiichi nuclear power plant, there was a time when highly contaminated water had been leaked into the sea, but now the cesium is rarely detected from the fish. Some say it’s because not only that the dose level of sea water has declined as it’s spread out to the Pacific ocean, but also that the sea fish excrete salt faster than the fresh water fish.
The contamination level of fish in the Lake Chuzenji, tends to become lower during spring and summer, and stay about the same during autumn and winter. This may be related to the water temperature; metabolism of fish accelerates when water is warm.
From analysis by the National Research Institute of Aquaculture, we now know that the fish of Lake Chuzenji is contaminated mostly by its food. They think that there is very small amount of cesium that fish take in through its surface on the body or its gills. Shoichiro Yamamoto, a researcher of NRIA, says ‘although fresh water fish metabolize slowly, their inner cesium level would go down quickly if not the environment was contaminated’.
Apparently there is a food chain in a pyramid of which large trout on top, in Lake Chuzenji. Since it’s a closed lake, its water and soil have been contaminated for a long period of time. Meantime, plankton absorb cesium and that cesium is added to the body of small fish that eat plankton. This food chain is keeping contaminating the trout even born after the accident.
