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閉伊川のサケ

 ワカサギ博士のブログにて、閉伊川の今年の状況について記事がありました。コメントが長くなったのと、自分の思うところもあったので、こちらでコメントを書きます。
 岩手県宮古市では津軽石川のサケ漁が有名ですが、市の中心部を流れる閉伊川でもかなりの遡上がありますし、中世の古文書にもその様子の一端が記されています。その閉伊川で例年以上の遡上があるようですね。
 で、ワカサギ博士がお書きになったのは、そのサケが採捕場を越えて大量に支流などにのぼり、産卵をし、死んでいる。これは自然状態では考えられないくらいの量であり、孵化事業で処理できないサケについては、もう少し活用について考えた方がよいのではないか、ということです。

 まず自分が把握している人工孵化事業の要点をまとめると、
①河川のサケは全て人工孵化事業に用いる
②河川のサケは孵化事業以外を目的に獲ってはならない
③自然に遡上し産卵するサケは放っておいてもよい(獲ってはいけないが)

 言い直すと、サケはごく一部の自然産卵をのぞき、人間が管理し、再生産する資源となっているということです。ですので、処理しきれないほどの遡上というのは基本的にあり得ず、目標とする放流(一部販売)尾数分を除き、遡上するサケもまた、事業者が責任を持って処理することが求められています。当然”ほっちゃれ”状態のサケは飼料用以外には処分できず、不法投棄という問題も起こることがありますが、基本的な考え方はそういうことになっています。もちろん、自然状態のサケを見てもらおうということで、わざと獲らないということもありますし、今年は想像以上の遡上だったため、閉伊川では自然産卵をさせようとサケを遡上させているのかもしれません。

 サケの利用方ですが、指摘されているように最近はサケ釣りを解禁することで一定の成果を上げている川はあります。ただ、いろいろと拝見していると、条件がなかなか厳しく、遡上するサケのごく一部を利用しているにすぎないような気がします。また、現在はごく一部の河川で解禁しているので人気が高いのですが、多くの川で、よりゆるい条件で解禁すると相対的に一つの川に集まる人が減るので、効果が薄れてしまうような気がします。
 さらに言えば、遡上量が非常に多い年はいろいろな利用ができるのでしょうが、少ない年は全てのサケを獲っても予定尾数に足りないという年もあるわけで、さらに遡上の量はシーズンがはじまり、また終わってみないとわからない(早い時期は多いけど、通年では平年以下ということもある)ので、事業者側もまた、利用まで考えにくいという側面があるように思います。その結果、余裕があれば自然産卵をさせるというぐらいの対処しかなくなっているのが現状ではないでしょうか。
 人工孵化事業と自然との関わりはなかなか難しいと思います。確かにSalangichthysのコメントのように有機栄養素なのですが、現状は飽和状態になるまで放流していますので、ほっちゃれをそのままにすれば過栄養になっているように思います。そういう意味ではやはり、採捕場でしっかりととり、管理をする必要があるのではないかと思います。それは、たとえば遺伝子の問題でも、宮城あたりでは早めに遡上するサケを千歳川から、遅めのサケを津軽石川からそれぞれ購入することで、安定したサケの遡上を実現しようとしていますが、それは母川回帰によって一定の遺伝子グループが自然状態であるはずのサケを改変していることになるわけですよね。それは全て管理している限りはある程度許されるのでしょうが、自然のサケだけど遺伝子的には千歳川と津軽石川の混血だよ、というのはやはり問題であるように思います。
 書いていて改めて思うのは、やはりサケは人工物であるということですね。

2004 12 22 10:07 AM | 固定リンク

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コメント

 サケはブリとならび,魚類の増養殖事業の成功例であるといわれていますこれだけ生産量が増えたのも,ふ化放流の努力と資源保護が徹底していたからでしょうね。
 過飽和状態の河川遡上は予想もつかなかったことではないでしょうか?産卵遡上が沢山あればいいのでしょうが,閉伊川のサケは上流まで行かずに,限られた河口域の伏流水でおもに産卵しています。
 川の漁師たちは,この時期になるとただ見ていることしか他に方法はありません。資源を有効に利用するための多方面にわたる調査が必要ですね。

投稿者: ワカサギ博士 (Dec 25, 2004 10:00:04 AM)

 本当にそう思います。記事にも書いたように毎年するのはなかなか厳しいのでしょうが、臨機応変に対応し、広くサケの姿を見てもらえるような体制は必要な気がします。
 あと、産卵場ですが、以前は新里村あたりでもサケを獲っていたという話ですが、サケは孵化場のある場所を目指して遡上する(そこが生まれた産卵場なので)とされるため、河口部で産卵するサケが多いのかも知れませんね。

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