Management procedure for determining the stocking efforts of salmon fry in the Toyohira River: Towards a balance between wild fish conservation and population persistence

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  • Morita Kentaro
    Hokkaido National Fisheries Research Institute, Japan Fisheries Research and Education Agency Sapporo Wild Salmon Project
  • Aruga Nozomi
    Sapporo Salmon Museum, Sapporo Foundation for Green Park Sapporo Wild Salmon Project

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  • オペレーティングモデルを用いた豊平川のサケ放流数を決める管理方式の検討 ―野生魚保全と個体数維持の両立を目指して―
  • オペレーティングモデル オ モチイタ トヨヒラガワ ノ サケ ホウリュウスウ オ キメル カンリ ホウシキ ノ ケントウ : ヤセイギョ ホゼン ト コタイスウ イジ ノ リョウリツ オ メザシテ

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Abstract

The Toyohira River, which flows through the Sapporo metropolitan area, is annually supplemented with ~200,000 fry of the chum salmon Oncorhynchus keta. The river additionally supports a naturally spawning chum salmon population composed of over half of all salmon returning to the river. Recent studies have suggested that hatchery releases may exert negative effects on wild salmon productions. However, it is unclear whether the salmon population of the Toyohira River would persist without hatchery supplementation. We proposed a management procedure for determining the stocking efforts of salmon fry with a feedback control loop to reduce the risk of population decline. The performance of the management procedure was evaluated by simulation using the operating model. The operating model was based on the age-structured Ricker-logistic population model with environmental and demographic stochasticity. Our results showed that implementation of the management procedure with a feedback control loop wherein the number of fry released was adjusted based on the number of returning adults resulted in relatively good performance with a small risk of population decline. Thus, this procedure, implemented in 2016, reduced the number of hatchery fry released in that year to 85,100 (43% of the normal level).

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