Effects of mass-stocking of the Kinu River with Sweetfish on the foraging distribution of Great Cormorant.

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  • 利根川水系鬼怒川中流域におけるアユの放流がカワウの採食分布に与える影響
  • トネガワ スイケイ キヌガワ チュウリュウイキ ニ オケル アユ ノ ホウリュウ ガ カワウ ノ サイショクブンプ ニ アタエル エイキョウ

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Abstract

The foraging distribution of Great Cormorants Phalacrocorax carbo was surveyed 19 times within a 46 km range along the Kinu River from March to August, 2005. During this period, the river was stocked, for angling, with Sweetfish Plecoglossus altivelis 26 times at 16 sites (total weight 2,417 kg). We assumed that released fish were vulnerable to cormorant predation due to their gregariousness for six days after the stocking. The number of cormorants found during each survey was not correlated with the amount of fish assumed to be vulnerable, and decreased sharply in late April, when Sweetfish seemed plentiful. The ratio of the number of cormorants foraging within 500 m of release sites to that in the study area was larger during the pre-stocking period than during the vulnerable period, contrary to expectation. Furthermore, no cormorants were found foraging around release sites during the vulnerable period at 12 of the 15 sites monitored. These results suggest that mass-stocking with Sweetfish did not affect the number of cormorants in the study area nor the number of foraging cormorants around the release sites.

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