Shelter effect of the gaps within rock piles to reduce the predation on Japanese eel Anguilla japonica

Published source details Sakanoue R., Satoh S., Matsushige K., Yasutake Y., Hibino Y., Manabe M., Utida K. & Mochioka N. (2021) 河川生活期のニホンウナギにおける浮き石による被食回避効果の検証. Nippon Suisan Gakkaishi (Japanese edition), 87, 255-264.
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This study is summarised as evidence for the following.

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Provide refuges

A replicated, controlled, before-and-after study in 2019 in eight experimental ponds in Ibusuki City, Japan (Sakanoue et al. 2021) found that placing baskets filled with rocks in the centre of ponds increased the survival rate of yellow Japanese eels Anguilla japonica and reduced the number of attacks by herons Ardea spp. Overall, 25–27 of 28 eels (89–96%) survived in ponds with baskets of rocks compared to 21–22 of 28 eels (75–79%) in ponds without. Eels were subjected to fewer attacks from herons in ponds with rocks (0.8 attacks/pond) compared to ponds without (3.9 attacks/pond), but there was no significant difference in the success rate of heron attacks (with rocks: 54%: without: 56%). Four experiments, each lasting seven days, ran from July–August 2019. In each experiment, seven wild yellow eels caught in an estuary were placed in each of eight concrete ponds (300 x 150 cm, depth: 78 cm). Four ponds contained a refuge comprising a mesh basket (100 x 100 × 50 cm) filled with 135 stones in the centre, while four other ponds contained no refuge. Before the experiment, 110 farmed loaches (Cobitoidea) were released into each pond to attract birds. Ponds were drained at the end of each experiment, and surviving eels counted. During the experiments, bird predation behaviour was monitored using a video camera.

(Summarised by: Vanessa Cutts)