Does construction in the spawning ground improve the riverbed conditions making it suitable for spawning of the Ryukyu-a
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Does construction in the spawning ground improve the riverbed conditions making it suitable for spawning of the Ryukyu‑ayu Plecoglossus altivelis ryukyuensis? Satoshi Awata1,2,3 · Tetsuya Tsuruta3,4 · Shin‑ichiro Abe3,5 · Toshihiko Yonezawa6 · Kei’ichiro Iguchi3,7 Received: 20 October 2019 / Revised: 22 March 2020 / Accepted: 23 March 2020 © The Ichthyological Society of Japan 2020
Abstract Maintaining a certain number of spawning grounds for endangered fish is particularly important in rivers and streams where the environment changes drastically through floods and exploitations. Ryukyu-ayu Plecoglossus altivelis ryukyuensis is an endangered annual fish inhabiting the few streams and rivers flowing through the Amami-Oshima Island, southern Japan, and spawns in the lower reaches at sites with the specific riverbed conditions. In the island, resident volunteers along with researchers modify riverbed manually to provide conditions suitable for the natural spawning of the fish every year. This study investigated the effects of these activities on the riverbed conditions and consequently on the spawning by Ryukyu-ayu in two rivers. Our results showed that riverbed hardness and the amount of silt significantly decreased in the modified areas, but not in the adjacent unmodified areas. In one of the two rivers, the modification suppressed the settlement of net-spinning caddisfly larvae Stenopsyche schmidi that promote consolidation of the riverbed. The riverbed conditions in the modified areas were similar to those in the sites where P. a. ryukyuensis indeed spawned. These results suggest that manual modification is efficient in improving riverbed conditions for the spawning of Ryukyu-ayu. However, the eggs of the fish were not found at the modified areas in both rivers. It will be necessary to develop effective and practical ways of selecting sites for modification without obstructing the natural spawning in this fish. Keywords Endangered fish · Spawning ground · Riverbed hardness · Silt · Caddisfly
Introduction * Satoshi Awata sa‑[email protected]‑cu.ac.jp 1
Laboratory of Animal Sociology, Graduate School of Science, Osaka City University, Osaka, Japan
2
Sado Marine Biological Station, Faculty of Science, Niigata University, Sado, Japan
3
Ueda Station, Freshwater Fisheries Research Division, National Research Institute of Fisheries Science, Fisheries Research Agency, Komaki, Ueda, Japan
4
Department of Human Life and Environment, Faculty of Human Environment, Osaka Sangyo University, Daito, Japan
5
College of Education, Ibaraki University, Mito, Japan
6
Kagoshima Environmental Research and Service, Kagoshima, Japan
7
Graduate School of Fisheries Science and Environmental Studies, Nagasaki University, Nagasaki, Japan
Ryukyu-ayu Plecoglossus altivelis ryukyuensis Nishida, a subspecies of ayu P. a. altivelis Temmink & Schlegel, is an amphidromous fish having an annual life history (Nishida 1988; Shinomiya 1997; Aritomi et al. 2017; Murase et al. 2018). This species is currently li
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