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.

中文翻译：

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产卵场的建设是否改善了河床条件，使其适合琉球香鱼 Plecoglossus altivelis ryukyuensis 的产卵？

保持一定数量的濒危鱼类产卵场在河流和溪流中尤为重要，因为洪水和开发使环境发生了剧烈变化。Ryukyu-ayu Plecoglossus altivelis ryukyuensis 是一种濒临灭绝的一年生鱼类，栖息在流经日本南部奄美大岛的少数溪流和河流中，并在下游特定河床条件的地点产卵。在岛上，居民志愿者和研究人员每年手动修改河床，为鱼类自然产卵提供合适的条件。本研究调查了这些活动对河床条件的影响，进而对琉球香鱼在两条河流中的产卵产生影响。我们的研究结果表明，河床硬度和淤泥量在改建区显着下降，但在邻近的未改建区没有显着降低。在两条河流之一，改造抑制了网纺石蛾幼虫 Stenopsyche schmidi 的沉降，促进了河床的固结。改造区河床条件与P. a. ryukyuensis 确实产卵了。这些结果表明，人工改造可以有效改善琉球香鱼产卵的河床条件。然而，在两条河流的改造区域均未发现鱼卵。有必要开发有效和实用的方法来选择改造地点，同时又不妨碍这种鱼的自然产卵。在两条河流之一，改造抑制了网纺石蛾幼虫 Stenopsyche schmidi 的沉降，促进了河床的固结。改造区河床条件与P. a. ryukyuensis 确实产卵了。这些结果表明，人工改造可以有效改善琉球香鱼产卵的河床条件。然而，在两条河流的改造区域均未发现鱼卵。有必要开发有效和实用的方法来选择改造地点，同时又不妨碍这种鱼的自然产卵。在两条河流之一，改造抑制了网纺石蛾幼虫 Stenopsyche schmidi 的沉降，促进了河床的固结。改造区河床条件与P. a. ryukyuensis 确实产卵了。这些结果表明，人工改造可以有效改善琉球香鱼产卵的河床条件。然而，在两条河流的改造区域均未发现鱼卵。有必要开发有效和实用的方法来选择改造地点，同时又不妨碍这种鱼的自然产卵。改造区河床条件与P. a. ryukyuensis 确实产卵了。这些结果表明，人工改造可以有效改善琉球香鱼产卵的河床条件。然而，在两条河流的改造区域均未发现鱼卵。有必要开发有效和实用的方法来选择改造地点，同时又不妨碍这种鱼的自然产卵。改造区河床条件与P. a. ryukyuensis 确实产卵了。这些结果表明，人工改造可以有效改善琉球香鱼产卵的河床条件。然而，在两条河流的改造区域均未发现鱼卵。有必要开发有效和实用的方法来选择改造地点，同时又不妨碍这种鱼的自然产卵。