How weirs affect three-lips migrating upstream from Lake Biwa to reproduce

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  • 琵琶湖の魚食魚ハスの遡上と産卵に対する定置罠「簗」の影響
  • ビワコ ノ ギョショクギョ ハス ノ ソジョウ ト サンラン ニ タイスル テイチワナ 「 ヤナ 」 ノ エイキョウ

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Abstract

<p>The freshwater fish community of Lake Biwa, Japan includes many potamodromous species, which migrate upstream seasonally to tributary rivers in order to reproduce. However, most of these rivers used by large numbers of migrating fish have weir fish traps that are set by local fisheries at the river mouths, which are thought to negatively affect the population density of potamodromous species. This study examined factors that are critical for conservation of three-lips (Opsariichthys uncirostris uncirostris), the only piscivorous cyprinid in Japan, which is vulnerable and endemic in Lake Biwa. During its reproductive season, we counted the number of individuals and determined their sex, body size, and frequency of spawning behaviours in a view of comparing them between rivers with and without weirs and between seasons when the weir is not operational. Illuminance, water temperature, turbidity, and current velocity were also measured to explore environmental factors that limit fish reproduction. The frequency of spawning behaviours, numbers of individuals, and fish body size were larger, while turbidity was lower in the river with a weir than in the river without a weir; thus, the former river is considered favourable for fish reproduction. When the weir was operational, the population density and frequency of spawning behaviours were negatively correlated and the proportion of females in the population was lowest among the three conditions examined. This suggests that excessive gathering of individuals (particularly males) during periods of an operational weir negatively affect their reproduction. For effective conservation of this species, further ingenuity is needed regarding the operation of weirs, such as minute changes in their position or season.</p>

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