Composition and Abundance of Drifting Fish Eggs on the Upper Reaches of Xijiang River, China, after the Formation of the Cascade Reservoirs

To develop effective management actions of riverine fisheries, it is important to monitor how fish resources (i.e., eggs) are recruited in the upper reaches of natural rivers, particularly where dams have been constructed, which potentially hinder life-history strategies. Here, we aimed to determine the State of drifting fish eggs resources, and the underlying environmental factors regulating the presence of fish eggs in the upper reaches of a river (Laibin section of Xijiang River, China). Based on surveys conducted over one spawning period (2016), we set out to: (1) describe the composition and abundance of drifting fish eggs in the 150 km Lainbin section under a dam control, and (2) analyze how the composition and distribution of fish eggs was correlated with environmental factors. A total of 15157 eggs belonging to two orders, four families, and 18 species were collected. Comparison of these data with historical records in the same area showed that the community structure of drifting eggs has changed considerably. Previously, the community was dominated by larger-bodied species, rather than the smaller species documented in 2016. Eggs were primarily detected between May and August. In the natural channel, the greatest abundance of eggs occurred during May and June. In comparison, the greatest abundance of eggs downstream of the dam was detected in July. The results of this study provide important information for water conservancy institutions towards managing regions containing dams to maintain the ecology of rivers and protect important fish resources.

Introduction 44 The diversity of species globally is declining at an alarming rate because of human influences 45 [1]. Global fishery resources are in decline due to many man-induced stressors, such as 46 overfishing, dam construction, biological invasions, and climate change [2]. Aquatic ecosystems have been severely degraded as a result of anthropogenic changes to landscapes 48 [3], with the damming of rivers representing a major anthropogenic factor impacting the ecology 49 of freshwater fish populations. In particular, dams cause habitat loss, affect fish reproductive    (Table 1); and V is the water velocity at that moment (km/h).

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To assess spatiotemporal variation in the distribution of eggs, a two-factor analysis of variance 140 (ANOVA) was applied, in which differences were regarded as significant at 0.05 probability.

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Canonical correspondence analysis (CCA) is a robust method that directly correlates 142 community data to environmental variables by constraining species ordination to a pattern that 143 is correlated with environmental variables [22]. CCA was used to analyze the correlation 144 between environmental factors and the temporal distribution of fish eggs. Only numerically 145 abundant species were tested to avoid any spurious effects caused by groups of rare species.  Drifting eggs were mainly found between May and July, with several spawning peaks 194 occurring in each year (Fig. 2). The species composition of eggs differed across the sampling 195 months (Fig. 3). The proportions of species belonging to Gobioninae increased from April to 196 August, whereas those belonging to Botiinae and Homalopteridae decreased over the same 197 period.  the key environmental factors affecting assemblages (P < 0.05).

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The CCA ordination plot of eggs (Fig. 4) showed that the correlation between  Basilewsky, and Saurogobio dabryi were positively correlated with water level and temperature, Variation in spawning sites 237 The developmental stages of the 18 species captured in this study are presented in Table 2  Reservoirs.

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Asian carp was previously the most economically important taxon in this region (Zhou, 2005); 264 however, we only found Hy. molitrix eggs at the Shilong sampling site in the Liujiang River during

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Yet, such information is essential for fisheries regulation and management to be effective.

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We confirmed the presence of reproduction at sampling sites located at a river junction and the the reproductive peak occurred in May and June at the junction and conventional channel, but in might be caused by cold water present below dams (Tan, 2010). Hydrodynamic conditions at river 293 junctions tend to be complex, particularly with respect to speed and the direction of currents. These The current study showed that the dominant species that spawn drifting eggs were Sinogastromyzon 306 wui, Botia robusta, Sinibotia pulchra, and Squalidus argentatus (Table 5). In particular, a significant 307 correlation was detected between the reproduction of these species and water level. Water level was 308 the main influencing factor in our study, and might facilitate the spawning of Sinogastromyzon wui,

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Botia robusta and Sinibotia pulchra in areas with cobblestones and sandbanks. With the planned 310 operation of Datengxia Dam, the rise in water levels will cause these spawning grounds to become of species with floating fish egg resources in this section of the river.

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The