Variation in the significant environmental factors affecting larval abundance of four majorChinese carp species: fish spawning response to the Three Gorges Dam

Abstract

Summary Larval abundance of fish species downstream of large dams generally declines with changes in flow and thermal regime caused by dam operation. Fish conservation strategies in dammed rivers may need to be modified because adjustments in life‐history or spawning behaviour are necessary for adult fishes to survive in modified environments. We used genetic programming to identify significant pre‐ and post‐dam environmental factors affecting larval abundance of four major Chinese carp species in the Yangtze River downstream of the Three Gorges Dam and to quantify their relationships with larval abundance during a 13‐year period (1997–2009) that encompasses seven pre‐dam years and six post‐dam years. Significant pre‐dam factors included duration and rise rate of flow increase. With a nonlinear correlation, larval abundance slightly increased with duration of flow increase (Duration) when 2 days ≤ Duration ≤ 4 days, but significantly increased with Duration when 5 days ≤ Duration ≤ 8 days. With an approximate linear correlation, larval abundance increased with average daily rise rate of flow increase (QRavg) when QRavg ≤ 2000 m3 s−1 day−1. Timing of flow increase was the most significant post‐dam factor, with the majority of adult carp spawning 55–80 days after water temperature at Yichang gauge station reached 15 °C. Duration of flow increase remained significant and its relationship with larval abundance remained unchanged, but rise rate of flow increase was no longer identified as a significant factor. Water temperature became significant with larval abundance decreasing slightly with maximum water temperature of flow increase (WTmax) when 22 °C ≤ WTmax ≤ 26 °C. Changes in the significance of individual environmental factors were caused by adaptation to thermal regime alterations through a gradual delay in spawning time. Because the delayed spawning period was much closer to, or even overlapped with the main flood season, magnitude and rise rate of flow increase and water temperature all increased. These changes, coupled with a lack of long flow increase (Duration ≥ 5 days), the occurrence of dissolved gas super‐saturation and the disappearance of most spawning grounds in the Three Gorges Reservoir, resulted in a substantial decline in carp larval abundance of the middle Yangtze River. Variation in the significant factors and their relationships to larval abundance implies that mimicking a natural flow regime without accounting for fish response to environmental changes may not provide the environmental flows needed to enhance native fish spawning. Based on pre‐ and post‐dam relationships between significant factors and larval abundance, we recommend that in order to enhance the four carp species spawning, the Three Gorges Reservoir should plan at least one flow release with increased discharges (Duration ≥ 5 days, 900 ≤ QRavg ≤ 3000 m3 s−1 day−1 and discharge ≤ 30 000 m3 s−1) between 15 June and 20 July when water temperature is <25 °C.

Using abiotic drivers of fish spawning to inform environmental flow management

“…Enhancing native fish populations is a common objective of many environmental flow programmes as fish have high social, cultural and conservation value (Bradford et al 2011;Koehn et al 2014). Flow is also thought to influence key aspects of fish life cycles such as condition and maturation of spawning adults, cue for movement and reproduction, egg survival, dispersal of young and improvement of larval and juvenile survival by promoting suitable food and habitat (Junk, Bayley & Sparks 1998;King et al 2010;Wang et al 2014), making them a promising tool for fish conservation.…”

Quantitative response of leaf‐litter decomposition rate to water abstraction in a gradient: Implications for environmental flow management

“…This can be used to inform adaptive management and thereby refine e‐flow allocations for water abstraction. Future studies should develop other method (Wang et al, ; Webb, Stewardson, & Koster, ) and examine components such as frequency, timing, duration, rate of change, and predictability of flow events (Arthington et al, ; Langhans & Tockner, ), with the ultimate aim of mimicking natural river flow variability in order to maintain ecosystem health.…”

Distribution of Botia superciliaris (Günther, 1892) ichthyoplankton in the upper mainstem of the Yangtze River preimpoundment and postimpoundment of the upstream dam cascade

“…Inundation of the upstream spawning grounds by a dam cascade in the upper Paraná River reduced the recruitment success of several migrating fishes (Agostinho et al, ). Impoundment of the TGR led to the loss of a major component of the spawning grounds for the four major Chinese carps (i.e., black carp Mylopharyngodon piceus , grass carp Ctenopharyngodon idella , silver carp Hypophthalmichthys molitrix , and bighead carp Aristichthys nobilis ) above the dam, which is considered a major reason for the dramatic decreases in the larval production and population size of these fishes in the middle reach of the Yangtze River (Duan et al, ; Wang et al, ; Xie et al, ).…”