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

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 2daysDuration4days, but significantly increased with Duration when 5daysDuration8days. With an approximate linear correlation, larval abundance increased with average daily rise rate of flow increase (QRavg) when QRavg2000m3s-1day-1. Timing of flow increase was the most significant post-dam factor, with the majority of adult carp spawning 55-80days after water temperature at Yichang gauge station reached 15 degrees 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 degrees CWTmax26 degrees 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 (Duration5days), 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 (Duration5days, 900 QRavg3000m3s-1day-1 and discharge30000m3s-1) between 15 June and 20 July when water temperature is <25 degrees C.