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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

Large dams often alter flow and thermal regimes downstream, resulting in fish spawning delays and larval abundance declines. Accurate prediction of the delayed spawning period under changed thermal regime is critical for selecting a correct timing to modify dam operation and to release the environmental flow needed to enhance fish spawning. We used correlation analysis to investigate the relationship between thermal regime alteration and spawning delay of the four major Chinese carps (FMCC) below the Three Gorges Dam in the Yangtze River, China, using a 13-year ecological data series (1997–2009). Eighteen variables were defined to quantify thermal regime and related to two variables representing spawning timing. Our results demonstrate that the start of FMCC spawning has been delayed from early May to middle June since the Three Gorges Dam initiated operation in 2003. Water temperature declines of 2 °C–4 °C in March, April and May (a critical period for gonad development) were the principal reason for the observed spawning delay. The variable most associated with spawning timing was the arrival date of the cumulative temperature needed for gonad development from stages IV to V (DDegDayIV–V), which describes the long-term impacts of the timing, magnitude and duration of thermal regime upon gonad development. Only the cumulative temperature for gonad development and the minimum temperature for FMCC spawning (18 °C) are both satisfied, the occurrence of suitable flow conditions, that is, flow increase or flash flood, would produce a successful spawning event. Consequently, we suggest that the experimental flow increase process of the Three Gorges Reservoir aimed at enhancing FMCC spawning should initiate after 15 June when the requisite thermal regime can be met. Copyright © 2013 John Wiley & Sons, Ltd.