Effects of thermal regime variability on adult rainbow trout (Oncorhynchus mykiss) mortality in Afromontane river systems

Abstract

Water temperature variability in river systems is a significant environmental concern. This study aimed to assess the effects of thermal regime variability on adult Oncorhynchus mykiss mortality in Afromontane river systems; using time series data spanning from 1994 to 2024. The objectives of the study were to analyze temporal and seasonal trends in water temperature (1994–2024) in Nyanga National Park, Zimbabwe, evaluate how temperature variability relates to adult O. mykiss mortality, and identify critical thermal thresholds that increase mortalityThe research used both secondary and primary data. Time series approach with a 5-year moving average analyzed temperature trends, while correlation and regression assessed the relationship between water temperature and rainbow trout mortality, and threshold analysis identified critical temperature limits increasing adult O. mykiss mortality. Seasonal comparisons revealed marked differences in both water temperature and mortality of O. mykiss. Mean water temperature was significantly higher in summer (20.13 ± 0.10 °C) than in winter (13.30 ± 0.13 °C; Welch’s t-test, t = 17.91, p < 0.01; n = 93 per season). Similarly, mean mortality was substantially greater during summer (1272 ± 44) compared to winter (229 ± 27), with this difference also highly significant (Welch’s t-test, t = 13.17, p < 0.001). Correlation and regression analyses revealed a strong positive association between temperature anomalies and mortality anomalies (ρ = 0.713; R2 = 0.453) indicating that nearly half of the variability in mortality anomalies is explained by temperature. The positive regression slope (134.1 fish per 1 °C anomaly) highlights the biological sensitivity of the population to even modest warming departures from baseline conditions. A critical thermal threshold of 15.20 °C was identified where O. mykiss mortality increased disproportionately above this threshold. Although mean temperatures have not shifted significantly, intra-seasonal variability acts as a primary driver of mortality. Implementing strategies that mitigate the effect ofis imperative to support economic growth and food security. water temperature changes on rainbow trout mortality