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
