Biological indicators of environmental stress: Condition and population dynamics of Enteromius paludinosus in Lake Naivasha, Kenya

Freshwater ecosystems are increasingly exposed to multiple stressors, necessitating the use of sensitive biological indicators to assess ecosystem health. This study examines key biological aspects of the straightfin barb (Enteromius paludinosus) during the rainy season in Lake Naivasha, Kenya, to evaluate population condition under prevailing environmental pressures. A total of 1,279 individuals were sampled from the mouth of River Malewa using seine nets (1.2 mm mesh size). Standard morphometric measurements were recorded, and gonadal examinations conducted to determine sex. The length-weight relationship (LWR), condition factor (K) and sex ratio were analyzed as indicators of physiological status and environmental stress. The LWR equations for combined sexes, males and females were Log W = 3.23 Log L + Log 0.1073, Log W = 2.83 Log L + Log 0.1500 and Log W = 2.87 Log L + Log 0.1522, respectively. The mean condition factor was below unity for all groups (combined: 0.5897; males: 0.5729; females: 0.5981), indicating suboptimal condition. The sex ratio was skewed towards females (1:1.981, males: females). The observed patterns suggest that E. paludinosus populations in Lake Naivasha are experiencing sub-lethal stress, potentially linked to environmental degradation associated with increasing anthropogenic pressures. These findings highlight the sensitivity of fish-based biological metrics in detecting ecosystem health changes and underscore the importance of integrating such indicators into long-term monitoring frameworks. Strengthening ecosystem-based management approaches is critical for enhancing the resilience of freshwater systems under changing environmental conditions.