A total of 174 specimens of the spined loach Cobitis taenia Linnaeus, 1758, comprising 117 females, 45 males and 12 juveniles, from Lake Klawoj (Poland) were identified as C. taenia from their karyotype (2n = 48 chromosomes). The overall sex ratio (M:F = 1:2.6) showed that females dominated in the population, but this varied with increasing fish size. Age and growth were determined based on otolith measurements. Standard lengths were back- calculated from the regression of fish standard length on the otolith radius and the growth pattern was described using the von Bertalanffy model separately for males and females, which revealed a strong fit for both females (Lt =92.4(1– exp(–0.278 (t – 0.456)); r2 = 0.803) and males (Lt=120.6(1 – exp(– 0.227 (t – 0.558); r2= 0.772). Comparison of asymptotic length with maximum observed size of males and females revealed that all values were a bit overestimated (as per the Taylor criteria). The SL – weight equations for males (W = 0.0003 SL3.8377; r2 = 0.8961) and females (W = 0.0039 SL3.1683; r2 = 0.9616) indicated allometric growth in both sexes, though more so in males than in females.
The variability of morphological characters was studied in a population of spined loach from Klawój Lake karyologically identified as a pure diploid C. taenia (2n = 48). 24 metric features were used to calculate correlation coefficients and linear regressions to determine the relationships between metric features with total length. As regards 11 meristic features, females had more lateral and predorsal spots than males. Statistically significant differences between males and females were also found in mean values of 15 metric indices. Age variability in the shape of the lamina circularis of males was revealed.