Growth and reproductive biology of loaches Cobitis sp. in Lake Lucień, Poland.

Five age-classes and their corresponding average body lengths of loaches from Lake Lucień were determined by the Bhattacharya method. The maximum observed length was 112 mm for females and 91 mm for males. The von Bertalanffy equation defining growth pattern was L(T) = 116 (1-exp (-0.401(t+0.02. Body length distribution of females and males differed significantly (chi2 = 91.295; df=9; P<0.00). Sex ratio showed the dominance of females in the studied population (M:F = 1:1.75; chi2 =18.00; P<0.01). Females were sexually mature at 56 mm TL and males at 52 mm TL. Female gonad weight increased with body size. The frequency distribution of egg diameters revealed 3 groups of oocytes. The average absolute fecundity was 2180 eggs and ranged from 418 to 6800 per gonad. The number of the largest oocytes (>0.6 mm in diameter) ranged from 208 to 975 (average 501) and was used to estimate fecundity at the moment of spawning. Both fecundity measures are related to body length of females their regression lines were parallel and did not differ in the coefficient of slope. The gonadosomatic index value, as an approximate measure of reproductive effort, was rather small (average IGS = 10.44 ranged from 5.12 to 17.88).