The aim of the study is to investigate Aphanius fasciatus biology in the northern section of its distribution
area. Within the Natural Regional Reserve of the Isonzo River Mouth, 8 sampling sites were monitored
between late spring (May) and autumn (November) in 2015 and 2016: 257 specimens were collected, and
five age classes were determined by scale reading (0+ to 4+). No significant differences were observed for
total length and weight between sexes. Sex ratio was 2.37 and favored females. Seven meristic characters
were analyzed on 60 specimens found dead: values showed no significant differences between sexes, except
for the number of vertical flank bars in 0+ and 1+ age classes. The gonadosomatic index was analyzed on 33
specimens, showing maximum values in June for females (11.44%) and in July for males (5.74%). Values
decrease over following months and reaching a minimum in late August for females (0.42%); no data were
available for males in August. Histological analyses suggest that A. fasciatus is a multiple serial spawner: the
reproductive period starts in June, while the first recruitment phase occurs in August. Female gonads showed
asynchronous development, with germinal cells at each maturation stage, but no dominant phases were
observed.
