Background. Limited information is available on the reproductive biology of grey mullets mainly because it is very difficult to collect fully mature specimens. In the presently reported study the reproductive biology of two mugilid species, namely Liza saliens and Liza ramada, was studied in the Vistonis and Porto Lagos estuarine systems. Materials and methods. Gonadosomatic index (GSI) and standard length at first maturity (SL50) were calculated. In order to estimate which oocytes are going to mature, oocyte size-frequency profiles were constructed for each fish and only the most advanced yolked oocytes were considered. The fecundity estimates were obtained using the volumetric method. Results. The results indicated that the sex ratio was in favour of females for both species. The gonadosomatic index indicated that the spawning season of L. saliens extends from June through October and that of L. ramada—from September through January. L. ramada sexually matures at a greater size and older age than L. saliens. For a correct estimate of the fecundity, the smallest group of eggs in both species (<0.20 mm) was rejected. For L. saliens the absolute fecundity varied from 245 000 to 555 000 eggs, while for L. ramada from 150 000 to 685 000 eggs. Fecundity is probably more correlated with the weight of the fish for both species. Conclusion. For the estimation of fecundity the total number of the oocytes is often used. However, this can be misleading as shown from the frequency distribution of the oocytes in the two grey mullet species, where a large proportion of the oocytes will never mature and therefore they do not contribute to the reproduction process. The percentage of these oocytes can be high, as shown in the current study (3.9%–28.1%).
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