Some reproductive patterns of the sardine, Sardina pilchardus (Walb, 1792), in Boka Kotorska Bay (Montenegro, southern Adriatic Sea)

The annual alteration of gonad morphology in sardine (Sardina pilchardus, Walbaum 1792) caught in the period from November 2006 to October 2007 using beach seines in the region of Boka Kotorska Bay (Montenegro, southern Adriatic) was studied. Samples were taken in the middle of each month. Total length of individuals ranged from 8.7 to 14.7 cm and weight ranged from 4.67 to 22.61 g. Their gonads were extracted and weighed, and a piece of gonad tissue was sampled for histological analysis. The length–weight relationship of all sardine specimens was described by the equation: W=0.0059 LT; (r2 = 0.963). The lowest gonadosomatic index (GSI) values, below 1, were found in June, July and August, corresponding to the state of gonad rest. The GSI increased gradually from September and October and reached its highest value in February, at 5.22 for females and 6.58 for males. After February the GSI started to decrease throughout March (4.5), April (2.4) and May (2.4). In June all gonads had a GSI bellow 1. Although primary oocytes (stage I) were present during all months, their percentage increased from May and was the highest during the summer months (June-September 100%). Mature stage IV oocytes were recorded from November to April, with the highest percentage recorded during January (26.7%). An increase in the percentage of oocytes in the yolk vesicle (II) and yolk (III) stages occurred in October, remaining almost unchanged until May. During the summer months (May – September), only stage I spermatogonia were present in the testicles. In October, spermatocytes (stage II) started to appear, while from November the spermatids (stage III) appeared as well. Spermatozoa (stage IV) appeared in December and reached their maximum level in January and February.