Reproductive biology of shorthead anchovy Encrasicholina heteroloba off Rayong Coast, the Gulf of Thailand

Abstract

The reproductive biology of shorthead anchovy Encrasicholina heteroloba was studied over one year period in 1995 from the mature population off Rayong waters, the eastern coast of the Gulf of Thailand. Gonad development cycle was investigated using a 5- stage gonad maturity classification, gonadosomatic index, and histological examination of oocyte development, oocyte size and stage frequency distributions. The incidences of postovulatory follicles and atretic oocytes were also studied. The shorthead anchovy had typical ovary of multiple spawning species with asynchronous oocyte development and oocyte size frequency distributions. Oocytes were released continuously demonstrated by the presence of partially spent fish with ovaries containing postovulatory follicles along with mature oocytes. Atresia was dominant during low peak spawning period. Overall sex ratio of female to male was 1:1.75. Batch fecundity was expressed as log (batch fecundity) = 2.09 + 1.21 log (midpoint of 0.5 cm total length class). The shorthead anchovy spawned throughout the year with high peak during Northeast Monsoon season. Rainfall seemed to create unflavourable condition for gonad development of the fish as the decrease of maturity proportion and high number of atretic oocytes was evident in this period.