Preliminary data on the ovarian histological structures observed in black hakes (M. polli and M. senegalensis) off Mauritania

Abstract

The black hakes, Merluccius polli and M. senegalensis, are target species of a trawl Spanish fleet in Mauritanian waters. M. polli is a deep-water hake while M. senegalensis shows a shallower distribution. Both species are mixed in the landings, although the deeper hake totals a greater fraction (85%) of the catch due to the fishing strategy of the trawl fleet. The female gonads of both hakes were collected during an experimental survey carried out in the Mauritanian EEZ. Mature ovaries were examined from the start of the reproductive season in this area. Ovaries were processed by a standard histological technique. Histological characteristics of ovarian tissues and oocyte stages were studied by light microscopy. The ovaries of black hakes showed various stages of oocyte development from the small sized, chromatin nuclear and perinucleolar oocytes, and the medium sized, cortical alveoli and vitellogenic oocytes. This last stage presents yolk granules occupying the cytoplasm at different levels. In the case of M. senegalensis, the presence of large sized oocytes with early migration of the nucleus to the animal pole, as well as fusion of oil droplets indicate maturation and imminent spawning, thus, indicating an earlier spawning than M. polli. Furthermore, postovulatory follicles observed in M. senegalensis indicate a recent batch spawning episode. This result is concordant with the values of GSI obtained for both species in these waters. The reproductive period extends from October to Mars with a defined peak in December - January for females of both hakes, although the shallower species, M. senegalensis, shows an earlier spawning in this study. Atretic stages were observed, but their incidence was low because the sampling corresponds to the beginning of the spawning period. The species are multiple batch spawners as suggested by their oocyte size frequency distribution in the ovary. The present study is a first approach to ovarian histology of M. polli and M. senegalensis, and intended to provide basic knowledge for further detailed studies on the reproductive biology of these species, essential for an adequate assessment of this stock.