Spawning pattern and type of fecundity in notothenioids collected around the Elephant and South Orkney Islands

The frequency distribution of oocytes diameter and relative fecundity (FR) of 4 notothenioids (Chaenocephalus aceratus, Chaenocephalus gunnari, Notothenia rossi and Pseudochaenichthys georgianus) was carried out, in females collected during a fishery research survey around the Elephant Island and South Orkney Islands), between 21th January and 14th February 2015. Measurement and oocyte counting were carried out using an image analyzer in sub-samples of ovaries at different stages of development, while the determination of RF was performed by a gravimetric method. Five were the main distinctive finding derived from this study: (i) in mature ovaries of all species immature oocytes or pre-vitellogenic appeared clearly separated from the most advance mode of mature oocytes; (ii) in hydrated ovaries the less developed group showed a polymodal pattern in C. aceratus and N. rossi; (iii) in hydrated ovaries of P. georgianus a trimodal structure was visible, containing immature oocytes (<0.9 mm), mature (range: 1.16 to 1.86 mm) and ovulated oocytes (range: 4.5 to 5.387 mm; average FR: 3 oocytes·g-1), respectively; and (iv) the FR tended to be higher when estimates were derived from mature ovaries (range: 5-12 oocytes·g-1) than form hydrated ovaries (range: 1-2 oocytes·g-1). The results confirm these species have a group-synchronous ovarian organization, but suggest there may be variables strategies, without dismissing is serial spawning, particularly due to the physical limitations that would impose the abdominal cavity to spawn large eggs, a characteristic ecological trait in notothenioid. Additionally, the occurrence of hydrated ovaries suggests reproductive activity in summer season for species commonly spawning in autumn and winter, which may be indicative of an either longer spawning season or alternatively is associated to some environmental factor that triggered an anticipation of the spawning season.