Oocyte development and fecundity type of the skipjack, Katsuwonus pelamis, in the Western Indian Ocean

The study aims to define the oogenesis pattern of the skipjack (Katsuwonus pelamis) of the Western Indian Ocean basin in terms of oocyte growth and recruitment style. The main objective is to define the type of fecundity regulation (i.e. determinate or indeterminate) based on four lines of evidence: (a) oocyte size-frequency distribution; (b) seasonal variation of the relative number and percentage of oocyte stages, (c) diameter of the advanced vitellogenic oocytes in females in the spawning capable phase; and (d) incidence of atresia throughout the spawning season. The samples were collected from 2009 to 2010 in the Western Indian Ocean, and 673 ovaries were classified in the different reproductive phases using histological staging. Moreover, the oocyte size distribution of 93 mature individuals was described by the newly implemented image analysis method. This species showed a broad oocyte size frequency distribution with no gap formation between the primary and secondary oocyte growth stages. There was no seasonal variation in the percentage of oocyte stages in ovaries in the spawning capable phase, and the diameter of those oocytes at the most advanced vitellogenic stage was approximately constant during the sampling period. These facts provide evidence of continuous oocyte recruitment into the standing stock of developing oocytes. Moreover, when reaching the end of the active reproductive period (i.e. February and March) the prevalence of atresia increased. This is a mechanism adopted by fishes of the indeterminate fecundity type to reabsorb the surplus oocyte production. Based on the findings, we state that the skipjack in the Western Indian Ocean shows asynchronous oocyte growth and an indeterminate fecundity type.

► We study the fecundity type of the skipjack in the Western Indian Ocean.
► We examine the oocyte recruitment pattern using histology and image analysis.
► Ovaries showed continuous oocyte recruitment during the spawning season.
► The prevalence of atresia increased at the end of the active reproductive period.
► These findings support the idea of an indeterminate fecundity type.