Contrasting post-ovulatory follicle production in fishes with different spawning dynamics

Summary

The assessment of postovulatory follicles (POFs) is of considerable importance when applying the Daily Egg Production Method (DEPM) since it is used to estimate both spawning frequency (S) and batch fecundity (F_B), provided that daily cohorts of POFs are discernible. Atlantic sardine (Sardina pilchardus) and Atlantic mackerel (Scomber scombrus), two commercially valuable pelagic species with different spawning dynamics were investigated principally to evaluate the appropriateness of the POF method to properly reflect S and F_B. For appropriate quantification, both the Weibel and the postovulatory follicles packing density (POFPD) method, the latter described in this paper, were applied and compared. In sardine, not only was the existence of one single daily POF cohort confirmed but also the estimated number of POFs in the cohort matched gravimetric F_B results. Furthermore, the Weibel method and POFPD led to similar results. However, several daily POF cohorts co-occurred in the mackerel ovary. Therefore, POF-based estimation of F_B only works for species such as sardine with a single, daily POF cohort. Likewise, in relation to estimation of S, the occurrence of single versus multiple POF cohorts makes the histological assignment of spawners to daily spawning classes either straightforward or difficult for species comparable to sardine or mackerel, respectively. In the latter case we suggest focusing in the largest, thus newest, POFs which correspond to the latest spawning event. Therefore, the appropriateness of the POF method in applications of the DEPM should be judged based on the spawning dynamics of the assessed species.