Background/Aims
Understanding the intricate link between cellular biology and spawning dynamics is pivotal for improving the application of the Daily Egg Production Method (DEPM), a widely used approach for estimating spawning stock biomass (SSB) in multiple batch-spawning fish species. This study investigated the ovarian dynamics of three commercially significant small pelagic species with indeterminate fecundity, and distinct spawning dynamics: Atlantic sardine (Sardina pilchardus), Atlantic horse mackerel (Trachurus trachurus), and Atlantic mackerel (Scomber scombrus).
Methods
The ovarian dynamics of these species were examined using innovative methods such as oocyte size frequency distribution, the ratio of total to batch fecundity, the number of oocyte cohorts, and the recruitment of early secondary growth oocytes. Histological and whole-mount procedures were applied to estimate key adult parameters, including spawning frequency (S) and relative batch fecundity (RFb), which are crucial for robust SSB estimation using the DEPM.
Results
The investigation revealed significant variations in the recruitment and dynamics of secondary growth oocytes and postovulatory follicles (POFs) among the species. Atlantic sardine, characterised by long intervals between spawning events and rapid oocyte growth rates, exhibited few (five) clearly separated oocyte cohorts. In contrast, Atlantic mackerel, with shorter spawning intervals, displayed approximately 15 coexisting secondary growth oocyte cohorts, complicating the estimation of relative batch fecundity (RFb) for DEPM application. The study also identified a single daily POF cohort in sardine ovaries, whereas mackerel ovaries contained multiple daily POF cohorts, making the estimation of spawning frequency (S) more challenging.
Conclusion
These findings underscore the challenges in DEPM application in species with complex ovarian dynamics and highlight the importance of understanding species-specific reproductive biology. In my current role at IMAS, I aim to apply these methods to four commercially important small pelagic fish species in Australia—blue mackerel (Scomber australasicus), jack mackerel (Trachurus declivis), redbait (Emmelichthys nitidus), and sardine (Sardinops sagax)—to explore opportunities for improving DEPM application off south-eastern Australia.