Atlantic mackerel daily spawning dynamics and implications for batch fecundity estimations

Abstract

The present study contributes to a better understanding of the daily spawning dynamics of southern NEA mackerel (Scomber scombrus) with implications for the estimation of batch fecundity. It shows that there is a time window during the day, mainly in the afternoon, during which the advanced oocyte mode in imminent spawners separates from the remaining, smaller oocytes. This synchronicity in the separation of the spawning batch amongst imminent spawners corroborates evidence for the existence of daily spawning synchronicity in the population. This is particularly important for applications of the daily egg production method, DEPM, because such pattern facilitates both the ageing of eggs for the estimation of the daily egg production at sea and the ageing of postovulatory follicles for the estimation of spawning frequency. For NEA mackerel, batch fecundity could only be measured when a clear hiatus was established between the spawning batch and the smaller oocytes. Hydrated females that do not show such hiatus would not be valid for batch fecundity measurements suggesting that the “hydrated oocytes method” is not fully applicable for this stock. Knowing the time of day at which the batch is separated, will facilitate the sampling of valid females for the estimation of batch fecundity.

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

“…The value of RF POF for sardine, estimated using the Weibel and the POFPD methods, was close to both the gravimetric estimate of RF B and to the corresponding historic mean value of 384 oocytes g -1 for Atlantic sardine stocks (Ganias et al, 2014) clearly suggesting the existence of a single daily POF cohort in this species. Contrary, RF POF values in mackerel were much higher than the present and previous RF B estimates for the western (31-46 oocytes g -1 ; Priede and Watson 1993) and the southern Northeast Atlantic (49.6 oocytes g -1 ; Ganias et al 2018) mackerel stocks, implying the presence of more than one daily POF cohorts in this species. The differences in the number of POF cohorts was also shown through differences of POF resorption across ovarian stages in the two species.…”

“…Batch fecundity (F B ) was estimated gravimetrically using ovaries at the late VTG and GVM stages for sardine (Ganias, 2012) and at the GVBD stage for mackerel (Ganias et al, 2018). A small portion of the fixed ovarian tissue was dissected and weighed (SOW, 0.001 g), oocytes were gently separated ultrasonically and stained with toluidine blue (Anderson et al, 2020).…”

First thorough assessment of de novo oocyte recruitment in a teleost serial spawner, the Northeast Atlantic mackerel (Scomber scombrus) case

“…Studies within applied fish reproductive biology have mostly targeted advanced developing oocytes, i.e. primarily VOs or FOM/HO (hydrated oocytes), when aiming at studying temporal and/or spatial fecundity dynamics 9 , 13 , 14 . However, over the recent years, several studies have pointed at the importance of focusing on the actual PVO production as well to better understand oocyte recruitment processes and thereby the underlying fecundity formation as such 10 , 13 , 21 , 24 , 25 , 28 , 29 .…”

“…the assumed determinate Atlantic cod ( Gadus morhua ) shows individual examples of OSFD with missing or indistinct hiatus 10 , in the indeterminate horse mackerel ( Trachurus trachurus ) the hiatus first emerges in late spawners 11 , whilst hiatus formation (cortical oocyte growth) in the indeterminate round herring ( Etrumeus teres ) coincides with hydration of the advanced oocyte cohort 12 . Neverthless, for many years, “the hiatus method” 10 has been used as a semiquantitative tool to decide upon the fecundity type, though most research consortia focus in these respects on the VOs rather than the PVOs, the latter an emerging field 13 , 14 .…”