Development of a new ‘ultrametric’ method for assessing spawning progression in female teleost serial spawners

Abstract

The collection and presentation of accurate reproductive data from wild fish has historically been somewhat problematic, especially for serially spawning species. Therefore, the aim of the current study was to develop a novel method of assessing female spawning status that is robust to variation in oocyte dynamics between specimens. Atlantic cod (Barents Sea stock) were used to develop the new 'ultrametric' method, that is based on the progressive depletion of the vitellogenic oocyte pool relative to the rather constant previtellogenic oocyte (PVO) pool. Fish were subsequently partitioned into one of four categories that accurately reflected changes in their oocyte size frequency distribution characteristics and gonadosomatic index throughout spawning. the ultrametric method overcomes difficulties associated with presence of bimodal oocyte distributions, oocyte tails, lack of clear hiatus region, and presence of free ova, and can be implemented at a single sampling point. Much of the workflow is fully automated, and the technique may circumvent the need for histological analysis depending on the desired outcome. The ultrametric method differs from the traditional autodiametric method in that PVOs can be separated by ultrasonication and then enumerated, and ovarian homogeneity is not a mandatory requirement per se. The method is designed for determinate spawners but might be extended to include indeterminate spawners.

Ovarian dynamics and fecundity regulation in blueback herring, Alosa aestivalis , from the Connecticut River, US

“…In order to estimate relative total fecundity (i.e., total number of SG oocytes/g of W ev ; RF T ) it was necessary to distinguish primary growth oocytes (PG) from SG oocytes. Because PG oocytes are translucent and closely attached to each other (Anderson et al, 2020) the whole mount procedure that included oocytes smaller than 200 μm needed some extra steps and was thus held in 10 out of the 91 females. The size threshold between PG and SG oocytes was estimated statistically as the mean cutpoint value between the mode of very small oocytes and the SM in these 10 females.…”

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

“…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 . In the stock assessment of mackerel, one of the biggest problem up today has been to unveil the actual fecundity type and thereby run the most adequate EPM 20 , 30 to provide the SSB index possibly nearest to the real SSB 18 , 20 .…”

“…This distinction may, however, be met with interpretation problems, e.g. 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 .…”

“…Thus, any associated macroscopic staging classification (see below) was in this particular situation considered too imprecise and thereby debateable 13 . This article aims at resolving this de novo oocyte recruitment assessment problem by merging state-of-the-art techniques, represented by the oocyte packing density (OPD) theory 15 —providing phase-specific oocyte numbers—and the most recent ultrametric method 10 —detailing the “stage of spawning” 4 , 16 represented by the oocyte ratio category. In short, earlier PVO and resulting de novo recruitment investigations have lacked these detailed insights, which are paramount to reconstruct the oocyte dynamics in the target species.…”

“…Based on the outlined controversy about the mackerel reproductive strategy and important discrepancies in estimates among primary stock assessment methods 20 , we combined OPD theory, applied previously on both determinate 15 , 21 and indeterminate species 15 , 22 – 25 with the new ultrametric method 10 to clarify whether the mackerel fecundity type is determinate or indeterminate, or an in-between combination of both fecundity types 4 . Ultimately, we accurately quantified how many eggs each mackerel de facto releases over the season by adding in the advanced, relevant PVO production.…”