Abstract
The understanding of teleost fecundity type (determinate or indeterminate) is essential when deciding which egg production method should be applied to ultimately estimate spawning stock biomass. The fecundity type is, however, unknown or controversial for several commercial stocks, including the Northeast Atlantic mackerel (Scomber scombrus). Aiming at solving this problem, we applied state-of-the-art laboratory methods to document the mackerel fecundity type, including any de novo oocyte recruitment during spawning. Initially, active mackerel spawning females were precisely classified according to their spawning status. The number and size of all phasei-specific oocytes (12 phases), with a special attention to previtellogenic oocytes phases (PVO [PVO2 to PVO4a–c]), were also thoroughly investigated. Examinations of relative fecundity (RFi) clarified that the latest phase of PVOs (PVO4c) are de novo recruited to the cortical alveoli–vitellogenic pool during the spawning period, resulting in a dome-shaped seasonal pattern in RFi. Hence, we unequivocally classify mackerel as a true indeterminate spawner. As PVO4c oocytes were currently identified around 230 µm, mackerel fecundity counts should rather use this diameter as the lower threshold instead of historically 185 µm. Any use of a too low threshold value in this context will inevitably lead to an overestimation of RFi and thereby underestimated spawning stock biomass.
Highlights
The population egg abundance divided by individual fecundity is used in many regions of the world’s ocean to provide indices of spawning stock biomass (SSB) of commercially important teleosts—so-called Egg Production Methods (EPMs)[1,2]
Northeast Atlantic mackerel (Scomber scombrus, hereafter referred to as mackerel), our study model, has a controversial fecundity type—rooted in the unclear de novo r ecruitment[4] speaking for the optional application of either the annual egg production method (AEPM) or the DEPM7
Our data on mackerel suggest that the so-called “lines of evidence” used as conceptual criteria to distinguish between determinate and indeterminate s pawners[4,17] should be revisited as they, informative, have not been sufficient enough to classify the mackerel into the right fecundity type
Summary
The population egg abundance divided by individual fecundity is used in many regions of the world’s ocean to provide indices of spawning stock biomass (SSB) of commercially important teleosts—so-called Egg Production Methods (EPMs)[1,2]. 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 If succeeding in these regards, today’s strict definitions of dichotomic fecundity types should become of less importance, e.g. one might adjust for advanced PVO production and the following de novo recruitment to the vitellogenic pool when using the AEPM on an indeterminate spawner. The mackerel fecundity type was comprehensively investigated by GreerWalker et al.[4], using a total of eight criteria—partly adopted from Hunter et al.17—among them any presence of a hiatus between PVOs and VOs, variation in oocyte number and diameter during the course of spawning, VO growth rate, and trend in atresia presence According to these authors, the mackerel fecundity should be classified as determinate “for all practical purposes”, even though some ovarian development features evaluated were typical of an indeterminate spawner species[4]. The methodological routes taken along with the refreshed conceptual framework regarding fecundity type should be transferable to oviparous teleosts in general, and, possibly, other marine ectotherms exhibiting related reproductive strategies
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