Abstract
Information on temporal variations in stock reproductive potential (SRP) is essential in fisheries management. Despite this relevance, fundamental understanding of egg production variability remains largely unclear due to difficulties in tracking the underlying complex fluctuations in early oocyte recruitment that determines fecundity. We applied advanced oocyte packing density theory to get in-depth, quantitative insights across oocyte stages and seasons, selecting the commercially valuable European hake (Merluccius merluccius) as a case study. Our work evidenced sophisticated seasonal oocyte recruitment dynamics and patterns, mostly driven by a low-cost predefinition of fecundity as a function of fish body size, likely influenced also by environmental cues. Fecundity seems to be defined at a much earlier stage of oocyte development than previously thought, implying a quasi-determinate – rather than indeterminate – fecundity type in hake. These results imply a major change in the conceptual approach to reproductive strategies in teleosts. These findings not only question the current binary classification of fecundity as either determinate or indeterminate, but also suggest that current practices regarding potential fecundity estimation in fishes should be complemented with studies on primary oocyte dynamics. Accordingly, the methodology and approach adopted in this study may be profitably applied for unravelling some of the complexities associated with oocyte recruitment and thereby SRP variability.
Highlights
Information on temporal variations in stock reproductive potential (SRP) is essential in fisheries management
Similar patterns were observed for subsequent secondary growth oocytes, i.e. in the stages defined by the presence of cortical alveoli (CAO), vitellogenic (EVO and vitellogenic oocytes (VTO)), migrating www.nature.com/scientificreports nucleus (MNO) and hydrating oocytes (HYO) (Fig. 2C and Supplementary, Table S2)
For European hake, oocyte recruitment – with direct consequences for the resulting egg production – occurs much earlier than previously thought, i.e. already during the gonadotropin-independent stage and that the standing stock of PVO4b-VTO stages reflects reasonably well the potential fecundity of this species
Summary
Information on temporal variations in stock reproductive potential (SRP) is essential in fisheries management. The use of OPD theory is certainly gaining momentum, it is rightly pointed out that its formulations require special attention by new experimenters if they are to be executed correctly Such efforts should, be balanced against the fact that existing studies on indeterminate species remain scarce, typically providing a limited view of this highly complex oocyte recruitment process, and focusing, for example, mostly on the spawning season, even though teleost reproductive cycles are well known to show circannual periodicity. There is a clear need to improve our understanding of the underlying regulation of oocyte recruitment and fecundity pattern, especially in species with an apparent indeterminate fecundity This requires extensive field sampling covering the full reproductive cycle and access to detailed environmental information, and advanced laboratory routines to track stage-specific oocyte development in reliable, quantitative terms. The specific aims of the research described in the present article were (i) to quantify the seasonal dynamics of the whole range of oocyte stages in European hake, applying OPD theory – with special emphasis on early oocyte recruitment, which is widely recognized as a “black box” in the teleost literature; (ii) to re-examine the traditional classification that classifies hake as a species with an indeterminate fecundity pattern; and (iii) to identify potential links between oocyte recruitment and environmental cues, while bearing in mind that multiple cues might be involved
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