Feasibility of using advanced vitellogenic oocytes to assess egg quality in the European sardine (Sardina pilchardus)

Abstract Egg quality can affect offspring survival and, therefore, recruitment, making this parameter as important as egg quantity in fish reproduction studies. It is determined by measuring the dry mass of hydrated oocytes. However, in the northwestern Mediterranean, sardines ( Sardina pilchardus ) are usually caught by purse seine at near dawn, when the spawn has just been released and the ovary therefore does not contain hydrated eggs. This study provides new insights into the use of advanced vitellogenic oocytes as alternatives for egg quality analyses. At this stage, oocytes no longer accumulate additional nutrients, and their dry mass does not differ significantly from that of hydrated oocytes. Only oocytes larger than 500 μm in diameter should be used, as significant differences in dry mass exist between early-mid and advanced vitellogenic stages. Moreover, within the same cohort, variation in egg quality is minimal. These findings demonstrate that cohorts of advanced vitellogenic oocytes are suitable for estimating egg quality and therefore have important implications for future research, given the challenges associated with obtaining hydrated ovaries in some species.