Quantification of Primary and Secondary Oocyte Production in Atlantic Cod by Simple Oocyte Packing Density Theory

Abstract

AbstractAs for other teleosts, the level of primary oocyte production ultimately determines the number of eggs shed by Atlantic cod Gadus morhua, but so far these minute cells have been little studied, probably due to methodological challenges. We established a quantitative “grid method” based on simple oocyte packing density (OPD) theory, accurate input data on ovary volume, oocyte‐stage‐specific ovarian volume fractions (from hits on grid‐overlaid sections), and individual oocyte volumes (from diameter measurements of transections). The histological OPD results were successfully validated by automated measurements in whole mounts. The analyzed material originated from cultured Atlantic cod held in tanks for 19 months through the first maturity cycle and part of the second maturity cycle. Prior to sexual maturity, none of the fish showed the so‐called circumnuclear ring (CNR; rich in RNA and organelles) in the cytoplasm of their primary oocytes, but this ring (phases 4a, 4b, and 4c) quickly appeared later on around the time of the autumnal equinox, followed by production of cortical alveolar oocytes (CAOs), early vitellogenic oocytes (EVOs), and late vitellogenic oocytes (LVOs). A very similar pattern was observed in the second maturity cycle. Thus, it is concluded that an autumnal night longer than 12 h generally triggers oocyte growth in Atlantic cod. A few immature individuals became arrested at the early CNR phase (phase 4a); hence, the use of CNR presence as a maturity marker should be treated with some caution. The maximum OPD was 250,000 oocytes/g of ovary for phase 4a; 100,000 oocytes/g for combined phases 4b and 4c; 100,000 oocytes/g for CAOs; 50,000 oocytes/g for EVOs; and 25,000 oocytes/g for LVOs. The relative somatic fecundity showed a dome‐shaped curve with oocyte development (from CAO to LVO). Production of CAOs appeared at a fresh oocyte diameter of 180 μm, which is significantly below the commonly accepted threshold value of 250 μm for developing Atlantic cod oocytes.