Oocyte growth and fecundity regulation by atresia of Atlantic herring (Clupea harengus) in relation to body condition throughout the maturation cycle

Oocyte growth, fecundity regulation by resorption of vitellogenic oocytes (atresia), and condition effects on fecundity for repeat spawners (≥32 cm in total length (TL)) of Norwegian spring-spawning (NSS) herring, Clupea harengus, were examined using samples collected periodically from July 1998 to February/March 1999. This period almost covered the maturation cycle of the fish, i.e., 67% (30/45) of the examined fish had started vitellogenesis as early as in July and 18% (7/40) showed hydrated oocytes in February/March. Oocyte diameter increased linearly over time. Average fecundity of 34 cm TL fish decreased by about 56% from 113 000 in July to 49 200 in February/March. Both prevalence of atresia (portion of fish with atresia) and average relative intensity of atresia (prevalence multiplied by geometric mean of relative intensity of atresia among only fish with atresia) were highest in October and November, i.e., following the summer feeding season when fish started to rely on accumulated body reserves. Estimated duration of atresia was 4.5, 6.8, 6.1 and 7.2 d for July–October, October–November, November–January and January–February/March, respectively. Atresia seemed to be limited to oocytes smaller than 1100 μm, which had lipid and solids (protein, ash and carbohydrates) contents that were only half of the values observed for fully matured oocytes (1400–1550 μm). Both the timing of intensive resorption and size of atretic oocytes seemed to optimise fecundity given available energetic reserves. There appeared a highly significant, positive correlation between ovary dry weight, a proxy of reproductive investment, and muscle dry weight condition factor (MDCF; 100×muscle dry weight/TL3) in the later maturation cycle. Relative fecundity also showed a significant, positive correlation with MDCF in February/March. In conclusion, this study demonstrates important energetic and cellular mechanisms for regulation of reproductive investment in NSS herring females, a long-lived, temperate capital breeder.