Fecundity regulation by atresia in turbot Scophthalmus maximus in the Baltic Sea

Abstract

Down-regulation of fecundity through oocyte resorption was assessed in Baltic Sea turbot Scophthalmus maximus at three locations in the period from late vitellogenesis in April to spawning during June to July. The mean ± s.d. total length of the sampled fish was 32.7 ± 3.1 cm and mean ± s.d. age was 6.2 ± 1.5 years. Measurements of atresia were performed using the 'profile method' with the intensity of atresia adjusted according to the 'dissector method' (10.6% adjustment; coefficient of determination was 0.675 between methods). Both prevalence (portion of fish with atresia) and intensity (calculated as the average proportion of atretic cells in fish displaying atresia) of atresia were low in prespawning fish, but high from onset of spawning throughout the spawning period. Atretic oocytes categorized as in early alpha and in late alpha state occurred irrespective of maturity stage from late prespawning individuals up to late spawning fish, showing that oocytes may become atretic throughout the spawning period. Observed prevalence of atresia throughout the spawning period was almost 40% with an intensity of c. 20%. This indicates extensive down-regulation, i.e. considerably lower realized (number of eggs spawned) v. potential fecundity (number of developing oocytes), suggesting significant variability in reproductive potential. The extent of fecundity regulation in relation to fish condition (Fulton's condition factor) is discussed, suggesting an association between levels of atresia and fish condition.

Atresia of ovarian follicles in fishes, and implications and uses in aquaculture and fisheries

“…Atretic degeneration was reported to involve 6 to 13% of western mackerel Scomber scombrus Linnaeus, 1758 oocytes (Greer Walker et al., 1994 ) and 35 to 55% of spring‐spawning herring Clupea harengus Linnaeus, 1758 oocytes (Óskarsson et al., 2002 ). Extensive down‐regulation of realized fecundity compared to potential fecundity was observed in turbot Scophthalmus maximus (Linnaeus, 1758) in the Baltic Sea (Nissling et al., 2016 ). On the contrary, atresia did not significantly affect fecundity in bluemouth Helicolenus dactylopterus dactylopterus (Delaroche, 1809) caught in the north‐western Mediterranean; indeed, a low percentage (1.45%) of atretic oocytes was observed in the spawning‐capable phase of this species (Muñoz et al., 2010 ).…”