The relationship of embryonic development, mortality, hatching success, and larval quality to normal or abnormal early embryonic cleavage in Atlantic cod, Gadus morhua

Abstract

A reliable method for assessing the viability of fertilized eggs early in development would be beneficial for the aquaculture industry, allowing egg batches with a high probability of low hatching success to be discarded before costly resources are devoted to their culture, and for recruitment models where egg viability is used predictively. During the last decade, the observation of cellular morphology during embryogenesis has received increased attention as a potential early indicator of embryo quality. However, most often, abnormally cleaving eggs are assessed en masse even though noticeable differences in cleavage patterns are generally present between individual eggs. We separated six egg batches of Atlantic cod, Gadus morhua Linnaeus, 1758, into normal and abnormal cleavage patterns, reared eggs individually in a temperature-controlled room, and recorded daily egg mortality until hatch, hatching success, larval deformation, and larval mortality. Seven abnormal cleavage patterns were readily distinguishable and all showed moderate variability in egg mortality. Both normal and abnormal eggs had similar mortality-rate trends, consisting of an initial high mortality that became asymptotic at about day 8 of development at 6.5 °C. Specific cleavage patterns showed variable mortality-rate trends. No significant differences in cumulative egg mortality were found between any abnormal cleavage patterns, but overall, abnormal eggs had significantly greater egg mortality than normal eggs. Hatching success was high in all groups and not significantly different between normal and abnormal eggs. Few larvae were deformed within any egg batch or pattern and no consistent trends were noted. A severity index was calculated and a suggested severity order determined as asymmetry < adhesions < margins ≤ inclusions < blastomere size.