Ovulatory Rhythm and a Method to Determine the Stage of Spawning in Atlantic Cod (Gadus morhua)

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Abstract
The size frequency distribution of the vitellogenic oocytes in Atlantic cod (Gadus morhua) changes in a predictable manner as spawning progresses. This change was studied by sampling successive batches of eggs from reared cod kept in the laboratory. The shape of the vitellogenic oocyte mode was described by measuring the diameter, standard deviation, and skewness and kurtosis. Linear regressions were derived which related the portion of the total number of eggs spawned per season (PES) to the number of vitellogenic oocytes per gram of the ovary and in relation to the standard deviation of the distribution. This method which allows the PES in a spawning fish to be estimated has the following applications. For fish stock assessment purposes fecundity estimates can be made on spawning rather than prespawning fish and residence times on spawning grounds can be calculated. In addition successive batches of eggs can be identified and studied. This is important in hormonal and larval studies. A detailed description is given of the oocyte growth during the ovulatory rhythm. The rhythm consists of four periods: arrest, final growth, hydration, and ovulation. It is suggested that the egg size is determined during the final growth period.