Egg quality in rainbow trout: The relation between egg viability, selected aspects of egg composition, and time of stripping

Abstract

Batches of eggs (1 batch/female) from 17 rainbow trout in their first spawning season were stripped and reared separately, and the percentage which hatched and the percentage which reached the stage of first feeding were determined. The fertilised egg batches were analysed for egg wet weight, egg dry weight, chorion weight and levels in the eggs of free, bound and total lipid, precipitable protein, protein phosphorus, lipid phosphorus, calcium and iron. All determinations were made on single eggs, and six eggs from each batch were analysed for each variable. Results were expressed in absolute terms (as weight of component per egg) and as percentage of egg dry weight. Highly significant variations in all these aspects of egg composition were shown to occur between parent females. However, there was no significant correlation between the percentage of the original number of eggs which hatched and any one aspect of egg composition, except for a weak positive correlation (P<0.05) with the percentage of protein phosphorus in the eggs. Similarly, there was no significant correlation between the percentage of the original number of eggs which reached first feeding and any one aspect of egg composition except for weak positive correlations (P<0.05) with egg wet weight and with both the absolute level and the percentage of protein phosphorus in the egg. There was significant positive correlation (P<0.05) between the percentage of hatched eggs (alevins) surviving to first feeding and each of the following: egg wet weight, egg dry weight and absolute levels in the egg of bound lipid, precipitable protein and protein phosphorus. Egg batches with higher hatching percentage (>50%) differed significantly from those with zero hatching percentage in having (in absolute terms) higher egg weight, chorion weight, protein phosphorus (all P<0.001), egg dry weight, bound lipid and precipitable protein (all P<0.01) and (percentages) higher chorion weight (P<0.05) and protein phosphorus (P<0.001), and lower free and total lipid (both P<0.01) and iron (P<0.05). In a separate experiment to investigate the effects of allowing the eggs to be retained by the female within the abdominal cavity for increasing periods of time after ovulation, eggs were obtained from three females on three or four successive occasions 2–11 days apart. Although the above aspects of egg composition remained almost constant when the eggs were held in the female for up to 18 days after ovulation, the hatching percentage declined sharply within this period, in two females falling from over 90% to near zero. These results together indicate that the time of stripping of the eggs in relation to the date of ovulation is a much more significant determinant of egg quality than any of the chemical and physical aspects of egg composition which were investigated.