Research Document 1997/93

Follicular atresia in Atlantic salmon (Salmo salar L.) in Newfoundland rivers

By O'Connell and J.B. Dempson

Abstract

Follicular atresia is a degenerative process which can occur at any stage of oocyte development, resulting in a reduction in the number of mature eggs available for spawning. Fecundity values used in the calculation of Atlantic salmon egg depositions for most Newfoundland rivers are derived from ovaries collected in the recreational fishery during the summer. Eggs are in early stages of development at this time. In this study, the fecundity of small salmon (<63 cm in fork length) determined from ovaries collected during the summer was compared to that derived from fish sampled during broodstock stripping in the fall for the same river. The rivers involved were Indian River, Exploits River, Conne River, and Little Salmonier River. A reduction in the number of eggs between summer and fall was attributed to atresia. Fecundity expressed in terms of length (eggs/cm) was a better indicator of atresia than weight (eggs/kg). There was a decrease in the number of eggs/cm between summer and fall for Indian River in 1984 (11.2%) and 1985 (5.0%); the average was 8.2%. Decreases for Exploits River in 1985 and 1986 were 16.6 and 14.2% with an average of 13.7%. The greatest reduction (28.5%) occurred in the case of Conne River in 1987. The onset of atresia has been attributed to adverse environmental conditions resulting in stress, among which is water temperature. During the summer of 1987, severe drought conditions affected most rivers in Newfoundland, including Conne River. Sustained low water levels and high water temperatures in 1987 could explain the high rate of atresia observed for this river. An increase was noted for Little Salmonier River in 1985 (16.9%) but this result might have been due to the small sample sizes involved. Results show that atresia can occur to varying degrees depending on environmental conditions. Fecundity values derived from ovaries collected in the recreational fishery therefore have to be regarded as potential and their use could result in underestimates of conservation spawner requirements and overestimates of egg depositions.

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