The reproductive cycle of Coregonus lavaretus (L) in Loch Lomond, Scotland, inrelation to seasonal changes in plasma cortisol concentration

Abstract

The reproductive cycle of Coregonus lavaretus (L.) in Loch Lomond, Scotland, was investigated by monthly sampling. Spawing takes place during the first 3 weeks of January, on offshore gravel banks. Males congregate on the spawning‐grounds throughout the spawning period, while ripe females cruise in unisexual shoals in deeper water. Individual females migrate to the spawning‐grounds as ovulation takes place, spawn, and return to deep water. After spawning, the gonadosomatic ratio and somatic condition factor of both sexes decrease until June or July. Gonad recrudescence occurs in males between July and October, and in females between July and December. The somatic condition factor of both sexes rises from its minimum in June or July to its maximum in September. A competitive protein binding assay was used to determine cortisol levels in 0–1 ml plasma samples of individual fish. The lowest cortisol levels occurred in fish caught by seine‐netting and killed immediately by anaesthesia in MS 222 (Sandoz). Seine‐netting and killing by concussion induced higher cortisol levels, in the range of 3–5 ng/100 ml. Cortisol levels in fish caught by gill‐netting for 18 h were 3–6 times higher than in seine‐netted fish, and the method of killing was immaterial in this case. The length of time spent in the gill‐net had no significant effect on cortisol level, but maintenance of the fish in aquaria for 24–80 h elicited high cortisol levels of over 50 μg/100 ml. Post‐mortem delay before blood‐sampling resulted in lowered cortisol levels. Seasonal variations in cortisol level were determined in gill‐netted fish throughout the year, and in seine‐netted fish when available. Cortisol levels were high in both sexes in September; and very high in ovulating females caught on the spawning‐grounds, though not in ovulating females caught off the spawning‐grounds.