INTER-SPECIFIC AND INTRA-SPECIFIC VARIABILITY IN FEMALE SALMONID SPAWNING BEHAVIOR

Egg pocket characteristics determined from the excavation of 29 strand ed redds made by Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in the River Vosso in western Norway are presented. The egg po ckets of Atlantic salmon were, on average, deeper (27.0 vs. 14.8 cm) a nd contained more eggs (707 vs. 148) than those of brown trout. An ada ptive ''test-digging'' behaviour was indicated by the finding of a sig nificant association between false redds and gravel classified as low- quality spawning substrate. Based on fecundity data and number of eggs per redd, it was estimated that individual Atlantic salmon and brown trout deposited their eggs in 8.4 and 5.7 different redds, respectivel y. A 100-year data series indicates that the high proportion of strand ed salmon redds (23%) can be attributed to the high frequency (27% of the years) and random occurrence of low water discharge rates. Estimat ed probabilities of extinction and associated variances in fitness ass igned different spawning behaviours suggest that the temporal and spat ial variation in offspring survival produces a selective advantage for the multiple-redd tactics over the single-redd tactic. Moreover, in c ontrast to iteroparous salmonids, semelparous Oncorhynchus species inv ariably place all their offspring in one redd. This dichotomy can most likely be ascribed to the provision of parental care (i.e., redd guar ding) by semelparous salmonids.