Inter- and intra-specific variability in female salmonid spawning behaviour

Egg pocket characteristics determined from the excavation of 29 stranded redds made by Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in the River Vosso in western Norway are presented. The egg pockets of Atlantic salmon were, on average, deeper (27.0 vs. 14.8 cm) and contained more eggs (707 vs. 148) than those of brown trout. An adaptive "test-digging" behaviour was indicated by the finding of a significant 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, respectively. A 100-year data series indicates that the high proportion of stranded salmon redds (23%) can be attributed to the high frequency (27% of the years) and random occurrence of low water discharge rates. Estimated probabilities of extinction and associated variances in fitness assigned different spawning behaviours suggest that the temporal and spatial variation in offspring survival produces a selective advantage for the multiple-redd tactics over the single-redd tactic. Moreover, in contrast to iteroparous salmonids, semelparous Oncorhynchus species invariably place all their offspring in one redd. This dichotomy can most likely be ascribed to the provision of parental care (i.e., redd guarding) by semelparous salmonids.