Cross-tributary analysis of parr to smolt recruitment of Atlantic salmon (Salmo salar)

We used estimates of Atlantic salmon (Salmo salar) parr and smolt density, estimated in three tributaries of the West River, Vermont, U.S.A., to deter mine (i) if smolt recruitment is density dependent or independent of parr d ensity, (ii) if the proportion of parr migrating as smolts and cohort survi val differ among tributaries, and (iii) the effect of parr maturity on smol t production and recruitment variability. We found that parr to smolt recru itment was best described with a linear function providing no evidence for density dependence in the recruitment dynamics of parr and smolts at the tr ibutary scale. The proportion of age-1 parr recruiting to age-2 smolts did not systematically differ among tributaries or years (overall mean +/- 95% CL: 18 +/- 11%, range = 9-37%), and mean age-1 to age-2 survival ranged les s than twofold among tributaries (27-46%) and was independent of cohort den sity. Survival of age-1 mature (39%) and immature (33%) parr was similar, b ut probability of smolting for mature parr (0.21) was threefold less than f or immature parr (0.76). Quantifying smolt recruitment pathways involving p arr maturation helped elucidate the population-level effect of parr maturat ion on smolt production and recruitment variability.