Research Document - 2006/016

Population Regulation in Eastern Canadian Atlantic salmon (Salmo salar) populations

By Gibson, A.J.F.

Abstract

The timing and nature of density-dependent survival in Atlantic salmon populations was analysed using electrofishing data from nine populations in the Maritime Provinces to evaluate its role in freshwater environments, and smolt-to-adult return-rate data from 15 populations in eastern Canada to evaluate its importance in marine habitat. Three spawner-recruit models, a Beverton-Holt, a Ricker and a one-parameter density-independent model, were fit to each data series using maximum likelihood, and model fits were compared using likelihood ratio tests. Within fresh water, no single, unequivocal pattern was evident with respect to the timing of density dependence. Of the six egg-to-age-0 transitions, the addition of a second parameter for density dependence resulted in a statistically better fit in three cases. In six of the nine age-0-to-age-1 transitions, the addition of second parameter provided a statistically better fit, as was also the case in three of the nine age-1-to-age-2 comparisons. Of the nine populations, density dependence was not detected in two populations, was detected in only one transition in two populations, detected in two transitions in four populations and was detected in all three transitions in the remaining population. Overcompensation was not detected in these data. The Ricker model (which exhibits overcompensation) did not provide a statistically significantly better fit in any of the 25 comparisons, and when comparisons were made over all populations, the Beverton-Holt model (which does not exhibit overcompensation) provided a statistically better fit for the three age class transitions investigated here. In the marine environment, density dependence was potentially detected in three of the 15 return-rate data series for salmon maturing after one winter at sea, but was not detected in any of the nine return-rate data series for fish maturing after two winters at sea. Carrying capacity for age-1 salmon was found to be highly variable among populations. Using a mixed-effects model, the median carrying capacity was estimated to be 24.8 parr/100m² with 95% of the probability density falling between 3.8 and 165.9 parr/100m². The variability in both the timing of density dependence and carrying capacity for parr highlights the need for population-specific data for establishing reference points or when planning recovery or enhancement activities.

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