Research Document - 2012/142
Recovery Potential Assessment for Southern Upland Atlantic Salmon: Population Dynamics and Viability
By A.J.F. Gibson and H.D. Bowlby
Abstract
The purpose of this research document is to provide information about the population dynamics and viability of Southern Upland Atlantic salmon in support of recovery planning for this designatable unit. It covers the topics required for the Recovery Potential Assessment for Southern Upland Atlantic salmon relating to estimation of age- and stage-specific life history parameters (mortality rates and stage transition probabilities), the past and present population dynamics and viability of these populations, and scenario analyses to help identify and prioritize among recovery actions.
Analyses are presented for the LaHave River (above Morgans Falls) and the St. Mary’s River (West Branch) salmon populations. Life history parameter estimates were obtained by fitting a life history model to population-specific data including annual estimates of juvenile densities, egg depositions, the number and age composition of emigrating smolts and the numbers of returning adults. The resulting estimates of age- and stage-specific mortality rates, as well as age-specific probabilities of undergoing smoltification and of maturing, were used to estimate smolt abundance and smolt-to-adult return rates in the 1980’s, a time when populations were considered viable, and in the 2000’s, when populations are not. The results indicate that at-sea survival has decreased by a factor of about two to three between these time periods, but remains about 10 times higher than current survival for inner Bay of Fundy Atlantic salmon. Maximum lifetime reproductive rates decreased from an average of 3.59 in the 1980’s to 0.84 in the 2000’s for the LaHave population, and from 4.44 to 1.02 over the same period for the St. Mary’s population. These populations, two of the larger, more productive populations remaining in the Southern Upland, are expected to extirpate in the absence of human intervention or a change in survival rates for some other reason.
Population viability analyses indicate that relatively small increases in either freshwater productivity or at-sea survival are expected to markedly decrease extinction probabilities, although larger changes in at-sea survival will be required to restore populations to levels above their conservation requirements. In contrast with inner Bay of Fundy salmon populations, for which at-sea survival is so low that recovery actions in fresh water are expected to have little effect on overall viability, recovery actions focused on improving freshwater productivity are expected to increase population viability for Southern Upland Atlantic salmon. The analyses indicate that the loss of past resiliency to extreme environmental events is contributing to the high risk of extinction. Recreational fisheries have reduced productivity in the past, although their overall contribution to the abundance declines is thought to be small given the extent to which population dynamics have changed. Conditional on model assumptions, their impact in the 2000’s is thought to be negligible. Research on mortality of adults between spawning events, of smolts and kelts in estuaries, and as a result of acidification is summarized in the document. A sensitivity analysis about the effect of starting population size on population viability highlights the risks associated with delaying recovery actions: recovery is expected to become more difficult if abundance continues to decline, as is expected for these populations with the continued passage of time.
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