Research Document - 2016/032

Recovery Potential Assessment for Outer Bay of Fundy Atlantic Salmon (Salmo Salar): Population Dynamics and Viability

By A. Jamie F. Gibson, Ross A. Jones, and Gregor J. MacAskill

Abstract

The purpose of this research document is to provide information about the population dynamics and viability of Outer Bay of Fundy Atlantic Salmon in support of recovery planning for this designatable unit. It covers the topics in the Terms of Reference for the Recovery Potential Assessment for Outer Bay of Fundy 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 Nashwaak River and the Tobique River salmon populations. For the Nashwaak River population, 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 results indicate that at-sea survival for salmon maturing after two winters at sea has decreased by a factor of about two to three, whereas recent increases in the return rates for salmon maturing after one winter at sea are nearer the historical values. Maximum lifetime reproductive rates decreased from an average of 2.49 in the 1970’s to 1.13 in the 2000’s for the Nashwaak River population, and was estimated to be 0.18 for the Tobique River population using data from 1989 to 2005.  Based on these values, in the absence of human intervention or a change in these rates, the Tobique River population is expected to extirpate, whereas, although the Nashwaak River population has a equilibrium population size greater than zero, it has very little capacity to rebuild and is at risk of extirpation from random variability and stochastic events.

Population viability analyses indicate that relatively small increases in either freshwater productivity or at-sea survival are expected to markedly decrease extinction probabilities for the Nashwaak River population, although larger changes in at-sea survival will be required to restore the population to levels above their conservation requirements. Larger changes in survival are expected to be necessary to prevent the extirpation of the Tobique River population due to the reduced rate of survival for emigrating smolts.

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