Dynamics of Endangered Southern Upland Atlantic Salmon Populations: Implications for Recovery Planning

The Southern Upland region of Nova Scotia, Canada, contains at least 63 rivers known to have supported anadromous Atlantic salmon populations. These populations are thought to comprise a single unit recently designated “Endangered” by the Committee on the Status of Endangered Wildlife in Canada. This designation was based on evidence for river-specific extirpations (region-wide electrofishing surveys did not detect juvenile salmon in over 60% of the sampled rivers), as well as declines in adult abundance that range from 56% to more than 99% during the last 15 years. In support of recovery planning for these populations, we analyzed their dynamics by fitting a statistical life-history model, consisting of two main components, to population-specific data. For the freshwater component of the model, time varying, age-specific survival rates and stage-transition probabilities were estimated by fitting the model to juvenile, adult and smolt abundance time series using maximum likelihood. The model also provides annual estimates of past smolt abundance that can be used in the marine component of the model in place of shorter time series obtained via field studies. For the marine component of the model, smolt-to adult survival rates were estimated from the adult and smolt abundance time series, and survival estimates for post-spawning adults were obtained from a Bayesian model designed to separate survival from the confounding effects of consecutive and alternate year repeat spawning strategies. We then used equilibrium models and population viability analyses to determine how populations would be expected to respond to recovery activities. For well-studied threats such as river acidification or mortality at hydroelectric generating stations, we are able to predict how populations would be expected to respond to actions that address these threats. However, less is known about how activities such as forestry, agriculture, urban development, mining and transportation are impacting rivers throughout the region. The results highlight the complex dynamics currently limiting population growth, and illustrate that recovery actions focused on several parts of the life cycle will be required to restore populations to past abundance levels.