Loss of natural Baltic salmon populations can severely reduce metapopulation capacity for retaining genetic variation

Abstract

The Atlantic salmon of the Baltic Sea is a species of high ecological importance and socioeconomic value. Historically, 94 rivers entering the Baltic Sea held wild salmon populations. Hydro-electric power plant construction during the 19th and 20th centuries has obstructed the salmons’ migratory pathways to upstream spawning grounds, leading to limited reproduction in the wild, and a depletion of natural - likely unique - populations. Today, wild populations remain in 30 of the rivers, of which 11 are self-sustaining. The additional rivers contain wild and released salmon in various stages of self-sustainability, although a few rivers are completely void of salmon (1). This study addresses whether the loss of separate subpopulations has reduced the overall capacity for Baltic salmon to maintain genetic variation, and if so, what the expected magnitude of such a reduction is.

Effective population size (Ne) is a key term in conservation biology as it measures the rate of loss of genetic variation. It was initially developed for a single, isolated population. Although considerable research has been devoted to modelling Ne in subdivided populations (i.e. metapopulations), most of those efforts have used simplifying assumptions such as subpopulations of equal size and standardized models of migration. We apply a novel analytical tool (2, 3) which allows modeling of complex metapopulations with large flexibility in metapopulation structure. This permits modelling highly topical issues on the genetic dynamics of metapopulations that have previously not been possible to address, which we apply to a case study of the Baltic salmon. Our model is parameterized with empirical data from Baltic salmon populations in order to estimate the metapopulation Ne and inbreeding dynamics of Baltic salmon prior to and after the subpopulation decline.

The results show that with subpopulation sizes, migration rates, and migration models mimicking Baltic salmon population structure, metapopulation Ne is drastically reduced following the loss of separate river populations. Compared to the loss expected based on the proportion of populations lost, the observed reduction in metapopulation Ne is larger by orders of magnitude. This indicates that the capacity of the Baltic salmon to maintain genetic diversity and adapt is likely seriously compromised as a consequence of local extinctions. The future survival and adaptive potential of the Baltic salmon critically relies on management efforts retaining remaining wild populations and restoring populations in rivers that have lost their local stocks.

References 1. ICES. 2016. Report of the Baltic Salmon and Trout Assessment Working Group (WGBAST), 30 March–6 April 2016, Klaipeda, Lithuania. ICES CM 2016/ACOM:09. 257 pp. 2. Hössjer O, Olsson F, Laikre L, Ryman N. 2014. A new general analytical approach for modeling patterns of genetic differentiation and effective size of subdivided populations over time. Mathematical Bioscienc