Population regulatory processes in the Baltic salmon (Salmo salar)

Abstract

Knowledge and understanding of population regulation is the foundation that allows for sustainable harvest of natural populations. In this thesis, I explore potential sources of bias in stock recruitment relationships resulting from potential spawning behaviours, and investigate alternative methods that can be used in order to evaluate population regulatory processes and improve estimates of stock-assessments of the Atlantic salmon (Salmo salar) in the Baltic Sea. I show that the spawner behaviour affects the stock-recruit relationship, and that estimates of recruitment and stockrecruit parameters in two common stock-recruitment functions might be biased. Further, I show that empirical data can be used in order to define the spawner behaviour. Using electrofishing data and spawner count estimates, I found that the underlying within-river spawner behaviour for salmon in River Byskeälven was most probably homing. I also provided an alternative method to predict length and age at smoltification for the Atlantic salmon, and show that this method can be used to predict smolt length and age distributions in rivers where only data of parr lengths are available. Lastly, I use experiments to explore the competition between juveniles of Atlantic salmon and trout (Salmo trutta). I find that the two species outcompete each other and that this relationship is not density-dependent but rather seems to be temperature-dependent, where trout is the stronger competitor in warmer temperatures compared to salmon that outcompete trout in colder waters. The results in this thesis provides important knowledge and understanding of regulatory processes in Baltic salmon, which can be used to improve the stock assessments.