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Doctoral thesis, 2020

Size-dependent predator-prey interactions, distribution and mortality in salmon: effects on individuals and populations

Jacobson, Philip

Abstract

The biomass dynamics of populations is largely determined by the survival, growth, and reproduction of individuals. For fish and other animals, survival, growth, and reproduction depends on the amount of energy an individual obtains via feeding and are therefore food-dependent. As fish often consume whole prey, the potential for a fish to consume prey depends on both its own size and the size of the prey. Furthermore, predation requires that the predator and prey overlap in time and space. Anadromous fish are born in rivers, grow large at sea and return to rivers to spawn. Consequently, anadromous fish populations will be affected by size- and food-dependent processes in the river and at sea. However, we have limited knowledge about the importance of these processes at sea for the dynamics of anadromous fish. Therefore, the objective of this thesis was to assess whether and when size- and food-dependent processes at sea are important for anadromous fish dynamics. I approached this knowledge gap using empirical and theoretical methods with Atlantic salmon, Salmo salar, in the Baltic Sea as my study species. I show that the diet of salmon at sea depends on their body size, the prey community size-structure and where salmon feed at sea, which, in turn, is governed by their body size and population-of-origin. The body condition of salmon at sea increased with prey availability at low prey densities. Moreover, growth and survival at sea differed between salmon originating from different rivers and inter-annual growth variation at sea contributed to explain inter-annual variation in the reproductive potential of salmon populations. I further show that the responses of anadromous fish populations to changes in river productivity and mortality targeting adults depend on if individuals compete for resources at sea or not and on sea productivity. In summary, these findings demonstrate the importance of accounting for size- and food-dependent processes at sea, such as survival, growth, and size-dependent predatorprey interactions to increase our understanding of anadromous fish populations and how they will be affected by external drivers such as environmental change and fisheries.

Keywords

Anadromy; habitat shifts; population dynamics; prey availability; salmon; size-dependent predator-prey interactions; size-structure; spatial distribution

Published in

Acta Universitatis Agriculturae Sueciae
2020, number: 2020:32
ISBN: 978-91-7760-582-9, eISBN: 978-91-7760-583-6
Publisher: Department of Aquatic Resources, Swedish University of Agricultural Sciences