Aggregative response in white-tailed eagles- an initial study of the terrestrial implications of invasive Pacific pink salmons (Oncorhynchus gorbuscha) in northern Norway.

Abstract

Abstract The spawning population of invasive Pacific pink salmon (Oncorhynchus gorbuscha) has only continued to increase in the river systems on the Varanger peninsula in Norway, causing concerns across institutional boundaries and management communities. So far, the Norwegian research efforts on Pacific pink salmon have been centred on consequential effects in marine- and freshwater ecosystems, while terrestrial ecosystems have received less attention from both the scientific- and management community. The trophic linkage of riverine- and terrestrial ecosystems is likely to be the key determinant of transfers of marine-derived resources through Pacific pink salmon. This thesis shed light on how marine-derived resources became available to the wider scavenger community through white-tailed eagles (Haliaeetus albicilla). Data materials were collected at Skallelv on the Varanger peninsula in Norway from July 1st to September 5th in 2021 and 2022. I find that the relative abundance of white-tailed eagles was indeed much higher when Pacific pink salmon spawned in Skallelv in 2021 than in 2022, when no spawning occurred. The spatiotemporal synchrony of white-tailed eagles and Pacific pink salmon was observed at several scales, suggesting an aggregational response on part of the highly mobile white-tailed eagle. The aggregation of white-tailed eagles corresponded to the time that an increasing number of Pacific pink salmon had entered the post-spawning stage.

This study has identified the white-tailed eagle as a key driver in the cross-boundary transfer of marine-derived resources from invasive Pacific pink salmon spawning in river systems on the Varanger peninsula in Norway. The white-tailed eagle temporary extended the food supply for the wider community of scavenger, also after post-spawners were naturally contributing to the carcass pool as most were submerged and remained unavailable to terrestrial scavengers. A main aspect of my observations is the suggested importance of white-tailed eagle. This is presented in a model for the cross-boundary transfers of marine-derived energy, nutrients, and organic matter from the Pacific pink salmon that spawns in our northern river systems.

Keywords: Terrestrial, riparian, riverine, trophic linkages, cross-ecosystem boundary transfers, carcass-derived resources, scavenger community, Varanger, Norway, white-tailed eagle, Pacific pink salmon.