The impact of spawning pink salmon (Onchorhynchus gorbuscha) on the water quality of northern Norwegian rivers

Abstract

Since the 2017 invasion of pink salmon in northern Norwegian rivers, increasing populations of the odd-year stock have continued to return and are further expanding their range southwestwards. A relatively short time from its first invasion and the pink salmon's strictly 2-year life cycle, gives a unique opportunity to compare the pink salmon's effect on the riverine system, during a season with its presence, compared to a season without. The aim of this thesis is to elucidate the impacts of pink salmon on nutrient dynamics and bacterial communities in four Norwegian rivers, by comparing data from a peak year (2023), with high pink salmon abundance, to a non-peak year (2022). Decompositon of a large number of salmon carcasses after spawning, during a peak year will lead to significant increase of nutrients, potentially stimulating growth of bacterial communities. By analyzing water samples collected through both seasons, from river stations which of different degrees are affected by mitigation measurements, makes it possible to compare pink salmon abundance, expected timing of migration, spawning and senescence death, in contrast to fluctuating variables of water chemistry and bacterial water quality, in relation to the rivers normal seasonal pattern. With pink salmon presence, variables of water chemistry and bacterial water quality were generally increasing from middle of July, including one or several peaks of varying range, until late September. In general, there were few significant differences of variables when comparing a season of pink salmon present, to a season without, but some significant differences when comparing between stations during its presence.