Differential changes in growth patterns of anadromous brown trout and Atlantic salmon from the River Etneelva over a 25-year period

Abstract

Freshwater and marine growth of anadromous brown trout (Salmo trutta) and Atlantic salmon (Salmo salar) from the River Etneelva was analysed in relation to river, fjord and ocean temperatures during the periods 1976–1982 and 2000–2007. Anadromous brown trout grew more slowly through their first and second summers in the sea during the last observation period compared to the first period, and there were more growth checks in the scales sampled from the last period. The reduced growth in length corresponds to a weight reduction of about 20–40%. In fresh water, there was no change in growth rate of the trout parr, while in Atlantic salmon, growth rates of both parr and post-smolts increased from the first to the second period, possibly due to the corresponding increase in temperatures in the river and the marine environments. Anadromous brown trout stay in the fjord during the whole marine feeding period, and therefore the reduced growth in the sea must be caused by factors operating in the fjord environment. Although changes in food availability may also play a part in the observed changes, the negative trend for anadromous brown trout is most likely related to the high infection levels of salmon lice (Lepeophtheirus salmonis) observed in the central and outer parts of the Hardangerfjord.