Growth and smolting in anadromous brown trout (Salmo trutta) in Lærdalselva : responses to hydropower and Gyrodactylus salaris

Abstract

Lærdalselva has faced many environmental alterations caused by anthropogenic activity over the last four decades. This study has categorized these into four different treatment periods: “Before hydropower ” (1954-1974), “After hydropower” (1974-1996), “Hydropower, Gyrodactylus salaris and Gyrodactylus salaris treatments” (1996-2011) and “present times” (2012-2014). The objective of this study was to determine if these treatment periods have imposed growth alternations and altered age- and size at smolting in the anadromous brown trout population (Salmo trutta L.) of Lærdalselva. These objectives were analyses using scale analyses of scales sampled from all treatment periods. From the scale readings, freshwater- and sea age as well as age at smolting were determined at individual level along with back-calculated sizes at all ages. These size-at-age and smolting trajectory data were subjected to statistical analyses designed for elucidating the research objectives. There were no significant differences in May-October mean water temperature among treatment periods. The May-October mean water discharge has changed significantly among treatment periods, with less water discharge after hydropower. Back-calculated length at first year was significant reduced after launching hydropower compared to the preceding period. Back-calculated specific growth rate during second year in freshwater, showed no evidence of treatment period effect. There was a positive correlation between summer low-water discharge and second-year specific growth rate. There has been a significant shift in smolt reaction norms (SRN) where the anadromous brown trout during the hydropower period delayed smolting by almost a year and larger size than during the preceding period. During the more recent periods, the SRN has gradually shifted towards the original pattern. The marine growth has changed significantly among the treatment periods. First sea year back-calculated specific growth rate has significantly declined during the period after hydropower compared to the preceding period. However, Second-seayear back-calculated specific growth rate has significantly improved after hydropower when compared to the before period. For the hydropower, G. salaris and G. salaris treatment period, a decline in second-sea-year back-calculated specific growth rate was found – after correcting for the positive effect of mean summer air temperature. My study indicates that the anadromous brown in Lærdalsleva is capable of rapid adaptions to anthropogenic-induced environmental change.