Growth and smolting in anadromous brown trout (Salmo trutta) from Lærdalselva: Responses to hydropower and Gyrodactylus salaris Figure 1: Anadromous brown trout in Lærdalselva. Foto: Torstein Kristensen Preface This master thesis was written for the Department of Ecology and Natural Resource Management at the Norwegian University of Life Sciences (NMBU). The study was carried out in collaboration with NIVA (Norwegian Institute for Water Research) and the KUSTUS project. This project objective is to survey and increase the knowledge base on salmonoids in Sognefjorden, and is financed by both public sector and private companies. Human induced environmental amendments, and the following effect on wild salmonid populations is highly relevant. I hope my study can contribute by increasing our understanding of these complex systems, and be a fresh breath of air in the further management of the anadromous brown trout population of Lærdalselva and Sognefjorden. Unless otherwise mentioned, all fotos are taken by me. I would like to take the opportunity to thank my supervisor Thrond O. Haugen for his excellent mood, statistics-skills and good help. I also want to thank my co supervisors Torstein Kristensen for good discussions and input, and Bjørn Olav Rosseland for providing me scales. Atle Rustadbakken and Reidar Borgstrøm also deserves a thank for evaluation my scale readings, and good discussions around scale reading theory. NIVA and KUSTUS-people Henning A. Urke and Morten Bergan for skilfully and educational guiding me through fieldwork in the deep valleys of Lærdal. Håvard Lo, from the veterinary institute in Trondheim, for providing me scales. My parents also deserves a thanks for just being there, calling me every Sunday for five years. And last but not least my girlfriend Marte, the “breakfast-club”, and all my fellow students and good friends at Ås for bringing joy and happiness into my university life! May 2015 ___________________________________________ Oskar Pettersen I II CONTENTS ABSTRACT .......................................................................................................................................... V 1. INTRODUCTION ............................................................................................................................. 1 2. MATERIALS AND METHODS ...................................................................................................... 5 2.1 STUDY AREA ................................................................................................................................. 5 2.1.1 Sognefjorden .......................................................................................................................... 5 2.1.2 Lærdalselva ............................................................................................................................ 5 2.1.3 Water discharge and temperature .......................................................................................... 6 2.2 STUDY SPECIES .............................................................................................................................. 8 2.2.1 Temperature ........................................................................................................................... 8 2.2.2 Life histories and habitat preferences .................................................................................. 10 2.2.3 Competition and density ...................................................................................................... 11 2.3 DATA COLLECTION ...................................................................................................................... 12 2.3.1 Fish sampling ....................................................................................................................... 12 2.3.2 Age determination and back-calculation of growth ............................................................. 13 2.3.3 Measurement errors ............................................................................................................. 17 2.3.4 Environmental data .............................................................................................................. 17 2.4 STATISTICAL ANALYSIS ............................................................................................................... 18 2.4.1 Air-to-water temperature model .......................................................................................... 18 2.4.2 Growth analysis ................................................................................................................... 19 2.4.3 Smolt reaction norms ........................................................................................................... 19 3. RESULTS ......................................................................................................................................... 20 3.2. RIVER WATER DISCHARGE .......................................................................................................... 20 3.3 RIVER TEMPERATURES ................................................................................................................ 20 3.2. GROWTH ANALYSIS .................................................................................................................... 23 3.2.1. The 1st-year back-calculated length .................................................................................... 23 3.2.2. The 2nd-year back-calculated growth rate .......................................................................... 26 3.2.3 The 1st-year in sea back-calculated growth rate ................................................................... 28 3.2.4. The 2nd-year in sea back-calculated growth rate ................................................................. 29 3.2. SMOLT REACTION NORMS (SRN) ............................................................................................... 32 3.3. BETWEEN RIVER VARIATION ...................................................................................................... 34 4. DISCUSSION................................................................................................................................... 35 4.1. TEMPERATURE AND DISCHARGE VARIATION ............................................................................. 35 4.2. VARIATION IN GROWTH ............................................................................................................. 35 4.2.1. Fresh water environment .................................................................................................... 36 4.2.2. Marine environment ............................................................................................................ 38 4.3. SMOLT REACTION NORMS ........................................................................................................... 41 4.4. BETWEEN RIVER VARIATION ...................................................................................................... 43 4.5. SHORTCOMINGS .......................................................................................................................... 43 5. CONCLUDING REMARKS AND MANAGEMENT IMPLICATIONS .................................. 43 6. REFERENCES ................................................................................................................................ 46 III IV 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-sea- year back-calculated