Spatial and Temporal Distribution of Two Groups of Juvenile Atlantic Cod (Gadus Morhua) at the Norwegian Skagerrak Coast
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Spatial and temporal distribution of two groups of juvenile Atlantic cod (Gadus morhua) at the Norwegian Skagerrak Coast Environmental effects and mechanisms Ida Kristin Mellerud Master of Science Thesis Centre for Ecological and Evolutionary Synthesis Department of Biosciences Faculty of Mathematics and Natural Sciences UNIVERSITY OF OSLO 2016 I II © Ida Kristin Mellerud 2016 Spatial and temporal distribution of two groups of juvenile Atlantic cod (Gadus morhua) at the Norwegian Skagerrak coast Ida Kristin Mellerud http://www.duo.uio.no Print: Reprosentralen, Universitetet i Oslo III IV Acknowledgements First of all I want to thank Halvor Knutsen and Asbjørn Vøllestad for excellent (!) supervision over the last two years. I have learned so much from both of you! Halvor, I am so grateful to have been under your guidance, to be a part of your amazing project, and for all motivation and encourage you have given me throughout this project. And Asbjørn, you deserve a special thank you for always giving an amazingly quick feedback with such a calm and positive attitude. With your laughter together with an on-point critical comment in the same sentence, you have made me understand the importance to never settle for anything less than perfect. You both are incredible researchers, and it has been a continuous learning process working together with you. I am also grateful for all the help I got from my fellow students for valuable comments, discussion and moral support during my time as a master student. Most of the work on this thesis has been performed at the Institute of Marine Research, Flødevigen Marine Research Station. I was met with the most incredible hospitality and helpfulness from everyone I met, and it made me feel like a part of the research station. A special thank to Hanne Sannæs and Kate Enersen that helped me in the DNA-lab, to Per Erik Jorde, Esben Moland Olsen, and Jon Albretsen for helping me with statistical analysis and oceanographic modelling, without you this thesis would probably not been existing today. Also, I would like to thank everyone that sampled and stored all my fish used in my thesis. Thank you so much to all of you! Finally, I would like to thank my family and friends for their support and encouragement when things have been hard. You know who you are. An extra, super, special thanks to Sondre for emotional support throughout all 5 years as a biology student, and to always putting a smile on my face. You are truly amazing. Oslo, June 2016 Ida Kristin Mellerud V VI Abstract Spatial and temporal scales of phenotypic and genotypic variation in marine systems are fundamental to understand the ecological and evolutionary processes influencing the system. Until recently, local adaptation in marine fishes have been believed to occur on broad geographic scales due to lack of obvious physical barriers, mobile life stages and high dispersal ability compared to terrestrial and freshwater systems. However, recent studies have challenged these beliefs since strong evidence of geographically structured local populations have been found. Here, I study the spatial and temporal variation in two groups of juvenile Atlantic cod (Gadus morhua), coastal cod (CC) and North Sea cod (NS), both situated along the Norwegian Skagerrak coast. A total of 2113 juvenile cod were sampled in three different years (2000, 2001, and 2003) along the Skagerrak coast during an annual beach-seine monitoring program conducted by the Institute of Marine Research at Flødevigen Research station. By using genetic markers (Single Nucleotide Polymorphisms, or SNPs) that were designed to segregate well between the two groups, I was able to assign the individuals to either North Sea (NS) or coastal origin (CC) based upon adult reference samples from the two areas with a relatively high precision rate (<3%). There was detected a highly significant differentiation among 25 out of 26 SNPs. The average FST over the loci was relatively high (0.12), demonstrating that genetically different groups of juvenile cod coexist along the Norwegian Skagerrak coast. There was a higher frequency of NS cod in more exposed outer coastal areas compared to inner sheltered fjords, but this varied both temporally and spatially along the coast and seemed to be strongly influenced by the strength of the Norwegian Coastal Current (NCC) from North Sea and into coastal Skagerrak. Also, juvenile cod assigned to NS was found to be significantly larger (~2 cm) than CC for all three years, both separately and all years pooled together. This indicates either a phenotypic response to different environmental conditions during development, or genetic differences in growth between the two groups. Nevertheless, the observed genetic variation between CC and NS imply some degree of isolation between the two groups. Spawning and retention of egg – and larvae within fjord basins that are sheltered from coastal currents, high site fidelity and natal homing of older cod may act as barriers and yield genetic differentiation. However, it is premature to conclude if the pattern that emerges is caused by environmental variation or by genetic differences, but the results are discussed in the light of new research findings. VII VIII Table of contents 1 Introduction ............................................................................................................................... 1 2 Materials and Methods ........................................................................................................... 5 2.1 Study species .................................................................................................................................... 5 2.2 Study area .......................................................................................................................................... 5 2.3 Sampling............................................................................................................................................. 8 2.4 Oceanographic modelling and environmental data ........................................................... 9 2.5 Genetic analysis ............................................................................................................................... 9 2.6 Statistics ........................................................................................................................................... 10 2.6.1 Genetic differentiation ....................................................................................................................... 10 2.6.2 Outlier detection .................................................................................................................................. 11 2.6.3 Genetic assignment ............................................................................................................................. 11 2.6.4 Generalized Linear Model (GLM) .................................................................................................. 12 3 Results ........................................................................................................................................ 14 3.1 Genetic structuring ...................................................................................................................... 14 3.2 Frequency of CC and NS .............................................................................................................. 17 3.3 Spatial distribution of CC and NS ............................................................................................. 18 3.3.1 Large-scale geographical distribution ........................................................................................ 18 3.3.2 Small-scale geographical distribution ......................................................................................... 20 3.4 Oceanographic modelling and environmental data ......................................................... 23 3.4.1 Volume flux (106m3/s) ....................................................................................................................... 23 3.4.2 Wave Exposure (m) ............................................................................................................................ 25 3.4.3 Current speed (cm/s) ......................................................................................................................... 26 3.5 Generalized linear model effects ............................................................................................ 27 3.5.1 Body size (cm) variation between CC and NS .......................................................................... 27 3.5.2 Model structure .................................................................................................................................... 27 3.5.3 Most parsimonious model ................................................................................................................ 28 4 Discussion ................................................................................................................................. 30 4.1 Spatial and temporal patterns ................................................................................................. 31 4.1.1 Large-scale patterns ........................................................................................................................... 31 4.1.2 Small-scale patterns ..........................................................................................................................