Master thesis: Comparison of Fish Communities at Two Danish Coastal Boulder reefs studied through Snorkel Visual Census, Stomach Analyse and Stable Isotopes By Simone Lorange Jensen – 201206193 Department of Bioscience, Aarhus University 20 November 2017 Supervised by: Tomas Cedhagen & Peter Grønkjær Preface This study is a 60 ECTS master thesis at Aarhus university. This study was conducted from February to November 2017. The aim was to describe two fish communities and their feeding habits. Fieldwork was carried out from May to September at a newly established boulder reef at Aarhus Ø and an older boulder reef located at Aarhus east harbor. The methods used were snorkel visual census (SVC), stomach analysis and stable isotope analyses. Supervision and support during the thesis work was provided by Tomas Cedhagen and Peter Grønkjær, Department of Bioscience – Aquatic biology, Aarhus University. Simone Lorange Jensen, Stud.nr. 201206193 Bioscience, Aarhus University, Denmark Submission date: 20.11.2017 2 Contents Acknowledgements ......................................................................................................................................... 5 Abstract ............................................................................................................................................................. 6 Dansk Resumé ................................................................................................................................................. 7 1. Introduction ................................................................................................................................................. 8 1.1. Ecological quality of boulder reefs ................................................................................................................. 8 1.2. Biodiversity on Danish boulder reefs............................................................................................................. 9 1.3. Importance for fish communities ................................................................................................................. 10 1.3.1. Attraction or production ......................................................................................................................................... 10 1.3.2. Boulder reefs as a feeding ground ....................................................................................................................... 11 1.4. Study methods ................................................................................................................................................... 13 1.4.1. Visual census method .............................................................................................................................................. 13 1.4.2. Feeding analysis ........................................................................................................................................................ 13 1.5. Aim ....................................................................................................................................................................... 16 2. Method and Sampling ............................................................................................................................ 16 2.1. Locality Description ......................................................................................................................................... 16 2.1.1. Aarhus Bay ................................................................................................................................................................. 16 2.1.2. Aarhus Ø ...................................................................................................................................................................... 16 2.1.3. Aarhus Harbor ........................................................................................................................................................... 17 2.2. Sampling ............................................................................................................................................................. 19 2.2.1. Snorkeling visual census ........................................................................................................................................ 19 2.2.2. Stomach analysis and stable isotope sampling ................................................................................................ 19 2.2.3. Gut examinations ...................................................................................................................................................... 20 2.2.4. Stable isotope analysis ............................................................................................................................................ 20 2.2.5. Data analysis and statistics .................................................................................................................................... 21 3. Results ........................................................................................................................................................ 22 3.1. Fish species abundance analysis ................................................................................................................... 22 3.2. Stomach content analysis ................................................................................................................................ 26 3.2.1. Food composition ..................................................................................................................................................... 26 3.2.2. Prey type ...................................................................................................................................................................... 32 3.2.3. Trophic analysis ........................................................................................................................................................ 35 3.3. Stable isotope analysis ..................................................................................................................................... 36 3.3.1. Stable isotope composition of species ................................................................................................................ 36 3.3.2. Visualization of likely food composition .......................................................................................................... 39 3.3.3. Ctenolabrus rupestris .............................................................................................................................................. 41 4. Discussion .................................................................................................................................................. 43 4.1. Fish species analysis ......................................................................................................................................... 43 4.1.1. Fish Species abundance patterns .......................................................................................................................... 43 4.1.2. Diel variations ............................................................................................................................................................ 44 4.1.3. Physiochemical factors ........................................................................................................................................... 45 4.1.4. Method efficiency ..................................................................................................................................................... 47 4.1.5. Conclusion .................................................................................................................................................................. 48 4.2. Stomach content analysis ................................................................................................................................ 48 4.2.1. Diet analysis ............................................................................................................................................................... 48 4.2.2. Trophic categories .................................................................................................................................................... 53 4.2.3. Food species patterns ............................................................................................................................................... 53 4.2.4. Method efficiency ..................................................................................................................................................... 53 4.2.5. Conclusion .................................................................................................................................................................. 54 4.3. Stable isotope analyses .................................................................................................................................... 54 3 4.3.1. Difference between stations .................................................................................................................................. 54 4.3.2. Stable isotope composition of fish species ....................................................................................................... 54 4.3.3. Ctenolabrus rupestris .............................................................................................................................................