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Sealworms (Pseudoterranova Decipiens) in Shorthorn Sculpin (Myoxocephalus Scorpius) from the Outer Oslofjord

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Sealworms (Pseudoterranova Decipiens) in Shorthorn Sculpin (Myoxocephalus Scorpius) from the Outer Oslofjord Sealworms (Pseudoterranova decipiens) in shorthorn sculpin (Myoxocephalus scorpius) from the outer Oslofjord Emma Lähdekorpi Master Thesis Fall 2011 University of Oslo Department of Biology Program for Marine Biology and Limnology Front page photos 1st from the left by D. Ólafsdóttir, 2nd from www.fiskersiden.no, 3rd by Irish Seal Sanctuary, 4th by Emma Lähdekorpi Acknowledgements This master thesis was carried out in 2010 and 2011 at the Department of Biology in the University of Oslo. My principal supervisor was Karl Inne Ugland and co-supervisors Morten Bronndal and Morten Laane. In order to finish my studies and to write this thesis, I needed help and support from several people to whom I want to thank. First of all thanks to my supervisor Karl Inne Ugland for giving me the opportunity to be a part of this interesting project. Thanks for all the comments and support during the writing process. I also want to thank Morten Bronndal, Gunnar Gundersen and the fellow students for the help of collecting materials and carrying out the wonderful fieldtrips. In addition, I am grateful to Bjørn Berland and Karin Raamat who taught me something about identification of the nematodes. Special thanks to Mark and Janne for the comments and corrections. Finally I want to thank my family and friends for all the support during my studies and dear Ricardo thank you for being there for me. Abstract Sealworms, Pseudoterranova decipiens, are found in several fish species, and generally have a high prevalence in cod (Gadus morhua) and sculpins (Myoxocephalus scorpius). In the outer Oslofjord the final host is common seals (Phoca vitulina Linne, 1758), and here there are two colonies of these seals: a small population of 150 – 400 individuals in the outer Hvaler skerries in Norway and a large population of more than 2000 individuals in the Koster archipelago in Sweden. A feature of the sealworm life cycle in these habitats is that sculpins (Myoxocephalus scorpius) act as an important transmitter from invertebrates to cod. Heavily infested seals have eaten cod which have consumed heavily infected sculpins. The purpose of this project was to compare the sculpins found from Hvaler and Koster in order to investigate the effect of the number of common seals on the infection levels in sculpins. We investigated 107 sculpins from the Hvaler Islands in 2009 and 161 sculpins from the Koster archipelago in the 1990’s. Females constituted slightly less than 70 % of the samples and had a significant different age structure than males. While the females were between 1.5 and 4.5 years old, the males achieved one year less: 1.5 – 3.5 years. All fish were between 12 and 29 cm. Females were approx. 5 cm longer than males. In Hvaler around 25 % of the sculpins were infested by sealworms and the number of sealworms per fish varied between 0 and 4. On average, the number of sealworms per investigated fish (abundance) was 0.4 in Hvaler, and the number of sealworms per infested fish (intensity) was 1.6. In Koster around. 85 % of the sculpins were infested by sealworms and the number of sealworm per fish varied between 0 and 34 with the exception of 102 sealworms in one female. On average, the number of sealworms per investigated fish was 6.4, and the number of sealworms per infested fish (intensity) was 7.5. There was no correlation between the number of sealworm and weight of the fish. In the Hvaler Islands, with few common seals, the number of sealworms per fish followed a Poisson distribution. In the Koster Islands with many common seals, the number of sealworms per fish had a large deviation from the Poisson distribution. In addition to the abundance of seals, the diet of sculpins may contribute to the observed differences in the distribution of sealworms in sculpins in the two areas. TABLE OF CONTENTS 1 INTRODUCTION…….. .............................................................................................. 1 1.1 Biology of the species ................................................................................................ 5 1.1.1 The sealworm (Pseudoterranova decipiens) ....................................................... 5 1.1.2 Shorthorn sculpin (Myoxocephalus scorpius) ..................................................... 6 2 MATERIALS AND METHODS ................................................................................... 8 2.1 Study area ................................................................................................................... 8 2.2 Processing of the fish ............................................................................................... 10 2.2.1 Age determination ............................................................................................. 10 2.3 Detection and identification of the nematodes ......................................................... 11 2.3.1 Characteristics of the sealworm (Pseudoterranova decipiens) ......................... 12 2.4 Statistical methods ................................................................................................... 14 2.4.1 Infection rate ..................................................................................................... 14 2.4.2 Mortality rate ..................................................................................................... 14 2.4.3 Condition factor ................................................................................................ 15 3 RESULTS ...................................................................................................................... 16 3.1 Age distribution ....................................................................................................... 16 3.2 Mortality rate ........................................................................................................... 18 3.3 Length versus age .................................................................................................... 19 3.4 Weight versus age ................................................................................................... 22 3.5 Condition factor ....................................................................................................... 24 3.6 Infestation rate of sealworms ................................................................................... 26 3.7 Distribution of number of sealworms per fish ......................................................... 30 4 DISCUSSION ................................................................................................................ 34 4.1 Methods .................................................................................................................... 34 4.2 Age structure ............................................................................................................ 35 4.3 Growth ..................................................................................................................... 38 4.3.1 Condition of the fish ......................................................................................... 40 4.4 Infection rates of the sealworms .............................................................................. 41 4.5 Infection levels in relation to age and size of fish ................................................... 44 4.6 Distribution of sealworms in fish ............................................................................. 44 4.7 Conclusions .............................................................................................................. 47 4.8 Future research ......................................................................................................... 48 REFERENCES ................................................................................................................ 50 APPENDIX I .................................................................................................................... 58 APPENDIX II .................................................................................................................. 59 APPENDIX III ................................................................................................................. 67 APPENDIX IV ................................................................................................................. 73 1 Introduction This study is one part of a larger student project on sealworms conducted by Karl Inne Ugland at the Marine Biology Research Program of the Department of Biology at the University of Oslo. Previous studies have revealed that the degree of infestation by sealworms (Pseudoterranova decipiens) seems to be rather independent of the number of common seals in the outer Oslofjord. Large samples of some fish species and stomachs of common seals have been collected by several students under the leadership of Morten Bronndal in the last years. I was part of the team doing fieldwork in autumn 2010 at Torbjørnskjær in southern Norway and near Finnøy in the west coast of Norway. Together with the other participating students I took part in identifying the stomach contents of the seals and identifying parasites from the stomach, lung and heart. We also preserved the nematodes for future investigation. Bjørn Berland from the University of Bergen taught us the techniques for fixing and identifying the nematodes from fish and seals. In addition we used the otoliths for age determination of fish. The project is ongoing and more results will be presented in 2012. The third stage of the parasitic sealworm
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