The Feeding Ecology of the Harbour Porpoise Phocoena Phocoena L
Total Page:16
File Type:pdf, Size:1020Kb
9th Meeting of the North Sea Group ASCOBANS/NSG9/Inf.3.4 Online, 20 – 21 January 2021 Dist. 21 January 2021 Agenda Item 3.4 Investigation of the Health, Nutritional Status and Diet of Harbour Porpoises (Action 10) Information Document 3.4 The Feeding Ecology of the Harbour Porpoise Phocoena Phocoena L. in a Changing Environment Action Requested Take note Submitted by Belgium ‘OCEANS AND LAKES’ INTERUNIVERSITY MASTER OF SCIENCE IN MARINE AND LACUSTRINE SCIENCE AND MANAGEMENT (JH/RBINS) THE FEEDING ECOLOGY OF THE HARBOUR PORPOISE Phocoena phocoena L. IN A CHANGING ENVIRONMENT Elke Lambert June 2020 Thesis submitted in partial fulfillment for master’s degree in Marine and Lacustrine Science and Management PROMOTOR: Dr. Steven Degraer (UGhent; RBINS) CO-PROMOTOR: Dr. Bob Rumes (RBINS) SUPERVISOR: Jan Haelters (RBINS) ABSTRACT The stomach contents of 180 harbour porpoises (Phocoena phocoena) stranded or bycaught along the Belgian coastline between 1997 and 2018 were analysed to reconstruct the diet and study the factors shaping their feeding ecology. This was investigated combining two techniques used in diet studies: population averages (i.e. diet indices) and analysis of individual variation in the diet through multivariate analysing techniques (nMDS and PERMANOVA). More than 25 fish and invertebrate prey taxa were identified, highlighting the broad prey spectrum that these generalist predators can feed on. However, the majority of porpoises consumed between 1 and 4 prey groups. The diet was primarily dominated by four key prey guilds (i.e. “The big four”): gadoids (mostly whiting Merlangius merlangus), gobies (Pomatoschistus sp.), sandeels (Ammodytes sp.) and clupeids (both herring Clupea harengus and sprat Sprattus sprattus), whilst other taxa were of less importance. Harbour porpoises mainly consumed small prey species or the juveniles of larger sized gadoids (e.g. Atlantic cod). Even though the optimal foraging strategy expresses the need for porpoises to strive to feed on mainly prey with a high energetic return (i.e. high-quality prey), they do not solely feed on prey that are most profitable in Belgian and surrounding coastal waters (i.e. sandeels and clupeids) and tend to eat a considerable amount of relatively low-energy lean prey (i.e. gobies and gadoids). Our analysis recovered an ontogenetic development in prey choice, with juveniles eating mainly small lean gobies as opposed to larger gadoids, possibly complemented with energy rich sandeels in adult porpoises. Ample seasonal variation in the diet was also recovered, clearly linked to the changing availability of the different prey groups throughout the year in the Southern part of the North Sea. Our study could neither confirm nor reject an opportunistic or selective foraging strategy, though we present slightly more evidence for the former, with porpoises feeding on locally abundant and easy-to- access prey species. Though, more quantitative data on resource availability, especially for pelagic species, is needed in our waters to confirm or reject opportunistic feeding behaviour. A relatively low amount (15%) of empty stomachs was recovered during this study and preliminary analysis suggested that the highest chance of starving in harbour porpoises can be ascribed to juveniles during the summer months. Key words: Phocoena phocoena; diet; individual variation; stomach content analysis; multivariate analysis; empty stomachs 2 ACKNOWLEDGEMENTS The past year represented one of the most educational and rewarding experiences of my academic journey so far. It helped me to develop better skills, both on an academic and personal level. This study was the result of a great deal of motivation and passion for the topic and the exceptional guidance and support platform I obtained to get here. Therefore, there are many people to thank. First and foremost, I would like to thank my promotor Steven Degraer for granting me the opportunity to work on this topic and his constructive comments, on both the structure and content of this thesis, to elevate it to a higher level. I would especially like to thank my co-promotor Bob Rumes for his fundamental guidance, support, feedback, and time spent answering all my questions. You have been a great motivator, guiding me through all the steps of this thesis. Special thanks to my supervisor Jan Haelters for his availability, input, and his guidance and patience throughout the otolith identification process. Thank you for showing me stomach content analysis from scratch and making all the resources I needed available for me (including the drive to obtain more stomach samples). I would also like to thank Francis Kerckhof for his warm welcome in the lab and for making it a great environment in which to write a thesis. Furthermore, I would like to thank you for your sparked interest in otolith research (giving me small side jobs on sprat and herring otoliths), identifications of various invertebrates found in the stomach and believing in my capabilities. A special thanks to Danae Kapasakali, for her time showing me around the lab and showing me how to work with the stereo microscope and its tools. I also want to thank Wim Wouters, for helping to identify several ‘difficult’ otoliths and giving me tips on how to distinguish ‘look-a-like’ otoliths. Thank you especially to Jan Vanaverbeke for the feedback on my R scripts and statistical interpretations. I would also like to thank Thierry Jauniaux for showing me the standardised necropsy techniques on harbour porpoises during the workshop and for making a great deal of samples available to me. Thank you to Kris Hostens for his calculations on seasonal density of dominant prey species in Belgian waters and to Elise Toussaint for making her data available to me to complement my samples. Finally, I am immensely grateful for my family, friends and boyfriend. To my mom and dad, who always believed in me and wanted me to become the best version of myself during this period of academic and personal growth, reassuring and supporting me whenever I needed it. To my boyfriend, for always encouraging me and for helping me blow off steam during the hardest moments. You showed me that I am capable of achieving much more than I often think, as you always told me, and I am incredibly grateful to you. To my friends Paulien, Evelien, Amber, Niels, Lara and Cédric who kept in touch almost daily during the COVID-19 crisis, always believed in me, and were always available whenever I needed them, day or night. 3 TABLE OF CONTENT ABSTRACT ................................................................................................................................................ 2 ACKNOWLEDGEMENTS .......................................................................................................................... 3 INTRODUCTION ....................................................................................................................................... 5 HARBOUR PORPOISES IN THE SOUTHERN NORTH SEA ..................................................................... 5 FEEDING ECOLOGY ........................................................................................................................... 8 RESEARCH FRAMEWORK AND RELEVANCE .................................................................................. 10 SPECIFIC OBJECTIVES OF THIS STUDY ............................................................................................. 11 MATERIAL AND METHODS .................................................................................................................... 12 COLLECTION AND PREPARATION STOMACH CONTENTS ............................................................ 12 ANALYSIS OF THE STOMACH CONTENTS (SCA) ............................................................................. 14 ESTIMATING ORIGINAL CHARACTERISTICS .................................................................................... 18 EXPRESSING PREY COMPOSITION: PREY GUILDS & DIET INDICES ................................................ 18 SOURCES OF BIAS ATTACHED TO METHODOLOGY ...................................................................... 20 DATA ANALYSIS ................................................................................................................................ 20 PREY AVAILABILITY IN THE BELGIAN PART OF THE NORTH SEA ..................................................... 22 RESULTS .................................................................................................................................................. 23 OVERALL PREY COMPOSITION ....................................................................................................... 23 SIZE AND WEIGHT OF PREY TAKEN .................................................................................................. 29 CORRELATION WITH PORPOISE CHARACTERISTICS ...................................................................... 31 EMPTY STOMACHS ........................................................................................................................... 34 NON-EMPTY STOMACHS: MULTIVARIATE ANALYSIS ...................................................................... 37 nMDS ............................................................................................................................................. 37 PERMANOVA ................................................................................................................................ 38 DISCUSSION .........................................................................................................................................