THE TROPHIC ECOLOGY OF THE ENDANGERED ENDEMIC BARAU’S PETREL (PTERODROMA BARAUI) FROM RÉUNION ISLAND, SOUTH-WESTERN INDIAN OCEAN. A thesis submitted in the fulfilment of the requirements for the degree of: Masters in Marine Biology at: Rhodes University By DANIEL KEITH DANCKWERTS BSc(Hons) African vertebrate ecology, Rhodes University DECEMBER 2014 “The richness I achieve in life comes from nature, the source of my inspiration” :- Claude Monet -: Abstract ABSTRACT: Réunion is the only tropical island that supports two endemic gadfly petrels. Population modelling has indicated that the current threats to the Barau’s Petrel (Pterodroma baraui) may drive it to extinction; this fate is almost definite for the Mascarene Petrel (Pseudobulweria aterrima). Management interventions have therefore been implemented, but conservation potential is handicapped since virtually nothing is known about the former species’ at-sea biology. Thus, following numerous recommendations, this study aimed to combine stomach content, stable isotope, and fatty acid analyses so to provide new information on the at-sea ecology of Barau’s Petrel. Breeding colonies were repeatedly visited over the same season and samples gathered from adults, fledglings, and downy chicks. Stomach contents consisted mostly of accumulated cephalopod beaks whereas structures from fishes, molluscs, arthropods, and crustaceans were less frequently encountered. Fatty acid profiles of blood varied greatly among individuals and the low incidences of monounsaturated and n-3 fatty acids discounted fish as an important dietary component. δ15N of blood indicated a niche between the fourth and fifth trophic levels, which proposes that these birds scavenge to a greater degree than other sympatric seabirds and suggests that adults are also reliant on their endogenous reserves during breeding. δ13C values confirmed the migratory behaviour of adults since birds returning from the non- breeding grounds were enriched relative to individuals sampled through the breeding period. Significant intra-breeding season variations in δ13C were also observed, which matched this species’ patterns of habitat use as have recently described. These results collectively indicate an opportunistic behaviour, which implies some degree of resilience against shifts in prey availability/accessibility, and suggest that this species’ reproduction is i Abstract dependent on distant foraging areas. This breeding strategy is synonymous with great vulnerability as over-investing into a single breeding episode may jeopardize future survival and fecundity. Thus, in light of environmental conditions that are becoming increasingly more susceptible to dramatic changes, the birds could rather be prioritising adult survival, over reproductive output. Further work is obviously necessary and should benefit from databases of fatty acid profiles and isotope signatures of potential prey species. ii Table of contents TABLE OF CONTENTS: ABSTRACT: ...................................................................................................................................i ACKNOWLEDGEMENTS: .......................................................................................................... viii LIST OF NON-STANDARD ABBREVIATIONS: ................................................................................ x LIST OF FIGURES: ...................................................................................................................... xii LIST OF TABLES: ...................................................................................................................... xvii CHAPTER 1: GENERAL INTRODUCTION AND MOTIVATION ...................................................... 1 1.1. Seabirds as biological models ..................................................................................... 1 1.2. Research, management, and conservation priorities for procellariiform seabirds .... 3 1.3. Réunion Island’s endemic Petrels ............................................................................... 8 1.3.1. Description ...................................................................................................... 8 1.3.2. History and taxonomy ................................................................................... 10 1.3.3. At-sea distribution and habitat ..................................................................... 11 1.3.4. Reproductive behaviour ................................................................................ 14 1.3.5. Relationship with man and conservation...................................................... 18 1.3.6. Trophic ecology ............................................................................................. 19 1.4. Motivation, objectives, and hypotheses ................................................................... 20 CHAPTER 2: STUDY SITE AND GENERAL METHODOLOGY ....................................................... 23 2.1. Study site ................................................................................................................... 23 iii Table of contents 2.1.1. Geography and topography .......................................................................... 24 2.1.2. Flora ............................................................................................................... 25 2.1.3. Fauna ............................................................................................................. 25 2.2. General methodology ............................................................................................... 26 2.2.1. Pre-sample collection preparation ................................................................ 26 2.2.2. Sample collection from dead birds ............................................................... 27 2.2.3. Sample collection from live birds .................................................................. 28 2.2.4. Molecular sexing ........................................................................................... 30 CHAPTER 3: IDENTIFICATION OF THE KEY DIETARY RESOURCES TARGETED BY BARAU’S PETREL (PTERODROMA BARAUI) DURING THE BREEDING SEASON AS INFERRED FROM TRADITIONAL STOMACH CONTENT ANALYSIS. ....................................................................... 33 3.1. Introduction .............................................................................................................. 33 3.2. Materials and methods ............................................................................................. 36 3.2.1. Sample description ........................................................................................ 36 3.2.2. Stomach content analysis ............................................................................. 36 3.2.3. Prey group identification ............................................................................... 38 3.2.4. Data analysis .................................................................................................. 39 3.2.5. Size estimation of cephalopod prey .............................................................. 40 3.3. Results ....................................................................................................................... 41 3.3.1. Breadth of diet .............................................................................................. 41 3.3.2. Diet composition ........................................................................................... 42 3.3.3. Size structure of cephalopod prey ................................................................ 46 iv Table of contents 3.4. Discussion .................................................................................................................. 48 3.4.1. Overall diet composition of breeding Barau’s Petrel .................................... 49 3.4.2. Other inferences about the foraging behaviour of breeding adult Barau’s Petrel ............................................................................................................. 53 3.4.3. Limitations of stomach content analyses ...................................................... 57 3.4.4. Conclusions and recommendations .............................................................. 60 CHAPTER 4: INTRA-ANNUAL VARIATION IN THE FORAGING ECOLOGY OF A MIGRATORY, TRULY-PELAGIC SEABIRD: INSIGHTS FROM AN ANALYSIS OF CARBON AND NITROGEN STABLE ISOTOPES. ................................................................................................................................ 62 4.1. Introduction .............................................................................................................. 62 4.2. Materials and methods ............................................................................................. 67 4.2.1. Tissue selection ............................................................................................. 67 4.2.2. Sample description ........................................................................................ 68 4.2.3. Sample preparation ....................................................................................... 69 4.2.4. Sample analysis ............................................................................................. 70 4.2.5. Trophic level estimation ................................................................................ 71 4.2.6. Data analysis .................................................................................................. 72 4.3.