PREDATION BY ALIEN , salmoides LACEPÉDE 1802 (: ), ON INDIGENOUS MARINE IN THE KOWIE SYSTEM, SOUTH AFRICA

Submitted in Fulfilment of the Requirements for the Degree of

MASTER OF SCIENCE

at

Rhodes University

By

Mandla Leon Magoro

February 2014

ABSTRACT

Estuaries serve as nursery areas for a large number of estuary-associated fish species. Some of these taxa also use river catchments as nursery areas. During the upstream migration of this latter group, the juveniles are prone to predation by native and alien predatory fish inhabiting the system. The rate of invasion of ecosystems by alien organisms can be directly linked to anthropogenic influences, including both intentional and unintentional introductions by alien organisms into new regions. The largemouth bass, Micropterus salmoides, is a facultative piscivorous fish that has been successfully introduced worldwide for the main purpose of sport fishing. Where introduced, it has been found to negatively impact native fish and invertebrate species through predation, competitive exclusion and displacement of indigenous fish species. The aim of this thesis was to investigate the predatory impact of largemouth bass on the estuary-associated Cape moony Monodactylus falciformis, Cape stumpnose holubi and freshwater mullet Myxus capensis in the lower Kowie River of the Eastern Cape Province, South Africa. A combination of approaches was employed during this study. Stomach contents, stable isotopes and fatty acid analyses were employed for the reconstruction of the diet of largemouth bass. Acoustic telemetry was used to elucidate largemouth bass movements, particularly their ability to enter the upper reaches of the Kowie Estuary. Stomach contents and stable isotopes analyses showed that M. salmoides exhibit an ontogenic shift in diet, with small and medium sized individuals consuming the higher proportion of fish prey, while large sized individuals mostly consumed invertebrates such as crabs (Potamonautes sidneyi) and Odonata larvae, while consuming only a small proportion of estuary-associated fish. Fatty acid analysis only showed a direct connection between the fatty acid profiles of largemouth bass and those of M. capensis and M. falciformis. The acoustic telemetry results indicated that some M. salmoides individuals periodically move into the upper reaches of the estuary following river flood events. The results obtained from all these approaches highlight the risk posed by largemouth bass introductions on indigenous fish species, particularly those that enter the areas occupied by these top predators.

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TABLE OF CONTENTS

Contents ABSTRACT...... I TABLE OF CONTENTS ...... II LIST OF FIGURES ...... IV LIST OF TABLES ...... VIII ACKNOWLEDGEMENTS ...... XIII DECLARATION ...... XIV CHAPTER 1: GENERAL INTRODUCTION 1. INTRODUCED ORGANISMS ...... 1 2. LARGEMOUTH BASS ...... 2

2.1. Biology and ecology ...... 2 2.2. Impacts of Largemouth bass ...... 3 3. ESTUARY-ASSOCIATED FISH ...... 4 4. STUDY AREA...... 5 5. OBJECTIVES AND HYPOTHESES ...... 7 CHAPTER 2: CONTRIBUTION OF ESTUARY-ASSOCIATED FISH TO THE DIET OF LARGEMOUTH BASS IN THE LOWER KOWIE RIVER, AS DETERMINED BY STOMACH CONTENT AND STABLE ISOTOPE ANALYSES ...... 10 1. INTRODUCTION ...... 10 1.1. Stomach contents analysis ...... 10 1.2. The use of stable isotope analysis in ecological studies ...... 11 2. MATERIALS AND METHODS ...... 14 2.1. Sampling sites for M. salmoides ...... 14 2.2. Sample collection ...... 17 2.3. Laboratory work ...... 18 2.4. Data analysis ...... 21 3. RESULTS ...... 23 3.1. Stomach contents analysis ...... 23 3.2. Stable isotope data ...... 3