AN ECOLOGICAL STUDY ON THE TIGERFISH HYDROCYNUS VITTATUS IN THE OLIFANTS AND LETABA RIVERS WITH SPECIAL REFERENCE TO ARTIFICIAL REPRODUCTION By Christopher Lodewyk Gagiano THESIS submitted in fulfilment of the requirements for the degree MASTER IN SCIENCE in ZOOLOGY in the FACULTY OF SCIENCE at the RAND AFRIKAANS UNIVERSITY SUPERVISOR: Dr. G.J. STEYN CO-SUPERVISOR: Prof. H.H. Du PREEZ MAY 1997 This thesis is dedicated to my Parents, Koos and Letta. The Heavens gave to thee the mind, the soul oh so purely free but never shall we know where anger deepest flow Where shall it all go the creatures that we know in time and space will man ever after see what Heavens gave to thee With mind to rule the life of creatures all belove the breath off those depend on us to spare their end ACKNOWLEDGEMENTS To God, who blessed me to deeply love and work with His creatures, and who looked safely after me, and gave me the strength to do this project. My supervisor and mentor, Dr. Gert Steyn, who gave me this project to do. Without his help, motivation, advice, critique, and patience, this piece of work would not have seen the light. My co-supervisor Prof. Hein du Preez, who also made this possible through his patience, advice and support. My beloved wife Tanya, who's love and persistent encouragement certainly inspired me a lot to finish this piece of work. My dearest Parents Koos and Letta. What more can I say other than to thank you for to what you have done for me. Your love, inspiration and assistance throughout my long years of study certainly taught me more than books ever could. Dr. Andrew Deacon, friend and mentor who helped me in so many ways that can impossibly be mentioned. Thank you for teaching me so many things during our field trips. To Gerhard Strydom, colleague and friend, who also helped me in so many ways. Your organising of the logistics certainly contributed to the success of this project. My faithful field trip companion Charlie Nkuna, who accompanied me on many a hot day in the Lowveld sun. The evenings spent with you alone in the bush will always be with me. Naledi Mare, who spent many hours in the computer room to help me with all sorts of data processing. The National Parks Board who allowed me to do this project in the Kruger National Park. I also wish to thank the rangers on who's sections I have worked, Johan Steyn, Leighton Hare and Arrie Schreiber for their assistance. This project was supported by the Kruger National Park Rivers Research Programme and financed by the Sanlam Research Unit for Environmental Conservation. TABLE OF CONTENTS Pg Chapter 1. Introduction. 1 Chapter 2. Literature Review. 5 2.1. Representatives of the Characidae in Southern Africa 5 2.2. Conservation Status of Hydrocynus vittatus 7 2.3. Migration 10 2.4. Ecological position 13 2.5. Feeding and growth 13 2.6. Reproduction 18 2.6.1. Spawning 18 2.6.2. Sex ratio 20 2.6.3. Gamete production 21 2.7. Artificial breeding 22 Chapter 3. Aspects of the population dynamics of the tigerfish Hydrocynus vittatus from the Olifants and Letaba Rivers in the Kruger National Park. 27 3.1. Introduction 27 3.2. Materials and Methods 29 3.2.1. Choice and description of sampling localities 29 3.2.2. Sampling procedure and data collection 31 3.2.3. Data analysis 31 3.3. Results and discussion 33 3.3.1. Distribution and abundance 33 3.3.2. Age and growth 34 3.3.3. Mortality 38 3.3.4. Sex ratio 39 3.4. Conclusion 40 Chapter 4. Feeding habits of the tigerfish .(Hydrocynus vittatus) in the lower Olifants and Letaba Rivers, Kruger National Park. 41 4.1. Introduction 41 4.2. Materials and Methods 42 4.3. Results 42 4.4. Discussion 48 4.5. Conclusion 51 Chapter 5. Gonad development and fecundity of the tigerfish (Hydrocynus vittatus) in the lower Olifants and Letaba Rivers, Kruger National Park. 52 5.1. Introduction 52 5.2. Materials and Methods 53 5.3. Results 54 5.4. Discussion 58 5.5. Conclusion 61 Chapter 6. Notes on the induced reproduction and development of the tigerfish Hydrocynus vittatus (Characidae) embryos and larvae. 63 6.1. Introduction 63 6.2. Material and methods 66 6.2.1. Selection and handling of broodstock 66 6.2.2. Artificial stimulation 66 6.2.3. Artificial insemination and hatching 67 6.2.4. Development of embryos and larvae 68 6.2.5. Rearing of larvae and juveniles 68 6.3. Results and discussion 68 6.3.1. Selection and handling of broodstock 68 6.3.2. Induced spawning 69 6.3.3. Artificial insemination and incubation 70 6.3.4. Development of embryos and larvae 71 6.3.5. Free embryo and behaviour of larvae 72 6.3.6. Juveniles 73 6.4. Conclusion 74 Chapter 7. Tooth replacement of tigerfish Hydrocynus vinatus from the Kruger National Park 75 7.1. Introduction 75 7.2. Materials and Methods 76 7.2.1. Field observation 76 7.2.2. Laboratory observation 77 7.3. Results 77 7.3.1. Field observation 77 7.3.2. Laboratory observation 79 7.4. Discussion 79 Chapter 8. General discussion and conclusion. 81 Chapter 9. References cited. 86 ABSTRACT Hydrocynus vittatus, commonly known as the tigerfish, plays an important role in riverine ecology. It is a top predator which roams the open waters of most larger river systems in southern Africa. Their presence in a freshwater ecosystem has a dramatic impact on the fish community structure. It is known that dams and weirs have a negative effect on the migration of the tigerfish. It is also evident that tigerfish do not occur in certain areas in some of the rivers where they have been present historically. The Olifants and Letaba Rivers in the Kruger National Park (KNP) are two of a few rivers within South Africa where tigerfish do occur. The KNP represents the edge of the most southern distribution of tigerfish in southern Africa. It was therefore expected that the tigerfish do not function optimal in the Olifants and Letaba Rivers as they are subjected to waters with high concentrations of silt and low flow which influences migration and successful breeding. Breeding migrations does however take place during the summer months after which the tigerfish returns to the Massingire Dam in Mozambique to avoid the colder winter temperatures in the rivers. Gonad development coincide with the yearly summer rainfall patterns. A deviation of the expected 1:1 male:female sex ratio to favour the males was experienced in both rivers, which may be the result of over population. Females were found to grow to a larger size than the males and were extremely fecund. Although H. vittatus is believed to be mainly piscivorous, other food items such as invertebrates, played an important role in the diet of small and large tigerfish in both the Olifants and Letaba Rivers. Invertebrates were mostly preyed upon which implies that optimal feeding conditions for the tigerfish does not prevail in these systems and that they have to adapt to satisfy their feeding requirements. Tigerfish is more abundant in the Olifants than in the Letaba River. The overall growth performance or phi prime (4)) values for H. vittatus in the Olifants River was determined and compares well to the overall growth performance of tigerfish in the Okavango River and Lake Kariba. However the maximum length calculated for tigerfish in the Olifants River (Lco = 52.40 cm ) is smaller than the Lco values (56.06 cm) for the Okavango River. The mortality rate of tigerfish in the Olifants River exceeds those in the Letaba River which means that the life expectancy is longer in the Letaba as opposed to the Olifants River. Successful artificial spawning revealed some of the secrets of the reproduction strategy of this species. Tigerfish has semi pelagic eggs, are very small (0.65 mm), negatively buoyant and slightly adhesive for bentic and epibiotic incubation, and it is expected that tigerfish would spawn in open water, on a sandy substrate in the vicinity of aquatic vegetation. First hatching took place at 22 h 30 min after fertilization. Vertical movement of the larvae lasts for two days, which allows for downstream movement and dispersement of the larvae. It was found that tigerfish replace their teeth on a regular basis as they grow larger. Transition from conical to functional dentition takes place 45 days after hatching. Replacement of sets of teeth occurs during all phases of its lifespan. It is a quick proses of three to six days during which all teeth are replaced in both the upper and lower jaws. OPSOMMING Die tiervis, Hydrocynus villains, speel 'n belangrike rol in rivier ekologie. Tiervis is 'n roofvis wat in die meeste groot riviersisteme van Suidelike Afrika voorkom en die teenwoordigheid daarvan in 'n ekosisteem 'n groot potensiele impak op die visgemeenskapsstruktuur van daardie sisteem het. Dit is bekend dat damme en keerwalle 'n negatiewe invloed op die migrasie van tiervis het en gevolglik is tiervis the meer teenwoordig in sekere gedeeltes van riviere waar hulle histories wel gevind is nie. Die Olifants- en Letabarivier in die Nasionale Krugerwildtuin (NKW) is twee van 'n paar riviere in Suid Afrika waar die tiervis wel voorkom. Die NKW verteenwoordig feitlik die mees suidelikste verspreiding van tiervis in suidelike Afrika en daar kan verwag word dat tiervis the ekologies optimaal funksioneer in hierdie riviere nie.