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. Tiervis is onderhewig aan hoe slik konsentrasies gedurende vloede in die Olifantsriver en Iaagvloei toestande in die Letabarivier beinvloed die migrasie en broeisukses negatief. Broeimigrasies word gedurende die somermaande deur die tiervis onderneem, waarna hulk na die Massingiredam in Mosambiek gedurende die wintermaande terugbeweeg om die koue watertemperature van die riviere te vermy. Gonade ontwikkeling wind saam met die jaarlikse somerreenval plaas. Daar is 'n afwyking van die verwagte 1:1 mannetjie:wyfie geslagsverhouding wat die mannetjies bevoordeel in beide die Olifants- en Letabariviere. Hierdie afwyking kan moontlik te wyte wees aan oorbevolking in die betrokke sisteem. Wyfies behaal 'n groter liggaamsmassa as die mannetjies en is besonder vrugbaar in vergelyking met ander visspesies. Alhoewel H. villains hoofsaaklik piscivories is, speel invertebrate 'n groot rol in die dieet van beide groot en klein tiervisse in die Olifants- en Letabariviere. Die dieet van die tiervisse in die Olifants- en Letabariviere bestaan hoofsaaklik uit insekte en dui daarop dat optimal voedingstoestande the in hierdie riviere geld nie. Daar is gevind dat tiervis meer volop in die Olifantsrivier is in teenstelling met die Letabarivier. Die algehele groeiprestasie oftewel die phi prime (4)) van tiervis in die Olifantsrivier is bepaal en vergelyk goed met die phi prime waardes in die Okavangoriver en die Karibameer, alhoewel die maksimum lengte vir tiervis in die Olifantsrivier (Lao = 52.40 cm) kleiner is as die Loo waardes (56.06 cm) in die Okavangorivier. Die Die mortaliteitskoers van tiervis is hoer in die Olifantsrivier in vergelyking met die Letabarivier en bring gevolglik mee dat die verwagte lewensduur Langer is in die Letaba as in die Olifantsrivier. Die suksesvolle kunsmatige teel van die tiervis het sekere belangrike aspekte van die voortplantingstrategie bekend gemaak. Tierviseiers is baie klein (0.65 mm), semipelagies met dryfvermoe en is effens klewerig vir bentiese of epibiotiese inkubasie. Die vermoede bestaan dus dat tiervis op 'n sanderige bodem in die omgewing van akwatiese plantegroei broei. Eiers begin 22 uur 30 minute na bevrugting uitbroei waarna 'n aanhoudende vertikale migrasie vir twee dae gehandhaaf word. Tandwisseling geskied op 'n gereelde basis soos wat die tiervis groei. Koniese tande word 45 dae na bevrugting deur funksionele tande vervang. Tandwisseling geskied gedurende alle fases van sy lewensduur. Dit is 'n spoedige proses van drie tot ses dae en geskied gelyktydig in beide die onder- en bokaak. CHAPTER ONE
INTRODUCTION
The tigerfish, H. vittatus (Castelnau, 1861), is probably one of the most strikingly beautiful freshwater fishes that inhabits the warm waters of the east flowing rivers of Southern Africa. The colours of the tigerfish immediately draws the attention as the silvery flank and bluish sheen on the back are reflected in the sun. Parallel to the lateral line are a series of dark lines which nuns across the body. The tail fin is blood-red with a thin pitch-black edge, immediately catching the attention while the other fins are tinged with orange or red. The scales are large, 44-48 in the lateral line, and 15-16 scales around the caudal pendule. The dog like teeth are large, very sharp and set in hard bony upper and lower jaws (Jubb, 1967).
The tigerfish has earned itself a reputation as one of the most sought after freshwater sport fish in the world. This reputation is well earned as the tigerfish is a fierce fighter once hooked. With speed and agility it tests the competence of the angler. The most spectacular moment once the fish is hooked is the way it shatters the surface of the water when it jumps into the air in an attempt to free itself. The head of the fish is then shaken vigorously in an attempt to shake the hook from the mouth. This normally happens a few times before the fish is lured in. The Upper and Middle Zambezi River with Lake Kariba is probably the areas where most anglers go to catch tigerfish. Numerous fishing Lodges are situated on the banks of this river. An international championship in Zimbabwe on Lake Kariba is held each year whereby hundreds of anglers compete to catch the largest tigerfish during the International Tigerfish Fishing Tournament (I.T.F.T.).
The genus Hydrocynus occurs widely in Africa but is absent from Lake Malawi and the rivers of Kenya and Somalia, the Kunene and Kafue Rivers (Bell-Cross, 1965;1966), and coastal rivers of Angola (Poll, 1967a), but occurs in the Okavango River and upper swamp (Skelton et al, 1985) and throughout the Zaire basin to Senegal and the West African rivers as well as in Lake Chad and in the Nile (Daget & Durant, 1981). The Chobe River (locally called the Kwando) is the
1 largest of the west bank tributaries of the Zambezi, where Hydrocynus is present. The Lungwebungu is the next largest tributary, where Hydrocynus occurs well over 200 miles into Angola throughout its course (Bell-Cross, 1965;1966).
Hydrocynus villains is the only representative of this genus in southern Africa. There are, however, five species of Hydrocynus that occurs practically throughout the range of the family Characidae in Africa. Hydrocynus tanzaniae occurs in the eastward flowing rivers of Tanzania, the Wami Ruaha and Rufiji river systems. The lateral stripes of H. tanzaniae are distinct and differs from all other Hydrocynus species in the presence of elongated 3rd and 4th dorsal and anal fin rays (Brewster 1986). Hydrocynus brevis has a deeper body (mean =24.4% of the standard length) than the other species. They occur in Nilo-Sudan, Upper Guinea, Cameroun, Togo, Ghana and Ivory Coast (Brewster 1986). Hydrocynus goliath is the largest of the genus and can attain a mass of 50 kg! Hydrocynus goliath is restricted to the Oubangui River, and the central and upper Zaire (Congo) basin (Brewster, 1986).
In her review of the tigerfish genus of southern Africa (H. villains) and those in central Africa (H. forskahlii), Brewster (1986) concluded that they are the same and that the name H. forskahlii should prevail for both of them. A significant difference in their review of the genus was later found by Paugy & Guegan (1989) and confirmed that the species H. forskahlii to be the central African tigerfish, and H. villains the southern African tigerfish.
The Zambezi rises in the Mwinilunga District of Zambia and flow about 55 miles before entering Angola. Several fish collections in that area failed to produce Hydrocyrtus, as were confirmed by the local fisherman, who state that it is caught a little south in Angola. Water falls in the vicinity of Cazombo might restrict its upstream distribution. Tigerfish are plentiful where the Zambezi re- enters Zambia at Lingelengende, and occurs continuously southward to the Victoria Falls (Bell- Cross, 1965;1966). Hydrocynus vittatus occurs in extremely small channels in the main river of the Upper Zambezi River but apparently seldom penetrates tributaries of equivalent size. This apparent behavioral inhibition may be correlated with the danger of becoming cut of when waters recede after the rainy season. Hydrocynus vittatus is also abundant in Lake Kariba and southward below the dam in the Lower Zambezi. The most southern distribution of H. vittatus is the Pongola
2 System and in South Africa they are mainly found in the Kruger National Park. As of present, all the major rivers and some of the larger seasonal rivers in the Kruger National Park are inhabited by the tigerfish although the distribution of the tigerfish has changed. Earlier on, tigerfish were found as far as Mica in the Olifants river and in the Greater Letaba River at Tzaneen. Large numbers of tigerfish were also found in the Greater Letaba River at Letaba Ranch (Hugo, pers comm). Tigerfish were also found west of Hazyview in the Sabie river (Pienaar, pers comm).
Together with many other fish species, the tigerfish had been inhabiting the waters of the Lowveld of South-Africa for thousands of years. The identification of fish remains from a Late Iron Age site on the Letaba River certainly provides some information as to the fish community of the Letaba River more than a thousand years ago. Amongst some of the remains found were a number of identified skeletal parts of the tigerfish (Hydrocynus spp), which were included in the diet of the communities living there during those years (Plug & Skelton, 1991).
The distribution ofHydrocyma clearly indicates that a linkage between the Zambezi River System and Limpopo River System of some sort must have existed thousands of years ago. The sparse distribution of H. vittatus in the south and the presence of other species such as Alestes lateral's in Kwazulu-Natal confirm that these river systems were linked to the Zambezi River Systems. This was indeed the case as described by Balon & Coche (1974). The Victoria Falls were believed to become the isolating barrier of the previously separate rivers i.e. the pre-Upper and pre-Middle- Lower Zambezi. The Victoria Falls were formed 99 km downstream from its present position, approximately 500 000 years ago. In the Pliocene the Upper Zambezi River and its probable tributaries, the Upper Kafue-Kalomo systems, flowed south together with the present Chobe- Okavango into the Ngami-Makarikari Lake and Limpopo valley. The Middle Zambezi was then a separate river with headwaters somewhere in the region of the present Matetsi River. A tectonic upward in the Caprivi area broke the connection of the Upper Zambezi with the Ngami- Makarikari drainage and the water of the former escaped eastward along a depression and spilled over the sheer drop at the old Middle Zambezi headwaters. At this point of origin of the Batoka Gorge, the existence of the united Upper-Middle Zambezi River and Victoria Falls, began (Balon & Coche, 1974). The monolithic surface of the basalt rocks along the entire length of the gorges has a system of fissures probably formed when the lava cooled. These were later enlarged by
3 upward tectonic movements. The fissures formed distinct fault lines. The gorges are eroded along such fault lines were a protective barrier of solid lava had been dissipated. When the river was spilling over an edge of solid basalt the erosion was slowed down until the water found the weakest point in the edge. When the river succeeded in eroding its way into another fault line, the process of upstream cutting was greatly speeded up (Balon & Coche, 1974).
Although the Upper Zambezi River and the Limpopo System was linked earlier on, the habitat of these two systems differ considerably today. The Upper Zambezi System has many flood plains with clear swift flowing waters and a sandy bottom and banks. The Limpopo System in contrast has irregular flow and carries large volumes of silt with virtually no flood plains. Hydrocynus villains seems however to survive in both these systems in spite of these differences. It would however seem that the Zambezi River is the typical habitat for H. villains.
Many studies on the ecology of the tigerfish had been done in the Zambezi River (Jackson, 1961a; 1961b; Winemiller, and Kelso-Winemiller 1994) and in Lake Kariba, (Badenhuizen, 1967; Matthes, 1968; Balon, 1971; Bowmaker, 1973; Kenmuir, 1973, 1975; Langermati, 1980, 1984; Marshall, 1985;). Few work has however been done on the Limpopo System where the tigerfish still do occur (Gaigher, 1970; 1973). Surveys over the last decade have shown that the most successful tigerfish population in South Africa occurs in the Olifants River. In spite of the totally different character of the Olifants River, as compared to the Zambezi System this species is extremely successful in the Olifants River. This leads to the key question which motivated this study: How does the ecology of the tigerfish compare between these systems. Furthermore was the artificial reproduction studied in order to learn more about the unknown reproduction strategy of this species.
4 CHAPTER TWO
LITERATURE REVIEW
2.1 Representatives of the Characidae in southern Africa
The species Hydrocynus vittatus is a member of the Characidae (Order Characiformes), and is one of the largest families of freshwater fishes found in both Africa and the Neotropics (Brewster, 1986).
Throughout the world, freshwater fish faunas are dominated by Ostariophysan fishes, and the riverine faunas of Africa is no exception to this. Africa is the only major continental area where the three groups of Ostariophysan fishes namely: the Cypriniformes, Characiformes and Siluriformes all occur, and it seems clear that the Ostariophysi must have originated in Gondwanaland prior to the separation of Africa and South America (Lowe-McConnel, 1988). The widespread distribution of certain families and sub-familial taxa is one of the outstanding features of the African fish fauna. The Cyprinidae, Characidae, Bagridae, Schilbeidae, Amphiliidae, Clariidae, Mochokidae, Cyprinodontidae, Cichlidae and Gobiidae are found in most east and west Afro-tropical river systems (Greenwood, 1983).
The characins form a large and diverse group of fishes which are widespread in tropical waters of Africa (Skelton, 1988). The Characidae family consists of 18 genera and some W9 species (Lowe-McConnel, 1988). The Characidae family has big eyes, bony cheeks, sharp teeth, big silvery scales, and a short dorsal fin. They feed normally on a wide variety of food types and occurs in shoals (Skelton, 1993). There are approximately five genera which are represented by six species in southern Africa. Hydrocynus vittatus is the only representative of the genus Hydrocynus that occurs in this region. The genus Brycinus is represented by two species i.e. B. imberi and B. lateralis. Brycinus imberi (Peters, 1852) keeps to a wide variety of habitats including big rivers and marshes and pans of flood plains. They occur in the east coast rivers of the Phongolo northwards to the Rufigi in Tanzania. They are absent in the Upper Zambezi system
5 (Skelton, 1993). Brycinus lateralis (Boulenger, 1900) shoals in clear slow flowing or standing water with an abundance of vegetation. They occur in the Zambezi System, Okavango-, Kunene-, Busi River and in the St Lucia catchment in Natal (Skelton, 1993). Micralestes acutidens (Peters, 1852) shoals in clear lentic or lotic open waters. These small fishes occurs in the Kunene, Okavango, Zambezi and east coast Rivers southwards to the Phongolo (Skelton, 1993). The remaining two genera are absent in South Africa. Rhabdalestes maunensis (Fowler, 1935) keeps to flood plains with shallow waters and lots of vegetation. They are found in the Kunene, Okavango, Upper Zambezi and Kafue River Systems (Skelton, 1993). Hemigrammopetersius barnardi (Herre, 1936) are found amongst vegetation of rivers and lakes. They are confined to the Lower Zambezi, Pungwe and Busi Systems (Skelton, 1993).
The wide distribution of the Characidae family in Africa ensures that many different systems are inhabited. This includes the huge man made lakes in Africa such as Lake Kariba and Cabora Bassa. River systems had been changed to lentic habitats which have an enormous impact on the ichthyuofauna and their composition in relation to one another. The ecology of these species must subsequently differ largely from one genera to the other. Not only does differences occur between genera, but also between different species. This is evident in Brycinus imberi and Brycinus lateralis. Some riverine fishes disappear out of the new lake faunas in an regular sequence, generally because they cannot adapt to the changed breeding conditions. Lakes challenge species to exploit new sources of food and breeding conditions. Once the lake is stabilized, species new to the area appear to become abundant (Lowe-McConnel, 1988). The phenomenon of the sudden replacement of an original species by a new species in Lake Kariba is most probably explained by the breeding biology of the two characin species B. lateralis and B. imberi. Brycinus lateralis occurs mainly in the Upper Zambezi system and Kafue River, but has never been found in the Middle Zambezi. Brycinus imberi inhabited the Middle and Lower Zambezi. These two species has never been found together. After the creation of the lake, B. lateralis was found in Lake Kariba (Middle Zambezi) in 1963 for the first time. A few years later B. lateralis proved to be exclusive in Lake Kariba and has replaced B. imberi (Balon 1971). The creation of Lake Kariba proved to be an ideal habitat for B. lateralis which in turn affected the whole ecology of B. imberi negatively.
6 Brycinus imberi spawn during the raising of the water level (rainy season) on freshly inundated grassy shores. Contrary to B. imberi, B. lateralis spawn on submerged flora, roots of Salvinia etc. irrespective of the variation of the water level. B. lateralis therefore suddenly found not only free space but exceptionally good spawning conditions. Individual specimens driven over the edge of the Victoria Falls therefore created in a short time a prosperous population (Salon, 1971). Not only did the creation of Lake Kariba on itself changed the ecology of the above mentioned species, but also that of the tigerfish. Shortly after the introduction of the freshwater sardine Limnothrissa miodon, cichlids and B. lateralis were the major food item of the tigerfish. This situation soon changed where L. miodon became the major food item of the tigerfish. There has thus been a major shift from B. lateralis and the cichlids to L.miodon in the diet of H. vittatus (Kenmuir 1973).
It is clear that the ecology of any given species is not a set and final matter, but that changing environments necessitate changes in their behaviour to survive and sometimes to thrive.
2.2 Conservation status of Hydrocynus vittatus
Tigerfish H. villains in Southern Africa is not listed in the Red Data Book of Fishes (Skelton, 1987), and is thus currently of no concern. It is however important to note that the status may differ from one river system to another due to various factors including water quantity and quality, fishing pressure and the loss off habitat.
Apart from the limited distribution of H. vittatus in South Africa the tigerfish is rare in South Africa and not readily found in the rivers of the Kruger National Park. Due to aggregation at natural migration obstructions and isolation in clear deep pools, tigerfish are more frequently found in the Olifants and Letaba Rivers. The tigerfish is thus relatively protected in South Africa as they are isolated in the Kruger National Park, but extremely vulnerable to anthropogenic activities which can influence the water quantity and quality in their catchment. Gaigher (1973) and Steyn (1987) mentioned that the reproductive potential of H. vittatus is negatively influenced
7 due to migration problems and limited spawning localities due to the erection of weirs and the shortage of water. Similar problems were experienced by many other African freshwater fishes (Bell-Cross, 1965a).
The fact that the Letaba River had been extensively used for irrigation purposes, contributed to its present poor status, providing little or no suitable habitat for the tigerfish to survive, both in the higher reaches of the river and within the boundaries of the Kruger National Park.
The water quality of the Olifants River is also of concern. Although the water of the Olifants River entering the Park is highly mineralized, bioaccumulation of selected metals seems to be low. Concentrations of selected metals in certain tissues and organs of the tigerfish were studied by Du Preez and Steyn (1992). The selected metals were detected in all the tissue examined, but in variable concentrations indicating differential bioconcentration of the metals. The bioconcentration factors were generally low suggesting low bioavailability of the metals.
Extremely high silt deposits, especially during floods, in the Olifants River below the Phalaborwa Barrage certainly has a negative effect on the numbers of tigerfish close to the western boundary of the KNP, as they prefer clear clean water (author, personal observation). Silt are deposited along the river as it moves downstream, resulting in somewhat cleaner water further down. Van Loggerenberg (1981) also mentioned that the numbers of tigerfish higher up in the Olifants River was so few that it could not be netted with seine nets. Surveys conducted with seine nets and gill nets during 1990 until 1992 also produced very few tigerfish. The previous drought of 1991/92 certainly also must have had its negative effects on the river with regards to habitat loss as the water flow were drastically reduced. Large surviving pools also reduced significantly in size as the days got hotter and the water evaporated in the blazing sun.
There is an abundance of tigerfish in the Zambezi System, but is currently under heavy fishing pressure. As early as 1965-6 Bell-Cross reported that man is undoubtedly the heaviest predator on adult tigerfish, because 4 and 5 inch gill nets are commonly used by fisherman in Lake Kariba. However he did not consider the effects of angling significant. Langerman (1984) reported that the impact of the pelagic fishery on tigerfish stocks in Lake Kariba is potentially the most damaging of all three fisheries and that the gillnet fishery is likely to have the largest biological impact on tigerfish. The use of smaller gillnets to achieve greater catches in the short term, certainly contributes to negative long term effects (especially recruitment) of the population (Langerman, 1984). Kenmuir (1972) also mentioned that if gill netting were to have continued for a few years at Lakeside where they have worked, it might have eradicated the tigerfish population. The most serious problem relating to tigerfish in Lake Kariba has been and remains the illegal netting in river systems. The problem is not only that of cropping the adult stock but also of destroying a large portion of the egg pool (Kenmuir, 1972). Although fears that the annual I.T.F.T. (International Tigerfish Fishing Tournament) is already affecting the tigerfish resource, since catches may consist of up to 1000 large females all full of eggs, Langerman (1984) concluded that the recreational fishery is unlikely to have any adverse biological impact on the tigerfish population. Tigerfishing is a very popular ecotourism attraction along the Zambezi River and Lake Kariba and tigerfish is heavily exploited by anglers visiting the numerous Fishing Lodges and then kept as trophies. Angling safaris on the Zambezi is common and the larger trophy fishes are removed from the system. These larger fishes are most probably the females as they grow to a much larger size than the males. This simply means that the prime breeding material for the genus is removed which will undoubtedly have negative results for the species in the long term. This especially give reason for concern with respect to the future of the Upper Zambezi population as the tigerfish in the Upper Zambezi represent an unique colour variation for the species H. vittatus (Chapter 6).
The tigerfish in the Okavango River is currently under ecological pressure, most likely due to subsistence fishery as mentioned by Van Zyl (1992), although he states that the survival of H. vittatus is at this moment not endangered. Hay (1995) reported that only a few large individual specimens of H. vittatus were collected during a study in the Okavango River in an area where no fishing is allowed and concluded that large individuals were basically non-existent in the rest of the river. Nikolskii (1969) noted that a decline in a predator population may be directly correlated to fish exploitation.
9 Electrophoretic analysis of tigerfish liver, white muscle, heart and testis samples revealed genetic variation at 20% (Upper Zambezi River, Namibia) and 63% (Olifants River, South Africa) of the protein loci studied (Kotze et al, 1996). Average heterozygosity values ranged from 1.9% in the Upper Zambezi River to 6.4% in the Olifants River, with a genetic distance value of 0.005 between these populations. These results clearly shows the relative genetic vulnerability of the tigerfish in the Upper Zambezi River in comparison to the tigerfish in the Olifants River, should there be overexploitation, any sudden environmental changes, virus infections or diseases. It is however interesting to note that the genetic variation of the tigerfish in the ()Hants River is much higher than the Upper Zambezi River although the Olifants River between the Phalaborwa Barrage and the Massingire Dam in Mozambique is a considerable smaller biotope as compared to the Upper Zambezi System.
The low genetic variation in the Upper Zambezi River could be due to genetic isolation by the Victoria Falls which acts as a migration obstruction prohibiting exchange of genetic material. The population in the Olifants River has a far greater genetic variation altough the Massingire Dam also acts as a migrational obstruction. This obstruction however is very recent (28 years) as compared to the 500 000 years of the Upper Zambezi River.
The limited genetic variation clearly shows that inbreeding is already taking place in the Upper Zambezi River. Of great concern is the disappearance of genetic material from trophy fishes in a population where inbreeding is already happening. Conservation strategies is thus important to prevent the loss of genetic variation and subsequently a unique species.
2.3 Migration
The tigerfish is potamodromous which means that they do migrate to areas in freshwater to spawn otherwise than normally during the year (Bowmaker, 1973). The "triggering" mechanism for upstream migration in fish, as generally believed, can not be attributed to any single factor but results from the single combination of many physical and chemical factors associated with flooding (Bowmaker, 1973). A single substance "Petrichor" may provide the final stimulus for
10 spawning Lake (1967). "Petrichor" is an oil consisting of a complex mixture of organic compounds containing acidic, basic and neutral fractions which is released from the dry ground when it is dampened.
The two factors which is the main cause for the annual spawning migrations are believed to be adequacy of cover and sufficiency of food. The most important one seemed to be safe refuge from adult predation (Jackson, 1961b). Feeding activity of tigerfish however decreased during the spawning migrations in the Sanyati Gorge while it was not the case in Lake Kariba (Kenmuir 1973). Tigerfish were found not to feed at all during the annual spawning migrations, after which feeding activities increased once spawning had taken place (Matthes, 1968).
Spawning migrations of the tigerfish do occur each year during the height of the rainy season when the river is in flood (Soulsby, 1960; Jackson, 1961a; 1961b; Matthes, 1968; Kenmuir, 1973). Migrations does not always start during the height of the rainy season as a large congregation of Hydrocynus were witnessed at Parrot Bent some 1.5 km from the entry point of the Mwenda River, Lake Kariba, prior to the first rains in October and November (Bowmaker, 1973). It could have been that the tigerfish anticipated the floods by the change in water temperature. Jackson (1961a) however stated that temperature which normally plays a role in the onset of the reproductive activity, may almost be discounted as the variation between the summer and winter water temperature in the Middle Zambezi is almost the same or sometimes equal.
It was observed that Hydrocyon villains' migrate upstream in the Luapula River out of Lake Mweru during the height of the rainy season and that migrational runs of tigerfish occurs in waves as if shoals had been formed and were moving together (Soulsby, 1960). Large migrational runs were observed to take place on overcast days when the river was flowing sufficiently (Bowmaker, 1973) and shoals of migrating tigerfish had been witnessed by Kenmuir (1973) in the Sanyati Gorge. During these migrational runs, tigerfish up to 4 kg or more were leaping up to heights of 1.8 metres or more. Contrary to the above mentioned upstream migrations of the tigerfish Gaigher (1967, 1970) has found that the tigerfish migrate downstream to spawn in Lake Chualo, Mozambique in the Incomati River system. Gaigher (1967; 1970) witnessed that an upstream
.1 Previously synonymous with Hydrocynus vittatus
11 migration of the tigerfish take place after spawning and concluded that migration of the tigerfish is thought to be a feeding migration and not a spawning migration as were found to be the situation in other systems as mentioned previously.
No migration of H. villains from the Middle Zambezi River to the Upper Zambezi River can take place because of the obstruction caused by the Victoria Falls. This means that 11. villains in the Upper Zambezi River is an isolated population due a natural obstruction.
Natural migration of tigerfish is restricted in South Africa. This has been the reason for the disappearance of the tigerfish in certain stretches of the Lowveld rivers. Obstruction of migration routes have been caused by a shortage of water and the erection of dams and weirs, restricting the movement of tigerfish and other fish species (Gaigher, 1970, 1973; Kruger, 1972; Clay, 1976; Pienaar, 1978). Tigerfish spent only a part of the year in Swaziland and that the building of dams and weirs have greatly changed the distribution of a lot of fish species in those catchments (Clay, 1976). It had been found that tigerfish do make use of fish ladders to migrate upstream in the Shingwedzi River in the Kruger National Park (Olivier, 1994). Three tigerfish were collected in the Kanniedood fish ladder during surveys when the dam wall was overflowing. An interesting statement was made that there is indeed no fundamental distinction between physical barriers and ecological factors which limits dispersion of fish (Crass, 1962). Such ecological factors include sudden temperature variations of the water. Tigerfish is known to inhabit the warmer waters of Africa, and a sudden dramatic drop in temperature could kill them. Dead tigerfish had subsequently been observed in the Crocodile River after a hailstorm. It seemed that the sudden decline of the water temperature caused the death of the tigerfish. The Zambezi River is much larger in volume than the rivers of the Limpopo System. It is also situated more towards the equator which means that the water temperature is slightly warmer and more stable, with fewer dramatic temperature variations taking place.
12 2.4 Ecological position
Hydrocynus vittatus predominantly frequents the 2m and 5m depth zonations, with fewer being caught at a depth of 10m in Lake Kariba (Mitchell, 1978). Hydrocynus forskahlii were found to be restricted to the surface waters in the deeper parts of Lake Kainji, and that no specimen of H. forskahlii2 was captured below a depth of 10m (Lewis, 1974). Winemiller and Kelso-Winemiller (1994) reported that Hydrocynus forskahlii occupies almost exclusively the open waters of the main river channel in the Zambezi River. The fact that they are open water loving species was also observed by the author in the Olifants, Letaba, Sabie and Crocodile Rivers within the Kruger National Park. Tigerfish favour upper reaches of rapids for they would almost swim into the rapid before darting back into the more quiet waters.
2.5 Feeding and growth
Hydrocynus vittatus is an open water loving species and preference would naturally be given to prey species which is most abundant in the river or lake system occupying the same pelagic habitat (Jackson, 1961b; Kenmuir, 1973; Lewis, 1974; Lauzanne, 1975; Winemiller and Kelso- Winemiller, 1994). The availability of cover for prey species plays an important role for them not to be preyed upon. The advantage of availability of cover is two-fold; there is an abundance of insects, crustaceans and other food, and there is complete protection from large predators, especially H vittatus Jackson (1961b). As the level of Lake Kariba dropped with a subsequent decrease in cover the degree of fullness of tigerfish stomachs increased (Kenmuir, 1973). The importance of cover for prey species can clearly be seen in the findings of Kenmuir (1973) who found tigerfish in very poor condition in a pool in the Gache River. Only 14 out of a total of 139 tigerfish had stomach contents of some sort. A few boulders and a shallow sloping beach provided sufficient cover for the prey species despite the lack of any vegetal cover.
. 2 Previously synonymous with Hydrocynus vittatus
13 Bell-Cross (1965;1966) describe Hydrocynus as: a - partly insectivorous/partly piscivorous for young of the year while they do take adult Barbus and other "small" fish; b - piscivorous on fish which are less than 10 cm when adult, and c - piscivorous on fish which grows greater . than 10 cm when adult. Predation would be on adults of small species and young of larger species (Lauzanne, 1975), which is related to the size of the predator and the season of the year. The adult of small species are mainly being preyed upon by tigerfish with a length between 100 and 200 mm throughout the year while the young of large species are mainly being preyed upon during the flood season. In Lake Kariba the impact of predation by tigerfish is mainly restricted to the small and young fish of the families Characidae and Cichlidae (Matthes, 1968). The first year class is restricted to very shallow water in the main river and is the only year class subject to predation by all other classes ofHyd,rocynus (Bell-Cross, 1965;1966). Tigerfish, and other fish species has a critical period to which they are more susceptible to predation once they have exceeded that critical length.
When tigerfish reaches a length of approximately 50 mm, they prefer fish in their diet as was found in Lake Kariba and Lake Chad (Jackson, 1961b; Holden, 1970; Lauzanne, 1975). Tigerfish from the Incomati River and larger than 55 mm (FL) preferred fish in their diet (Gaigher, 1970). Insects, predominates as food item during seasons with low water levels while the dominant food item during the high water levels were found to be fish with insects virtually absent. Prey fishes within the suitable size-range for the tigerfish are not present in sufficient quantity during seasons of low flow prior to the breeding season (Jackson, 1961a). Insects as food item played a major role (86.8%, 63.6% and 60%) in the diet of tigerfish between 180-300 nun (FL) in Lake Kariba (Kenmuir, 1973). In Lake Albert, a small prawn, Cardinia nilotica, was present as a food item in 96% of tigerfish specimens caught at offshore stations (Holden, 1970).
Juvenile tigerfish has three major components in their diet namely zooplankton, insects and fish. Zooplankton were preyed upon after five days when the larvae were 5 mm in length. The change- over in diet from zooplankton to insects is approximately at 35 mm (FL) (Kenmuir, 1975). In the Chari River tigerfish up to 30 mm (SL), were almost exclusively zooplanktophytes, and from 30-45 mm (SL) they followed a mixed diet in which insects dominate while tigerfish larger than 50 mm (SL) were almost exclusively ichthyophagous (Lauzanne, 1975). Kenmuir (1973)
14 mentioned that tigerfish appears to become piscivorous at a length of 28 mm (FL). This is in line with the findings of Gaigher (1970) that tigerfish less than 25 mm (FL) fed almost exclusively on Entomostraca.
The change-over to a predominantly fish diet, occurs at a length of more or less 80 mm (TL) while the smallest specimen recorded feeding on fish was 52 mm (TL) (Jackson, 1961a). The change-over in diet to an almost ichthyophagous one occurs when the young tigerfish are between 40 and 50 mm (TL) with no intermediate stage of insect eating (Matthes, 1968). This was supported by Kenmuir (1975) who later found that fish-feeding starts at 40 mm (FL). The diet of tigerfish larger than 100 mm (SL) and longer consisted mainly of insects, shrimps and fish during the fall in the south-eastern Archipelago of Lake Chad. The importance of insects were evident in the 100-200 mm classes after which it became insignificant. Shrimps and fish were the main diet of fish with lengths between 200-300 mm. while fish only became the main diet for Hydrocyon3 larger than 300 mm (Lauzanne, 1975).
The diet of the tigerfish in the tributaries and the south-eastern Archipelago is totally different to that of the fish in the open waters of Lake Chad. Hydrocyon are exclusively ichthyophagous throughout the year in the open waters of the lake. Insects and shrimps that constituted an important part of the diet in the Archipelago were absent from the stomach contents, presumably because of their extreme rarity in this biotope. The same diet for the fish in the river system was observed as in the open waters (Lauzanne, 1975).
The impact of predation by tigerfish were restricted to fish generally not more than 40 percent of the predator's length, although this was not always the case (Jackson, 1961b; Matthes, 1968; Kenmuir, 1973). This ratio would not exceed 50% of the predator's length as was observed by Jackson (1961b) and Gaigher (1967). On one occasion a prey species of 64% of the length of a tigerfish was found by Matthes (1968). Winemiller and Kelso-Winemiller (1994) found the predator prey ratio between 7% and 62% with a mean of 27%. No significant relationship between the body lengths and prey could be derived for the tigerfish in the Upper Ogon River Nigeria, although maximum prey size increased with body length (Adebisi, 1981).
. 3 Hydrocyon Genus previously used synonymous to Hydrocynus
15 The effect of predation on prey fish by tigerfish is not to be taken lightly, and it seems probable that many of the spawning migrations of many African fishes are directly influenced by the impact of predation by this fish Jackson (1961b). The greatest predation pressure is caused by tigerfish with a length of 550 mm or 3 kg (Jackson, 1961b).
Although several species of Hydrocynus occurs in Africa, there seems to be a big difference in the normal feeding behaviour between some of the species. Hydrocynus brews has a specialind mode of feeding whereby they are biting pieces from, or mutilating large fish. In most instances the posterior or ventral part of the prey species had been bitten away, though occasionally the body of the prey, although badly lacerated, was still intact. Once maimed the prey fish usually floated to the surface to have pieces repeatedly bitten away by the predator (Lewis, 1974). The method described above is however not the only one by which H. brevis feeds. Typically of all the other Hydrocynus species they also catch smaller prey and swallow them head first, whole in, as described by many authors before.
A total of nine occasions had been witnessed during which H. vittatus attacked struggling fish on the end of a line (Bell-Cross, 1965;1966). Jackson (1961b) reported of a crocodile being shot in the head and emitting great quantities of blood, after which a number of large Hydrocyon were instantly on the spot and biting at the wound. This specific incident proves Lauzanne (1975) partially wrong when she stated that Hydrocyon forslcalii of all sizes and in all biotopes attack only living prey which they swallow whole. In this case, the tigerfish reacted purely to a blood stimulus in the water from the bullet wound in the head of the crocodile. Although they were biting at the wound they must have known that the blood was coming from something much bigger than them. Their reaction simply emphasises their whole nature as being that of ferocious predators roaming the waters in search for any given prey.
It was also witnessed by (Jackson, 1961b) that H. vittatus had been biting Tilapia macrochir indescriminantly as they passed by. On another occasion a 210 mm Labeo altivelis was removed from a tigerfish of 455 mm which was severed in two. The head and tail sections were lying side by side in the stomach of the tigerfish. The Labeo had a neat cut at right angles to the body. This however is not common feeding behaviour of H. vittatus. Tigerfish usually attacks from behind
16 and biting at their tails. The damaged fish are then finally eaten (Matches, 1968). It was observed by the author that prey species were bitten in the middle of the body before turned and swallowed, head first.
The effect of the area of free movement of fish in a body of water does play a role in their growth rate (Chen and Prowse, 1964). In other words, the larger the volume of water, the larger the fish will become in that body of water, in comparison to fish in a small volume of water. Growth of tigerfish is rapid when food supply is good and shelter from predators is available. It is also necessary to grow fast to exceed a critical length in order to avoid predation (Jackson, 1961a). It was estimated that the growth of tigerfish was between 140-180 nun in the first year and up to 250 mm in the second year in riverine conditions (Jackson, 1961a). A growth rate of 160-200 mm per year for the first year and 310-320 mm for the second year in Lake Kariba (Badenhuizen, 1967). A similar growth rate with the first year between 170-210 mm, the second year at 260-320 mm and the third year at 380 mm for the tigerfish in Lake Kariba was found by Kenmuir (1973). A much slower growth rate was documented for tigerfish in the Incomati System (Gaigher, 1967). A growth rate of 90 mm in the first year, about 170 mm for the second year and 250 mm for the third year was found by the latter author.
The growth rate found by Salon (1971) is very similar to that of all the above mentioned authors with the exception of Gaigher (1967). The growth rate was found to be between 140-180 mm for the first year and 250 mm for the second year. The second year class of these fish had the most active gonads, of which 93% were males. These males had a faster growth rate than the females, especially in the older age groups (Salon, 1971). Winemiller and Kelso-Winemiller (1994) found that the growth rate of the tigerfish in the Zambezi River floodplain was 170 mm and 230 mm for the first and second year classes respectively. Drought periods play an important role in the growth rate of tigerfish (Dansoko el al, 1976). It was pointed out that the young of the year suffered most from the drought. Balon (1971) concluded that tigerfish with a faster growth rate, has a better chance of survival. This observation was supported by Kenmuir (1973) who found that the larger sizes had a more rapid growth rate, probably due to food availability and space. The very high mortality rates (84%) of tigerfish in their first season of growth emphasises the faster growth rate and critical length of the fish to avoid predation (Baton, 1971).
17 There seems to be a difference in the relative maximum size of H. villa:us throughout Africa (Gaigher 1970, Kenmuir 1973, Van Zyl 1992). Tigerfish with a mass of 7.4 kg and 8.8 kg were sampled in the Upper Zambezi River System (Bell-Cross, 1965;1966) which is much smaller than the 17.7 kg tigerfish which was caught in Lake Kariba (Jubb, 1967). Tigerfish with a mass of 5.4 - 5.9 kg were caught during earlier years in the Sabie, Crocodile and Letaba rivers (Pienaar, 1978). Tigerfish oft 7 kg were also regularly caught during the earlier years in the Olifants River (South Africa) below the Olifants Wilderness Trails Camp in the Kruger National Park (Bryden, Kloppers, and Van Niekerk, pers comm). It would therefore appear that the tigerfish grow to larger sizes in the Zambezi River System than in the most river systems in South Africa, and the Zambezi River therefore seems to be the ideal habitat for H. vittalus.
2.6 Reproduction
2.6.1 Spawning Although the spawning period for all the fishes of the order Ostariophysi (including H. vitlatus) is of extremely short duration (Jackson, 1961a), the duration of breeding is closely linked with the duration of river flow and can be as long as five months (Kenmuir, 1973). No reference to time was given by Bowmaker (1973) when he stated that Hydrocynus breeds "more quickly" than Mormyrus, once the river had been entered. The actual spawning period of tigerfish coincides with the rainy season when the rivers are in flood (Jackson, 1961a;1961b; Gaigher, 1970; Bowmaker, 1973; Kenmuir, 1973; Lauzanne, 1975; Badenhuizen, 1976; Van Zyl et al, 1992 and Hay, 1995). It was however found that the tigerfish spawn prior to the floods in the Okavango Swamps (Merron and Bruton, 1989). Tigerfish experienced two major spawning cycles in the Okavango River (Van Zyl, 1992). A possible second breeding cycle was mentioned by Kenmuir (1973) in the Sanyati Gorge, although he could not confirm that due to a lack of data. On one occasion Jackson (1976) recorded a two month old juvenile tigerfish in early November in the Middle Zambezi River, indicating an abnormal spawning time. This is an indication that the gonads of the females had matured during abnormal seasons or conditions.
18 Surveys conducted by Van Loggerenberg (1982) indicated that the tigerfish have spawned at the confluence of the Olifants and Letaba Rivers in the Kruger National Park during January 1982. The Olifants and Letaba Rivers were in flood just after new year although the Kruger National Park itself has received little rains during that period. Water were released by the Phalaborwa Barrage in the Olifants River while the Fanie Botha Dam was overflowing in the Letaba River creating flood conditions in these two rivers. The manipulation of the water in the rivers have stimulated the tigerfish to spawn during that specific period. Surveys conducted revealed that only spent females were found (Van Loggerenberg, 1982). Steyn (1987) reported tigerfish spawning in a deep isolated pool in the Letaba River during February 1986. Females were holding back when the Letaba River was not flowing. When the river started flowing and the pool received some water it was enough to stimulate the fish to spawn. Spawning will however not always take place as the normal cycles is not always maintained. This could lead to the atrophy of eggs.
Bowmaker (1973) observed atrophy of Hydrocynus in the Mwenda area and speculates that this may result from too long a waiting period prior to the sufficient flooding of the river, but could also result from disease or some breakdown in the ethological breeding sequence. The presence of atrophied gonads in an area may be a reflection of the nature of river flow in that area, and atrophied gonads may occur more frequently in the vicinity of the smaller rivers (Kenmuir, 1973). Tigerfish with atrophied eggs as a result of no river flow was witnessed by Langerman (1984) in Lake Kariba.
The question as to the exact habitat the tigerfish prefers to spawn, has not yet been answered. Different habitats as possible ideal spawning, or definite spawning grounds have been given by various researchers. Hydrocynus brevis breeds in the flooded plains, while H. fors/obi is confined to smaller river beds (Daget, 1954). This view was also shared by Badenhuizen (1967) who found that tigerfish migrates to breeding grounds in shallows upstream in rivers. Flood plains are described as ideal spawning conditions for the tigerfish which is created with the influx of rising water in Lake Chualo in Mozambique (Gaigher, 1967; 1970). Tigerfish were found to reproduce both in the south-eastern Archipelago and river systems of Lake Chad (Blache, 1964), while the exact locality of spawning by tigerfish going up the Sanyati River during river flow is not known
19 (Kenmuir, 1973). With the rivers in flood, the need for vegetal cover always seems to be important. This would ensure a safe refuge for the larvae and abundance of food amongst the submerged vegetation while the larvae are transported downstream.
The behaviour of the tigerfish embryos and larvae are adapted specifically for downstream dispersion along the river course by the current (Bowmaker, 1969b; 1973; Kenmuir, 1973; Chapter 6). This rather strongly suggest that the tigerfish would select a spawning site upstream from the nursery area.
2.6.2 Sex ratio The expected sex ratio of a fish population is a 1:1 male:female ratio (Nikolskii, 1969). Certain factors however normally causes a deviation of this ratio. A shorter life span and higher mortality rate of males shifts the ratio towards the females (Kenmuir, 1973; 1984). Disproportions of the number of mature males and females, shortened breeding periods and unsynchronised maturation of the male and female tigerfish are associated with low recruitment in the waters of the Kruger National Park (Steyn, 1987).
The expected 1:1 sex ratio was found in the Okavango River except for the 1992 winter survey which favoured the females (Hay, 1995). This was a shift towards the 1:1 ratio since Van Zyl (1992) found the females to be favoured in the Okavango River. The deviation of the 1:1 sex ratio during different seasons was also evident in Lake Kariba (Kenmuir, 1973). The sex ratio favoured the females (1.35:1) during the non-breeding months, whereas the males were favoured (4:1) during the peak breeding season. The sex ratio favoured the males at 1.8:1. from Mwenda Bay in Lake Kariba (Langerman, 1984). This male:female ratio would however ensure fertilization of the eggs during the spawning periods.
20 2.6.3 Gamete production Sperm counts of fish is typically high which is needed to fertilize normally hundreds or thousands of eggs. Fish which spawn in running water tend to have high sperm counts with short motility duration when compared with still-water spawners (Ginsburg, 1972). The semen of tigerfish is white and has a water-like viscosity. The white colour of tigerfish semen reflects a high sperm count, while a low sperm count is reflected by a transparent appearance (Kruger, Smit, Van Vuren & Ferreira, 1984). The sperm count of the tigerfish (11.90 ± 4.81 x 10 6/nun3) is much higher than the sperm count (0.04 x 10 6/mm3 of O. mossambicus (Kruger et al, 1984; Steyn, 1993). The high sperm count of tigerfish may therefore either reflect a stream spawning activity or a high fecundity (Steyr!, 1993).
Fishes spawning in cold water tend to have a shorter duration of sperm motility than warm water species (Steyn, 1993). The duration of motility of the tigerfish spermatozoa (86 sec) is relatively short compared with that of Clarias gariepinus (120 sec) (Steyn and Van Vuren, 1987). Steyn (1993) concluded that the relatively short motility duration and high sperm count of the warm water tigerfish suggests that this species is a stream spawner.
The males in Lake Bengeullu, Zambia have a length of 390 nun (Griffith, 1975), and 300 mm (FL) in Lake Kariba when they become sexually mature (Kenmuir, 1972). According to Langerman (1984), forty percent of the males became sexually active at a length of 240 mm (FL). There seems to be a big difference in the lengths of males reaching sexual maturity as obtained by Kenmuir (1972) and Langerman (1984) respectively in Lake Kariba. The findings of Langerman (1984) is similar to that of Gaigher (1975) for the male tigerfish (200 mm FL) in the Incomati River system. Male tigerfish reaches sexual maturity at a minimum of 170 mm FL and 180 mm FL respectively in the Okavango River (Van Zyl, 1992; Hay, 1995). Although the average length when 50% of the males reaches sexual maturity was not given by Van Zyl (1992), one can assume that the length would be similar to the findings of Hay (1995), who found the length to be 260 nun (FL).
21 Females seems to reach sexual maturity later in life, compared to the males (Kenmuir, 1973; Langerman, 1984). Female tigerfish reaches lengths of up to 700 mm (FL) and more, while males hardly grows to lengths beyond 500 mm (FL) (Kenmuir, 1973). Females reaches sexual maturity at a length of 360 mm (FL) in the Incomati River system (Gaigher, 1970), but in Lake Kariba, 50 percent reaches sexual maturity at 260 mm (Langerman, 1984). The minimum length at which females reaches sexual maturity in the Okavango River is 420 mm (FL) (Van Zyl, 1992) and 275 mm (FL) (Hay, 1995) respectively. The length given by Van Zyl (1992) can certainly not be used as the minimum average length when females reaches sexual maturity in the Okavango River, as that was the only active female sampled. The findings of Hay (1995) for the minimum length at which females becomes sexually active in the Okavango River is more acceptable. The fact that female tigerfish grows larger than males suggests that egg counts would be high in order to ensure fertilization and the survival. Female tigerfish grows larger than the males (Jackson, 1961a; Kenmuir, 1973; Van Zyl, 1992).
Tigerfish were found to be very fecund, with large numbers of eggs (Pott, 1969; Kenmuir, 1973; Hay, 1995; Steyn, pers comm). Although egg counts varied in length groups, egg counts increased with length. A large specimen can produce up to a million eggs (Langerman, 1984). This would ensure the survival rate of the species as mortalities of the eggs and larvae are expected to be extremely high.
2.7 Artificial breeding
More than two decades ago the need to restock some of the Lowveld waters in South Africa was mentioned by Gaigher (1968), since the distribution of tigerfish was negatively influenced by the erection of dams and weirs. There are two possible methods of restocking the Lowveld waters with tigerfish. The first is to capture and translocate the tigerfish; and the second one is to propagate the species through induced reproduction. The first option would not be possible since there is no locality in the Transvaal were enough large and small tigerfish can be collected. The Incomati River System was known to have large numbers of tigerfish (Gaigher, 1968). This situation has changed dramatically, mainly due to weirs being built in Mozambique and the
22 absence of suitable breeding grounds in South African waters (Gaigher, 1968). The outflow of capital to the neighbouring countries to catch tigerfish can be counteracted if South Africa can create its own "Zambezi River" (Kruger, 1972). The need for propagation of tigerfish is thus very important, and can be truly justified.
There are, however, certain factors to take into account when dealing with tigerfish, as it is a very sensitive species. Water temperature fluctuations have to be kept to the minimum when transporting the fish. Water temperature where tigerfish are kept should be constant at all times and should not decrease below 15°C, although tigerfish do survive at that specific temperature. Kruger (1973) reported mortalities due to temperature shock after successfully transporting the fish to the Marble Hall fisheries research station. Mortalities of tigerfish occurred in the Piet Grobelaar dam in the Kruger National Park when the water temperature decreased to 14.5°C (Van der Merwe, pers comm). Dead tigerfish were also observed in a laboratory after the water temperature decreased to 14.5°C (Steyn, pers comm).
Probably the most important aspect of transporting tigerfish is to ensure high oxygen levels. The fish should be handled and transported with great care. An anaesthetic should also be applied to calm the fish when placed in a container and to prevent injuries against the sides of the container (Kruger, 1972).
The first breeding programme of H.vittatus was initiated in 1972. The first female was injected with a 2 mg carp (Cyprinus carpio) pituitary extract. The specimen died after 48 hours, but the gonads were very well developed with eggs which were loose from one another. Three females were injected a year later with pituitary extract but unfortunately died 3-5 days later. The gonads of these females did not show any signs of development and were green in colour. The constant loss of females when injected with carp pituitary could have been due to poisoning as the fish started losing scales and haemorrhage between the scales was visible.
23 Van Loggerenberg (1979) initiated a project to translocate tigerfish from the Kruger National Park to certain dams in the Lowveld. The main objective of the project was to determine where the tigerfish would spawn in the eight selected dams. The Klaserie Dam was first chosen because it is a relative small dam (123 ha) and situated close to the Olifants River from where the tigerfish would be collected. Tigerfish were collected close to the Gorge Picnic spot in the Olifants River and translocated to the Mala-Mala Dam of the Phalaborwa Mining Company. A minimum of mortalities were reported in the Mala-Mala dam after transportation (Van Loggerenberg, 1979). The tigerfish which were translocated to farm dams after being collected in the Komati River were also reported to be in good condition. Unfortunately, Van Loggerenberg (1980) did not report on the growth of the tigerfish in the Klaserie Dam but the growth of tigerfish in the dams close to Komatipoort were very satisfying (Van Loggerenberg, 1981).
A breeding experiment was conducted on tigerfish in the Transvaal Sugar Board (TSB) Dam at Ten Bosh (Van Loggerenberg, 1981). Five females were injected with a combination of (C. carpio) pituitary and Pregnyl for the first session. A second injection after nine hours of only (C. carpio) pituitary was then given to the females. One female died on the second day of treatment and two other females did not respond to the hormones. One other female did produce a portion of her eggs but after a dissection showed that the ovaries were only partially developed. The eggs of this female were fertilised in the same manner as it was done with the eggs of the Letaba River which is described in the next paragraph. The fifth female was in poor condition and also dissected, the eggs were removed, fertilised, and incubated. The eggs were inspected under a microscope on a hourly basis to see if any development had taken place. After one hour of fertilisation cleavage of one of the eggs was observed. No further development of any eggs were witnessed during the following inspections (Van Loggerenberg, 1981). According to Van Loggerenberg (1981) the fact that egg development was poor, could well have been because of a high concentration of DDE-insecticide as were found by Van Rensburg (1981) in the tissues of tigerfish and Clarias gariepinus at Komatipoort.
During their investigation of the tigerfish in the breeding season of 1979/80 in the Mala-Mala Dam at Phalaborwa no ripe running females were found while all the males were found to be ripe running (Van Loggerenberg, 1980). Due to the lack of ripe-running females in the previous year
24 it was thought that it would be better to obtain the breeding stock just below the Engelhardt Dam in the Letaba River. It would have been easier to keep the tigerfish there in 500 litre plastic tanks and to treat the fish with hormones on site. The plan was then to transport the fertilised eggs in plastic bags with water to Phalaborwa where they would be placed in a breeding apparatus. On 21 November 1980 two females were treated with a combination of Estrumate and Pregnyl in a ratio of 1:3. The dosage of hormones were calculated as for every one kilogram of biomass to
1000 International Units (I.U.) hormones in total for a single injection. Another female was in a poor physical state after transport and the eggs were removed. Most of the eggs were in a ripe condition. Only one male was treated with 150 I.U. Estrumate. The semen of a male was then mixed with the eggs of that female. The eggs which were removed from the female were rinsed with oxygen rich water to get rid of the unwanted semen and ovarian tissue, five minutes after fertilization. The eggs were put in plastic bags and oxygen was supplied to the eggs in such a way that a small turbulence was created. These eggs were kept for about thirty hours in the plastic bags. During this period the water was changed twice in the plastic bags with river water, and the dead egg material removed. One of the two females which were treated for a second time on 22
November with hormones were also dissected and the eggs removed and fertilised. The fertilised eggs were inspected on the 23 November and were suspect. Upon further inspection it was found that Paramecium Protozoa were feeding on empty eggs shells. A few larvae which were apparently dead were suspended in the water. The cause of the death of the eggs during that night could not be explained, although some speculations were made. The remaining female was treated again on 23 November, but did not produce any eggs when stripping was attempted (Van
Loggerenberg, 1981).
On 10 December 1981 two females were three times, with a nine hour interval, treated with hormones. Three more females were collected later and released in a pool in the river. A net was placed in the pool to prevent the fish from escaping into the rest of the pool. A hippopotamus walked through the net resulting in all the tigerfish to escape. No more females could be collected later during a follow up survey (Van Loggerenberg, 1982).
25 A few years later (1986) Steyn and Deacon had limited success with the breeding of tigerfish at the rangers post at Crocodile Bridge. The embryos developed up to 12 hours after which no further development took place. The same problems with development were also experienced during December 1991 at Charara next to Lake Kariba in Zimbabwe (Steyn, pers comm). These experiments were however extremely important as this had lead to the successful artificial spawning of the tigerfish which will be dealt with in Chapter Four.
26 CHAPTER THREE
ASPECTS OF THE POPULATION DYNAMICS AND DISTRIBUTION OF THE TIGERFISH HYDROCYNUS VITIATUS IN THE LOWER OLIFANTS AND LETABA RIVERS, KRUGER NATIONAL PARK.
3.1 Introduction
The study of population dynamics may be defined as the attempt to measure rates of birth, growth and death, and their inter responses in a precisely defined population. It may be thought of as including the quantitative aspects of the ecology of a population (Regier, 1966).
Tigerfish may attain a length of 140 mm to 180 mm in the first year and 250 nun in the second year in Lake Kariba (Balon, 1971). Tigerfish is a long lived species which may attain a length of 590 mm according to the Loo values calculated by Van Zyl (1992). The maximum size of Hydrocynus villains differ considerable from one system to another. Bell-Cross (1965;1966) reported tigerfish with a total length of 705 mm (7.4 kg) from the Upper Zambezi, while the Zambian Fish Records Association recorded a specimen of 8.8 kg from the Zambezi River. The maximum sizes reported from the Okavango and Lake Kariba were 620 mm and 800 mm (15.5 kg) respectively (Kenmuir, 1983; Van Zyl, 1992 & Hay, 1995). Tigerfish of 5.4 - 5.9 kg were regularly caught in the Sabie, Crocodile and Letaba Rivers until 1978 (Pienaar, 1978). Maximum sizes of 3.8 kg and 3.5 kg tigerfish has been recorded by (Van Loggerenberg, 1982; Steyn, 1987) respectively in the Crocodile, Letaba and Olifants Rivers. Consequently it appears as if there is a progressive decline in the maximum sizes of tigerfish in the KNP.
Tigerfish individuals within a group which has a faster growth rate, has a better chance of survival (Balon, 1971). The very high mortality rate (84%; Z= 0.851) of tigerfish in their first season of growth, emphasises the benefit of a faster growth rate and critical length to avoid predation (Balon, 1971; Van Zyl 1992). Females had a faster growth rate than the males, especially in the
27 older age groups, in Lake Kariba (Balon, 1971). These findings are also supported by the phi prime values calculated for males and females in the Okavango River (Van Zyl, 1995; Hay, 1995).
Due to the difference in growth rate between the sexes of the same age group, males and females separate naturally and arrange themselves in schools of similar length groups in order to avoid cannibalism within the same age group. This hypothesis is based on behaviourial observations within an aquarium supported by several years of field observations (Steyn, pers comm). A discrepancy between males and females often occurs in nature and a specific sex tend to aggregate at certain localities, possibly to a uniformity in size (Steyn, 1987). For example, the larger females within the same brood tend to stick together and predate on their smaller male counterparts which will shoal separately.
In the Incomati River System, Lake Kariba and the Okavango River, males reach sexual maturity at a minimum length between 170 and 200 mm (Gaigher, 1975; Langerman, 1984; Van Zyl, 1992; Hay, 1995). The average length when 50% of the males reaches sexual maturity in the Okavango River is 260 mm with the minimum length of the females slightly larger at 275 mm (Hay, 1995). Fifty percent of the females reaches sexual maturity at a length of 260 mm in Lake Kariba (Langerman, 1984), which is similar to the males in the Okavango River (Hay, 1995). A total of 84% of the females caught in Lake Kariba were larger than 400 mm, indicating that the older or larger tigerfish contribute to the breeding potential of the population (Kenmuir, 1973). Female tigerfish grows larger than males (Jackson, 1961a; Kenmuir, 1973; & Van Zyl, 1992), and also reaches sexual maturity later in life (Kenmuir, 1973; Langerman, 1984). Tigerfish has a high fecundity in comparison to other fish species (Pott, 1969; Kenmuir, 1973; Hay, 1995; Chapter 6). This greater fecundity is an evolutionary response to ensure the survival of the species as mortalities of the eggs, larvae and yearlings are very high which is demonstrated by the high mortality rate calculated (Balon, 1971; Van Zyl, 1992).
28 The purpose of this section of the study was to compare some of these aspects with other tigerfish populations, as the KNP tigerfish population is represents the edge of distribution of the species and it can therefore be expected that the values for population parameters will not be optimal. Furthermore it is evident that the distribution range of the tigerfish in the Northern Province and Mpumalanga has changed.
3.2 Materials and methods
3.2.1 Choice and description of sampling localities Tigerfish populations were studied over a period of thirteen months (October 1991 - September 1993) in the Olifants and Letaba Rivers in the Kruger National Park. Eight localities were selected along the Olifants and Letaba Rivers. These localities does not necessary represent different reaches (based on geology, geomorphology and rainfall) as identified by Venter (1991) in these rivers but was selected on the basis of obstacles which causes aggregation during migration and sometimes isolation at low flow and consequently serve as ideal sampling localities. Six surveys were done during the autumn and winter months (March; April; May; June; July; August) and seven during the spring and summer months (September; October; November; December; January; February) which will be referred to in this study as winter and summer. Four localities (localities 1-4) were selected along the Olifants River from Mamba weir on the western border of the KNP to the confluence of the Letaba River ± 5 km from the border with Mozambique and four localities (localities 5 - 8) were selected in the Letaba River between Engelhardt Dam and the confluence with the Olifants River (Fig 3.1).
Locality one is situated directly below Mamba weir within reach one according to the classification of Venter (1991). This reach has a single channel with mostly a flat river bed and shallow stream. Short rapids occur over firm or rounded rock, with deep pools only occurring occasionally. The river bed consists of sand and gravel, alternating with small rocky pools. Riparian vegetation is moderately dense with trees such as Fiats sycomorus, Trichilia emetica, Lonchocarpus capassa, Acacia robusta and Diospyros mespiliformes. Hanging reeds (Phragmytes spp.) are limited to isolated small patches.
29 localities in the Olifants an Locality two is downstream from the Balule low water bridge and above the potholes at the Olifants Wilderness Trail Base Camp. A natural obstruction to fish during periods of low flow occurs at the potholes. This locality falls within reach two. The channel of reach two is mostly irregular and branches off to form small (5 - 10 m) and sometimes deep channels between the islands. The river bed consists of irregular deposits of silt or sand on rock or islands. Dense reeds occur on islands and sometimes also trees such as F. sycomorus and Breonadia saligna. The riparian vegetation is scattered to moderately dense with trees such as F. sycomorus, T. emetica, L. capassa, A. robusta, D. mespiliformes and Colophospermum mopane. Hanging reeds (Phragmytes spp.) are very dense in some localities (Venter, 1991).
The third locality is below the potholes in the river stretch below the Wilderness Trail Camp. Reach three has a V - to U - shaped single channel with deep pools and short rapids. At the Olifants camp and Trails camp, the channel is deeply cut into the rock to form a series of low waterfalls and deep little ravines. The river bed consists of rock with relatively thick depositions of red silt in deep pools and round loose cobble-stones in the rapids. Virtually no vegetation occurs on the river bed and reeds are limited to rocky places or alluvial islets. The river banks are open with trees such as F. sycomorus, T. emetica, L capassa, A. robusta, and A. xanthophloea. Hanging reeds (Phragmytes spp.) occurs alternatively (Venter, 1991).
Locality four is at the Olifants/Letaba confluence downstream into the Olifants Gorge. This locality lies within reach four. This valley consists of a V- to U shaped, narrow and deep gorge in the Lebombo Mountains. The banks are steep (often vertical) and high (40 - 60 m). It has a single U- shaped channel with many deep pools and little rapids. There are no vegetation on the river bank and bed (Venter, 1991).
Locality five is directly downstream from the Engelhardt Dam with locality six five kilometres further down. The river bank is slightly steep and low with the channel as an irregular river bed that is highly divided. Dense reed beds occur with a larger tree composition of F. sycomorus, T. emetica, L capassa, C. mopane and Combretum imberbe. Dense reed beds over the whole width of the river bed and trees like F. sycomorus and Breonadia saligna are sometimes found in the middle of the river bed (Venter, 1991).
30 Figure 3.1 Locality one within reach one in the Olifants River.
Figure 3.2 Locality two within reach two in the Olifants Figure 3.3 Locality three within reach three in the Olifants River.
Figure 3.4 Locality four within reach four in the Olifants River. Figure 3.5 Locality five within reach four in the Letaba River.
Figure 3.6 Locality six within reach four in the Letaba River. Figure 3.7 Locality seven within reach five in the Letaba River.
Figure 3.8 Locality eight within reach five in the Letaba River.
c Localities seven and eight are situated within reach five, approximately two km upstream from the confluence with the Olifants River. It has a channel width of 30 - 50 metres with a gradient of 10.0 m/km. The riverbank and bed is solid rock with fallen rocky boulders. Little vegetation occurs along the river (Venter, 1991).
3.2.2 Sampling procedure and data collection Tigerfish were caught with rod and reel in the Olifants and Letaba Rivers (Fig 3.1). Brass lures of different sizes were used as artificial bait to prevent sample bias. The sampling method was however selective for specimens larger than 80 mm as this was the smallest size which could be obtained with lures.
Electro-narcosis as method was not used for most of the data collection took place in deep pools with great numbers of crocodiles. Since sampling took place in a National Park, and only tigerfish were sampled for, the use of rotenone was not deemed necessary. After specimen collection the standard length (SL) was measured (mm), and the mass (g) was determined with a Kern electronic balance (model 440/53). Scales were collected dorsolateral, between the head and'dorsal fin, for age determination. They were then placed in a small envelope and marked with all the relevant information of the specimen, date and locality of collection. The sex was determined by dissection.
3.2.3 Data analysis In order to analyze the data with the Length Frequency Stock Assessment (LFSA) software, the data sampled over a period of thirteen months was categorised under two sections namely September - February representing summer and March - August representing winter. Length data was sorted into 10 mm length groups to obtain length frequencies for the two seasons in the Letaba and Olifants Rivers respectively (Fig 3.2). These length frequencies were separated into cohorts to determine the age groups and population structure (Table 3.1).
Age determination was done after the scales were first soaked in water for 24 hours and then washed to remove excessive mucus and dead tissue. Age determination based on scale readings from tigerfish in the Olifants River were done while the scales were still wet and correlated with length groups (Table 3.2). The length frequencies of H. vittatus and scale readings were used to
31 determine the Von Bertalanffy growth parameters (Loo & k) with Bhattacharya's method, link of means and the programme VONBER in the Length Frequency Stock Assessment (LFSA) software.
The Von Bertalanffy growth curve was determined with the following formula: 1-t) = Lop (1 - exp 441-w)) where k = curvature constant, Dr = L - infinity t = time (years), to = t - zero
The overall growth performance as reflected by Munro's phi prime (4) was determined by using Loo and k. 4) (phi prime) = In k + 2 * In Loo
Total mortality (Z) was determined with the catch curve and Pauly's empirical formula (Sparre et al, 1989): Z = M + F where M = natural mortality coefficient & F = fishing mortality coefficient
The percentage survivors from the annual recruitment after one year was determined with the Exponential decay model (Sparre et al, 1989): -(Z*(t-Tr)) N(t) = N(Tr)exp where N = number of survivors, t = time (years), Tr = recruitment age, Z = total mortality coefficient
32 3.3 Results and discussion
3.3.1 Distribution and abundance The abundance and distribution of tigerfish in the Olifants and Letaba Rivers varies considerably in relation to the localities and the different seasons. Tigerfish are more abundant in the Olifants River in comparison with the Letaba River in spite of the intensive survey conducted during October 1991 in the Letaba River.
Although tigerfish are found along the entire Olifants River in the KNP they are very unevenly distributed in numbers but rare towards the western border near the Phalaborwa Barrage. Van Loggerenberg (1981) also mentioned that the numbers of tigerfish higher up in the Olifants River in the KNP was so few that it could not be netted with seine nets. It is however not possible to determine to what extent the abundance of the tigerfish below the Mamba weir has changed, for it was never recorded in detail. During the summer months tigerfish was more abundant (n=1249) than the winter months (n=209) in both the rivers. A grand total of 1100 tigerfish were collected in the Olifants River of which 932 and 168 tigerfish were collected during the summer and winter months respectively. Tigerfish is more abundant towards the eastern border in the Letaba River below the Engelhardt Dam. A total of 358 tigerfish with 317 in the summer and 41 in the winter months were collected.
Tigerfish seems to aggregate in large numbers below the Olifants Wilderness Trail Camp during certain periods of the year. It is evident that aggregation of tigerfish takes place before the onset of the summer at the natural migration obstruction below the Wilderness Trail Camp. The numbers of tigerfish decline after the floods due to distribution, indicating that the obstructions had been overcome. As the winter months approach, the numbers of tigerfish decline drastically, since only 168 tigerfish had been collected during the winter. This implies that tigerfish migrates downstream in order to tolerate winter temperatures. This assumption is supported by the observation of Van der Merwe (pers comm) whom observed tigerfish dying in the KNP when water temperature decreased to 14.5 °C at night during the winter of 1994. Tigerfish in the Letaba River were more readily caught during the summer months than the winter months in spite of the fact that the pools were also isolated during the summer and not only during the winter months.
33 A decline during the winter months could also be due to a reduced metabolism, urge to feed and activity associated with the lower winter temperatures. During this study it was found that the best locality for collecting tigerfish would be below the Olifants Wilderness Trail Camp during the months October until February before the water level rises to overcome migration obstacles.
Although the tigerfish is relatively protected in South Africa as this species is largely isolated in the Kruger National Park, it is vulnerable to anthropogenic activities which can influence the water quantity and quality in this seemingly haven. The reproductive potential of H. vittatus is negatively influenced due to migration problems and limited spawning localities due to the erection of weirs and the shortage of water (Gaigher, 1973; Steyn, 1987) and that the distribution of tigerfish in South Africa has diminished due to anthropogenic activities (Chapter 6).
3.3.2 Age and growth Although the sampling method used during this study was selective, a good representation of the population larger than 8.5 cm (SL) was obtained. A larger variety of length groups were represented in the length frequencies of the Letaba population and varied between 85 and 405 mm (Fig 3.2). Length frequencies displayed a maximum of three cohorts in the Olifants River with a minimum length group of 85 mm and a maximum length group of 375 mm. Although the first two cohorts for the winter season in the Olifants River are relatively close to each other, and can probably be seen as one, the separation index (Si.) of 2.078 is an indication that the two cohorts can statistically be separated (Table 3.1). Only two cohorts could be identified for the Letaba population sampled during summer. Inadequate data was obtained to separate more than one cohort from the winter data of the Letaba River. In both the rivers fish larger than 295 mm (SL) were scarce however a good representation of fish smaller than 295 mm (SL) was evident during the sampling periods.
The average lengths of cohorts separated in Table 3.1 demonstrate that the tigerfish in the Letaba
River are larger than the same age groups in the Olifants River. Tigerfish in the Letaba River were larger than those in the Olifants River during surveys in 1986 (Steyn, pers comm). This might be due to a smaller population size tigerfish in the Letaba as compared to the Olifants River.
Competition for food and living space is thus not as fierce as in the Olifants river. Furthermore
34
Olifants - Winter 0 0 i tisti JolaciumN R I 0 — I oLL —E —E
Letaba - Summer Letaba - Winter ID 0 0 S ID 11 qsy JolaqtatiN tisg PJaquint4 a n m1 I 71.p. N R ; 0 T -in 1 .5 a • O o-1 CU t.0 er; 0) CU Ctl col 0.4 Et 2 0 C N C a) C tin 0 -4 Cr 0 >-. 0 N OD .) far less crocodiles are also found in the Letaba River, which subject them to less predation by these efficient predators. The absence of large tigerfish from these rivers as documented by Pienaar (1978) is possibly a result of the massive Massingire Dam inside Mozambique. It could well be that the larger tigerfish prefer to stay within the larger volume of water in the dam. Consequently younger fish are then forced out of the dam to escape predation pressure and to expand their living space. This could also serve to explain the large numbers of the same age group of tigerfish that aggregate below obstacles in these rivers.
Table 3.1 Average length (mm) of the different cohorts obtained by the Bhattacharya method for H. villains in the Olifants and Letaba Rivers during two different seasons.
Season Cohort No. Average Standard deviation Cohort size Separation Index
length (nun) (s.d.) 00 (SS)
Olifants River
Summer 1 769 16 761 R96 44 --- 2 328 9.000 30.400 4.619 3 366 10.313 3.160 3.933
Winter 1 236 13.043 20.000 --- 2 270 18.835 137.93 2.078 3 327 7.238 9.070 4.379 Letaba River
Summer 1 270 17 56 275 73 --- 2 330 13.03 31.92 3.907
Winter 1 262 22.20 29.44 ---
The calculated maximum length (Lco = 52.40 cm) according to the Wetherall method, was much larger than the largest specimen sampled in the Olifants (375 mm) and Letaba (405 mm) Rivers. The L4 values calculated for the Olifants and Letaba Rivers is smaller than the L4 value (Lco = 56 06) calculated for the Okavango River (Van Zyl, 1992). The maximum sizes sampled from the
35 Olifants and Letaba Rivers respectively were also substantially smaller than the maximum lengths recorded in the Okavango River (620 mm) and Lake Kariba (570 mm) (Balon, 1971; Van Zyl, 1992). Table 3.2 Length frequencies of age groups as determined from scale readings of H. villains from the Olifants River.
Length group Age (years) (mm) 1+ 2+ 3+ 0+ 5+
121 - 130 1 151 - 160 1 181 - 190 1 201 - 210 1 211 - 220 1 221 - 230 1 231 - 240 4 7 241 - 250 26 3 251 - 260 46 9 261 - 270 49 16 271 - 280 12 19 281 - 290 3 12 1 291 - 300 3 6 3 301 - 310 6 311 - 320 4 1
321 - 330 2 2 331 - 340 1 361 - 370 1 1 371 - 380 1 391 - 400 441 - 450 1
36 Two methods, having a set asymptotic length of 52.40 cm, was used to calculate the curve (k) values: where Gullant and Holt Plot, k = 0.387; Munro's method, k = 0.40. These k values are significantly. The overall growth performance or phi prime (4)), as calculated with the different curvature parameters are: 4) = 6.97; k = 0.387 and 4) = 7.00; k = 0.40.
Annuli counted on the scales under a microscope showed that the tigerfish can reach an age of five + years in the Olifants River (Table 3.2), altough they are very scarce. The mean age group of all the specimens sampled from the Olifants River are those of two and three years old. Data from scales of H. vittatus in the Olifants River were also taken to determine the growth parameters, where Lao = 35.193 and k = 0.449 with 4) = 6.32 (scale readings) and 4) = 6.97 & 7.00 (length frequency).
The phi prime values as obtained by length frequencies and scales are supposed to be equal (Sparre et a1,1989). The values of the phi prime from the scale readings and that of the length frequencies are similar due to the fact that we are dealing with the same population, in spite of the different values obtained for the asymptotic lengths.
The overall growth performance of the H. vittatus in the Olifants River compare well to those found by Van Zyl (1992) and Hay (1995) in the Okavango River although the two systems can not be compared with each other. Similar growth rates were found by Beattie (1982) in Lake Kariba. It would therefore appear that tigerfish have the same growth performance irrespective of the system they occupy.
According to the catch curve the tigerfish population in the Olifants and Letaba Rivers can reach an age of six and seven years respectively. The age given by the catch curve compares well with the age determined by scale readings for the tigerfish in the Olifants River. Tigerfish could reach an age of 8 to 10 years in the Okavango River (Hay, 1995; Van Zyl, 1992) and seven years in Lake Kariba (Langerman, 1984). Annuli counted has indicated that H. vittatus can reach an age of five years in the Olifants River, which closely corresponds with the year class calculated by using the catch curve method. The fact that H. vittatus can reach an age of five years in the Olifants River, might also be a greater reflection of mortalities in that river.
37 3.3.3 Mortality Length frequencies are used when determining the catch curve values to determine the mortality of a population. The small sample size in the length frequencies in the Letaba River may therefore influence the values of the catch curve resulting in misinterpreted data.
The total mortality (Z) for the tigerfish population in the Olifants and Letaba Rivers is Z = 1.488, and Z = 1.067 respectively. The Z- values of a light, medium and heavy exploitation are: Z = 0.6, Z = 0.9 and Z = 1.2 respectively (Sparre et al, 1989). According to these values, there is a medium and high exploitation rate of tigerfish with 34.4% and 22.6% tigerfish recruits to survive after one year in the Letaba and Olifants River respectively. Although the tigerfish population in the Letaba River has a higher survival rate as opposed to those in the Olifants River, it is still lower than the survival of tigerfish in the Okavango River. The total mortality of the tigerfish in both the Olifants and Letaba Rivers is higher than the mortalities recorded in the Okavango River (Z = 0.851) and in Lake Kariba (Z = 0.557) by Van Zyl (1992) and Balon (1971) respectively, but still much lower than the total mortality in the Niger (Z = 2.58) as recorded by Dansoko (1976).
It is unknown in this stage why there is such a high mortality rate of tigerfish in the two rivers, but possible causes are: a - heavy predation by the large number of crocodiles and other predators in the river and area present; b - heavy fishing mortalities by the Mozambique residents in the Massingire Dam; c - unsuitable habitat for efficient recruitment; d - high silt loads during floods.
The nile crocodile (Crocodilus niloticus) were reported to be a predator of the tigerfish, although some contradiction as to that exists (Balon, 1971). If the nile crocodile does happen to prey on tigerfish in the Letaba and Olifants Rivers, they could have a big impact on the tigerfish population. The Olifants Gorge and River up to the Wilderness Trails Camp is probably one of the areas with the highest concentration of crocodiles per kilometre in a river system known in Africa (Viljoen, pers comm). In comparison the Letaba River has less crocodiles. Counts of up to 155 crocodiles 500 meters downstream from the Olifants Wilderness Trail Camp in a 150 meter river stretch had been made by the author. This could well explain the high exploitation rate of
38 the tigerfish in the Olifants River and not in the Letaba River. Other known predators of the tigerfish is the Cape Clawless Otter (Aonyx capensis), Fish Eagle (Habanns vocifer) and the Pel's Fishing Owl (Scotopelia peli) which occurs along the rivers.
The very high concentrations of silt in the Olifants River during floods (Buermann, Du Preez, Steyn, Harmse, and Deacon, 1995) could have a negative effect on the distribution and abundance of tigerfish towards the western border of the Kruger National Park. Recruitment and larval survival rate might also be negatively influenced by the high concentrations of silt.
3.3.4 Sex ratio The male:female (M:F) ratio differs somewhat in the Letaba and Olifants Rivers. In both rivers the males seems to be more abundant than the females during the summer and winter season. The sex ratio of H. villains in the Letaba River is 5.1:1 and 8:1 in the Olifants River respectively. During the non-breeding season the M:F ratio (7.1:1) favoured the males, although the ratio was much higher (9.9:1) during the breeding season (September - February) in the Olifants and Letaba Rivers combined. The very high sex ratio of males to females which was found in especially the Olifants River can not clearly be explained. More males than females are produced when a population is under stress (Holtzhauzen, 1989). These stress conditions seems to be a result of negative environmental conditions such as water temperature, photoperiod and over population on the parental fish. External factors such as photoperiod and especially stress as a result of over population was reported to be the main cause for the shift towards males in the sex ratio of a population (Tave, 1986), rather than fluctuations in water temperature (Mires, 1974). As a result of the high population of tigerfish in the Letaba and especially the Olifants River, and the stress associated with that, the high male:female ratio could therefore be easily explained.
The high male:female ratio in the Olifants and Letaba Rivers could also be as a result of the larger females preferring to stay within the larger volume of water in the Massingire Dam in Mozambique.
39 3.4 Conclusion
The phi prime values from length frequencies and scale readings were found to be similar due to the fact that we are dealing with the same population. These values compare well with the phi prime values for tigerfish in the Okavango River (Van Zyl, 1992) and Lake Kariba (Balon, 1971). The mortality rate of tigerfish in the Olifants and Letaba Rivers is larger than those in the Okavango River and Lake Kariba possibly indicating that reproduction requirements are not optimal. Tigerfish in the Olifants and Letaba Rivers have a life expectancy of six and seven years respectively as opposed to the ten to twelve years in the Okavango River (Van Zyl, 1992). A deviation from the expected 1:1 M:F sex ratio was experienced in both the Olifants and Letaba Rivers which could well be explained by stress as a result off over population. The population dynamic values obtained, indicates that conditions are not optimal for tigerfish in the KNP except for the growth parameters.
40 CHAPTER FOUR
FEEDING HABITS OF THE TIGERFISH (HYDROCYNUS VM'ATUS) IN THE LOWER OLIFANTS AND LETABA RIVERS, KRUGER NATIONAL PARK.
4.1 Introduction
There can be no doubt concerning the predatory impact of tigerfish on any given population of fish in a system. Their role in an ecosystem certainly has been the subject of many studies, and rightfully so too, not only because of their predatory impact, but also of their economical value. Most work has however been done in large rivers and lakes in Africa and include studies in Lake Kariba (Jackson, 1960; Matthes, 1968; Kenmuir, 1973; 1975), Lake Cabora Bassa (Jackson, 1976), Lake Albert (Holden, 1970), Lake Kainji (Lewis, 1974), Lake Chad (Lauzanne, 1975), large rivers such as the Zambezi river (Jackson, 1961a; 1961b; Winemiller and Kelso-Winemiller, 1994), the Okavango Swamps in Botswana (Van Zyl, 1992; Hay, 1995) and the central Delta of the Niger (Dansoko et al, 1976). Few work has however been done on smaller river systems towards the southern range of their distribution where they are relatively scarce (Gaigher, 1967; 1968; 1969; 1970).
Tigerfish is known to be a successful occupant of clear well oxygenated water if one consider for instance the Zambezi River. One should expect that the feeding behaviour of a predator, especially a piscivore, would be influenced in a system with a high turbidity or low visibility. Apart from the physical/ chemical environment, the population size of an organism is mainly determined by recruitment success and availability of food. During this investigation the feeding habits were compared with other systems.
41 4.2 Material and methods
Tigerfish were caught with a selection of artificial lures and angling gear in the Letaba and Olifants Rivers within the Kruger National Park. The size range of artificial lures used provided an equal catchability of all possible sizes larger than 80 mm (SL). Samples were collected on a monthly basis during the period October 1991 - September 1993. Specimens were gutted and the stomachs removed for laboratory analyses.
The stomachs, including the contents, were preserved and stored in plastic bottles with 10% formaldehyde. The fixed stomach contents were analyzed under a stereo microscope in the laboratory. Tigerfish sampled, were placed in different feeding categories due to different composition of their stomach contents. The categories were defined as: Invertivore - invertebrates as food item Piscivore - fish as food item Carnivore - fish and invertebrates as food item Opportunistic - Any other material such as plant material etc.
A linear regression analysis was used to determine if there was any correlation between the length of the tigerfish and the fullness of the stomach.
Four seasons were identified which the stomach contents were divided in: these included spring (September; October; November), summer (December; January; February), autumn (March; April; May) and winter (June; July; August).
4.3 Results
A total of 84% of all the stomachs examined with contents had invertebrates as food item. In both the Olifants and Letaba Rivers invertebrates were the most important food item (54% and 32%) followed by fish (7% and 11%) (Fig 4.1). Four specimens examined (3%), had more than one food item in their stomachs however invertebrates were present in all four stomachs. Only one
42 tigerfish stomach (1%) contained both invertebrates and fish. Other food items included plant material and invertebrate eggs. The seed of an exotic invader plant, Xanthium Strumarium which grows abundantly on the banks of the Olifants River were also found in the stomach of a tigerfish. Three teeth of a tigerfish were found in a its own stomach, which must have been swallowed after replacing a set of teeth (Chapter 7). A large percentage of the stomachs in the Olifants and Letaba Rivers (37% and 55%) were empty (Table 4.1).
The stomach contents of only thee tigerfish had fish remains that could be identified during this study. Prey species that were identified, were Synodontis zambezensis and Brycinus imberi. A 74 mm (TL) Synodontis zambezensis was obtained from a 230 mm (SL) tigerfish, which comprises 32% of the predator's length. The other specimens were in an advance stage of digestion and could not be measured. Consequently, no correlations between predator length and prey could be made.
The percentage of invertebrates taken by tigerfish in the Olifants River, decreased progressively from spring to winter. However, the relative percentages of fish that could be sampled also decreased in the same order as described above (Table 4.2). The presence of invertebrates in stomach contents did not show the same tendency in the Letaba River. In both the rivers a definitive trend could be observed as to the feeding dependence on invertebrates during the different seasons of the year (Table 4.2). With a correlation coefficient of 0.0386 and 0.0523, no correlation could be found between length classes and feeding preference for tigerfish in the Olifants and Letaba Rivers respectively, although the peak of number of fish with full stomachs were found to be between length groups of 250 and 280 mm (Table 4.3).
Identification of the invertebrates preyed upon, were not always possible, as they were in an advanced state of digestion. Sixteen percent of the invertebrates could thus be not identified. A total of 59% of all the stomachs examined had contents, and the remaining 41% stomachs were found to be empty.
Nine invertebrate Orders were present of which the Ephemeroptera (30%), and Hemiptera (29%) were the most abundant (Figure 4.1). Twelve families are represented in the nine orders of
43 •▪
invertebrates on which the tigerfish preyed. The invertebrate families notonectidae, corixidae and chironomidae represented the most food important items for the tigerfish (Table 4.4).
Table 4.1. Food preference of H. vittatus in the Letaba and Olifants Rivers. Percentage and number of fish in each category Composition of diet category Letaba River Olifants River
n= % n=
Stomach empty 31 55 66 37
Invertivore 18 32 97 54
Piscivore 6 11 12 7
Carnivore 0 0 1 1
Opportunistic 1 2 2 1