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The Fishes of Tristan Da Cunha and Gough Island (South Atlantic), and the Effects of Environmental Seasonality on the Biology of Selected Species

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The Fishes of Tristan Da Cunha and Gough Island (South Atlantic), and the Effects of Environmental Seasonality on the Biology of Selected Species ; 't- THE FISHES OF TRISTAN DA CUNHA AND GOUGH ISLAND (SOUTH ATLANTIC), AND THE EFFECTS OF ENVIRONMENTAL SEASONALITY ON THE BIOLOGY OF SELECTED SPECIES Submitted in Fulfilment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY of Rhodes University by TIMOTHY GILBERT ANDREW December 1992 ACKNOWLEDGEMENTS Numerous people have made this programme of fish research on Tristan possible and successful. The Tristan islanders have offered their utmost support for work in the field, both in gathering information and the collection of specimens. Their tremendous hospitality will not be forgotten. Jimmy Glass and his family are especially thanked for adopting us on Tristan and making us feel so much at home while on the island. Jimmy was also extremely helpful as a source of local knowledge and as a diving partner. The Administrator of Tristan during the period on the island, Mr Bernard Pauncefort, is thanked for his enthusiastic support and interest in our work. Captain Cecil Pohlman and his crew on the rock lobster vessel MFV TRISTANIA II were extremely helpful during the course of the research and their warm-hearted acceptance of myself aboard is greatly appreciated. Permission to work aboard this vessel and MFV HEKLA, granted by the owners, Tristan Investments (Pty) Ltd. was essential to the success of the project. Robin Matthews of the South African Meteorological Team at Gough Island (1988/89) is thanked for his assistance in collecting specimens. Billy Ranchod and Edward Matama of the J.L.B. Smith Institute of Ichthyology are thanked for their efficiency in handling the Tristan fish collection. I am grateful to Robin Stobbs for his excellent photographic and radiographic work. Elaine Heemstra is thanked for her illustrations of Bassanago nielseni, Conger wilsoni, Gnathophis capensis, Brama australis and Hyperoglyphe perciforma. Joan Wright also provided important assistance with the taxonomic part of the study. Doctors P.A. Hulley, Leonard Compagno and Mr Cedric G0liath facilitated the examination of specimens in the fish collection of the South African Museum. Dr Guy Duhamel of the National Museum of Natural History in Paris is thanked for the gift of two specimens of Acantholatris monodactylus from Amsterdam Island. The knowledge imparted to me by Dr Phil Heemstra, not only on taxonomic issues but on all aspects of fish biology, is gratefully acknowledged. Dr Andre Punt of the Department of Applied Mathematics, University of Cape Town helped to sort out problems with the ageing of A. monodactylus. His help is appreciated. Larry Oellennann and Linda Coetzee of the Department of Ichthyology and Fisheries Science (DIFS), Rhodes University, were always willing to assist with computer problems. Dr Rob Tilney, also of the DIFS, provided valuable guidance through many discussions. All the staff and students of the DIFS and the J.L.B. Smith Institute of Ichthyology have supported this project through discussion, moral support and friendship. They have provided a great environment in which to work. Dr Colin Buxton of the DIFS has given of his expertise whenever approached and was particularly helpful with discussions on hermaphroditism in fish. Professor Tom Hecht, my supervisor, has repeatedly restored my self-confidence on those occasions when doubts have crept into my mind concerning aspects of this research. He has also stimulated me academically especially by insisting that I ask the question "Why?" about observed phenomena. Tom is also the only other person in the Department who was on Tristan with me for a period and shared the initial excitement of new discoveries during the first month on the island. He has therefore been an essential person with whom ideas could be discussed. Tom also initiated this project and I am grateful that he chose me to carry it out. Professor Johan Lutjehanns and Mr Henry Valentine of the Department of Oceanography, University of Cape Town put a great deal of effort into the oceanographic part of this study. Henry prepared all the equipment necessary for field measurements and analyzed all the salinity samples while Johan provided important academic insights into the oceanography of the Tristan region, a field about which I had very little knowledge. John Cooper and Peter Ryan of the Percy Fitzpatrick Institute of African Ornithology at the University of Cape Town, two colleagues who also have a deep interest in the natural history of Tristan and Gough, supported this project in any way that they could. John has corresponded with me regularly over the last few years and his interest in my work has been stimulating. Peter collected otoliths for me while on Inaccessible Island which was of great value. Financial support was received from the South African Department of Environmental Affairs, the Foundation for Research Development, Rhodes University, and the South African Breweries. In addition bursaries were granted by Irvin and Johnson (Pty) Ltd. and by the late Margaret Smith. The Government of Tristan da Cunha assisted by allowing us to use facilities on Tristan, either free of charge or at a reduced rate. The assistance of all these parties is gratefull y acknowledged. ii I would like to thank my parents, Ron and Gill, for making it possible for me to arrive at the point of undertaking a Doctoral degree. I am particularly grateful to them for allowing me to freely choose my direction in life and for supporting any wise choices that I have made. My wife, Maura, has supported me morally throughout this study and has been extremely patient with my fluctuating moods towards the end of the write-up stage. Her love and understanding is enormously appreciated. Finally, I dedicate this thesis to the islanders of Tristan da Cunha from whom I have learnt so much. 111 ABSTRACT This study focuses on the taxonomy, biogeography and biological aspects of the fishes of Tristan da Cunha and Gough Island in the cental South Atlantic Ocean. Oceanographic measurements were undertaken to detennine the position and nature of the Subtropical Convergence (STC) in the vicinity of the islands. The Tristan da Cunha group is thought to be situated on the northern edge of the STC while Gough Island is situated on the southern edge of the front. The seasonal environmental cycle at the islands is characterised by an annual sea surface temperature fluctuation of approximately 5 °C and an annual change in stratification of the water column. It is suggested that this increase in stratification at the STC, brought about in the summer by insolation warming the surface layers of the ocean, enhances primary production. The STC is identified as a unique habitat for fishes and as an important barrier to dispersal of species in the Southern Ocean. The present study has produced 18 new distributional records from the shelf waters of Tristan da Cunha and Gough Island. The fish fauna known from the shelf waters of these islands is reviewed, and a diagnosis, synonymy, and in some cases an illustration are provided. For certain species, brief notes on their biology, relative abundance and seasonal distribution are included. An analysis of the ichthyofaunal relationships between shallow water areas in the vicinity of the STC has resulted in the identification of a characteristic neritic STC ichthyofauna. The circumglobal distribution of many of these species is thought to be a consequence of a dispersive pelagic phase in their life-cycles. Biological processes in fishes at these islands are temporally and spatially affected by seasonal changes in oceanographic conditions. Seasonal sea temperature variation was identified as being an indicator of other oceanographic phenomena that may affect processes such as growth, reproduction and feeding in fishes at the islands. Most species have a summer spawning season and growth rate increased during the summer months. It was hypothesised that enhanced primary production and a related increase in food quality and availability during the summer were the major factors temporally regulating the growth and reproduction of fishes at the islands. iv Twenty families of neritic fishes have been recorded in the shelf waters of Tristan da Cunha and Gough Island. Most families are represented by only one species. Because of this a wide range of life-history strategies are apparent in the fauna. Nevertheless, there are a number of common life-history characteristics shared by most of the neritic species. Firstly, the early life­ history of many species included an extended pelagic phase which might have facilitated colonisation throughout the STC zone in the past, thereby regulating extant species diversity in this region. Secondly, the common occurrence of an extended summer breeding season and iteroparity suggested that these characters also contributed to survival in what is considered to be an unpredictable environment. It would appear that these common characteristics hold the key to the success of many fishes in the STC zone. This study has contributed to our understanding of oceanic island ecosystems and has revealed distributional patterns of fishes that were unknown previously. By focusing on the biological processes of selected species the regulatory role played by local environmental conditions became apparent. The results of this study are also relevant to proposed conservation measures for Tristan da Cunha and Gough Island. v TABLE OF CONTENTS CHAPTER 1. GENERAL INTRODUCTION .................................................................. 1 CHAPTER 2. A DESCRIPTION OF THE STUDY AREA AND
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