Biology of age 0+ sand flounder Rhombosolea plebeia in the Avon-Heathcote estuary
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The sand flounder Rhombosolea plebeia (Richardson) (Pleuronectidae) inhabits shallow coastal marine and brackish water regions throughout New Zealand, Auckland Islands and possibly Australia. Relatively little research has been carried out on this important commercial species. The principal investigation up to the present time is an unpublished Ph.D. thesis by Mundy (1968), who carried out a population study involving tagging of adult fish off the Canterbury coast. Some data were provided as part of a study on the fish population of the Avon-Heathcote Estuary by Webb (1966, 1972, 1973a, 1973b). A Ph.D. study on osmoregulation of sand flounder from Otago Harbour has been completed by Raj (1973), and a study of flounder spawning in the Hauraki Gulf has also been recently published by Colman (1973). The shallow water inlets and estuaries around the Canterbury coast act as a nursery for young stages of sand flounder. While some population and feeding studies have been carried out on the older age classes, little is known of its early development, natural history and physiology. The Avon-Heathcote Estuary (latitude 43°32'S., longitude 172°43'E.), the study area in the present investigation (Fig. 1), is one of these nursery areas for the young stages. It is probably the principal area for producing fish to restock the offshore grounds which provide the commercial catches on the east coast of the South Island (Mundy, 1968). Sand flounder is an important commercial species, both in terms of public demand and in the volume of landings. The annual landings of' flounder of all species in New Zealand from 1944-1970 was about 907,730 kg, and of this total sand flounder formed about 50 percent (New Zealand Marine Department, Reports on Fisheries, 1944-1970). Hence the study of its biology in a major nursery area and of the effects of man-made environmental changes and pollution is important. The Christchurch Drainage Board proposes to control flooding in the City of Christchurch and have had several flood control schemes tested by the Hydraulics Research Station, Wallingford, England. They have also commissioned an intensive biological research programme on the Avon-Heathcote Estuary headed by Professor G.A. Knox, Department of Zoology, University of Canterbury to assess the effects of' these flood control schemes. The present study forms part of this biological research programme. The flood control schemes include proposals to erect a barrier near the Estuary mouth. If spring tides coincide with heavy rainfall, surface flooding in parts of Christchurch becomes a problem. When flooding is imminent it is proposed that the barrier gates will be closed at low water to prevent tidal penetration so that flood water can drain into the Estuary without being retarded by the incoming tide. Any such scheme affecting the biology of the Estuary must be carefully considered as undesirable effects may result. The first general study of the biology of the Estuary was made by Thompson (1930), whose work forms a useful basis with which to compare changes that have occurred since then. More specific studies have been made on polychaetes (Estcourt, 1962), fish populations (Webb, 1966), sand flounder (Mundy, 1968) and benthic macrofauna (Voller, 1973), and a number of surveys directed at the effect of pollution in the Estuary have been carried out by Bruce (1953), Williams (1959), Rosenburg (1963), Webb (1965) and Cameron (1969). Pollution studies of the Avon and Heathcote Rivers have been published by Hogan and Wilkinson (1959) and Cameron (1970). Other studies including Estuary populations as part of more extensive research have also been carried out (for a complete bibliography see Knox and Kilner, 1973). Research currently in progress, in addition to the O+ sand flounder study, include studies on the algal communities, a water chemical analysis programme and an investigation of the biology of the Christchurch Drainage Board’s Sewage Oxidation Ponds. A report on the general ecology of the Estuary has been prepared recently (Knox and Kilner, 1973), and this summarizes much of the above material and includes other relevant information.