Life of a flatfish, the yellowbelly flounder, Rhombosolea leporina Günther, 1873, in Auckland's sheltered waters

Abstract

Rhombosolea leporina Günther, 1873 is a right-eyed flatfish endemic to New Zealand coastal waters. This study examines this flounder species as an indicator of the importance to fish of the fringe habitats, and its early life stages in those habitats as an indicator of human disturbance. The study was carried out in disturbed areas of the Manukau and Waitemata Harbours, and Tamaki Estuary, which have an urban catchment from the Auckland metropolitan city area, and less disturbed areas in the Kaipara Harbour and Waiwera Estuary with rural catchments. Juvenile fish on mud/sand flats were caught by scoopnet and in channels by a dragnet. The adult fish were taken by a setnet. Aging was estimated by use of the otolith. Analysis of the otolith and length relationship shows that R. leporina has a short life span of about four years. This species spends most of its life in shallow water and starts tidal migration during the 1+age. At low tide R. leporina <5cm total length remain on the mud / sand flats, but at localities where there is no standing water this size group occurs along the edge of the retreating tide. Fishes>20cm migrate with the ebb tide to the deeper parts of the channels which in turn lead to the main, deepest channels of the harbour. Length and weight data were used in the analysis of growth of fish from different localities. The Condition Factor index (CF) calculated from length and weight data of juvenile R. leporina reveals that the Manukau Harbour fish are in poorer condition than those of the Waitemata and Kaipara Harbours. Growth estimates by Von Bertalanffy (1938) curves show fast growth in the 1+ and 2+ age groups. The Von Bertalanffy parameters and Walford plots reveal that the female R. leporina may grow larger than the male fish. Sexual maturity is attained at age 2+, females at a length >25cm and males at 22cm.Spawning of this species takes place offshore. Ovary development can be divided into five stages; immature / resting, ripening, ripe, running, and spent. Gonadosomatic index (GSI), founded on ovary development, shows a peak spawning season from winter to spring. GSI is estimated from ovary and fish weight. The juvenile stages of 0+ and 1+ age groups feed on assorted invertebrates. Adults from >20cm length size, the 2+ age group, start to specialise in their diet, mainly feeding on three species of crabs, Halicarcinus cooki, Helice crassa and Macrophthalmus hirtipes. The early life stages of the 0+ and 1+ age groups are protected by the minimum legal size of fish that can be taken commercially not being reached until 2+. This enables the fish to reach adulthood and spawn at least once before being taken. The species grows fast and has high fecundity. The fish collected from the Manukau Harbour are more affected by pollution related diseases than those from the Waitemata and Kaipara Harbours. The sediment and water samples were analysed for heavy metals, organo-chemicals, faecal coliforms and Enterococci coli. Those fish analyzed for heavy metals and organo chemicals had their liver loaded with these toxins more than any other organ or tissue. The Manukau, Waitemata Harbours and Tamaki Estuary are more impacted than the Kaipara Harbour. The other flatfish species caught were Rhombosolea plebeia (Richardson, l842) and Peltorhamphus latus (James, 1972). The Auckland inshore areas are dominated by the juvenile and adult yellow eye mullet Aldrichetta forsteri which occurs throughout the year. The other fish species caught were only present seasonally, particularly from spring to summer. Nineteen other fish species were recorded at the study sites.