Aspects of the early life history of flatfish (Pleuronectidae) in Wellington Harbour, New Zealand

Abstract

Flatfish larvae and juveniles occur abundantly in Wellington Harbour throughout the year. Though four adult species of Pleuronectidae (and one Bothid) are known to occur there, an electrophoretic study has shown that as larvae, Peltorhamphus latus (speckled sole) is the predominant sole, while Rhombosolea plebeia (sand flounder) is the predominant flounder. Over 80% of the larval flatfish catch taken by Bongo plankton net in the harbour is normally sole. An extensive study, principally based on movements of larvae, and food and feeding of the larvae and juveniles, was carried out for sole and flounder to establish the main features of the early life history of these two species. Diurnal vertical migration was found to occur, following the general pattern of larvae away from the surface during the day and near the surface at night. Tide also affected the vertical distribution of larvae, 'causing' movement away from the surface. The role of diurnal vertical migration is probably to retain larvae in the harbour system. An analysis of larval diet from gut contents of 1505 sole and 334 flounder shows that these two species eat essentially the same prey, though sole favours polychaete larvae and flounder favours the harpacticoid crustacean Euterpina acutifrons. Larvae are discriminate feeders, with larger individuals appearing to 'prefer' specific prey. Prey composition changed according to site, depth and season of capture, as well as larval size. Larval size has the greatest effect on stomach fullness, and capture site, time of day and season also influence fullness. Feeding changes from a generalised to a specialised feeding mode with increasing size. It occurrs in 'bursts', dawn and early afternoon being the most important periods. Minimum gut fullness occurrs before dawn and maximum fullness at dusk. Settlement of juveniles follows high larval densities over spring/summer and autumn. Harpacticoids, polychaetes, amphipods and molluscs are all eaten by juveniles. Prey type differs for both site and size ranges of the flounder, feeding differences exist between sole and flounder, with ostracods and fish not eaten by flounder. Meiofauna1 availability appears to be the critical determinant of juvenile diet. The larval/juvenile transition period is discussed. During metamorhposis, no major change in the taxonomic diversity of prey types consumed by the flatfish occurs.