Characteristics and consequences of movement patterns of King George whiting (Perciformes : Sillaginodes punctata) in South Australia

Abstract

The characteristics of movement of the South Australian population of King George whiting (Sillaginodes punctata) were determined through analysis of tag and recapture records collected from three tagging periods: (i) 1968–1969; (ii) 1978–1985; and (iii) 1986–1987. The characteristics were compared between the northern and southern parts of two large fishery regions, and determined for fish tagged at different sizes and ages. Fish tagged in the northern Gulf St Vincent and Spencer Gulf moved southwards up to several hundred kilometres, but those tagged in the southern areas showed no systematic directional displacement. Most fish tagged were subadults or young, immature adults at 2–4 years of age. Small, young fish did not move far until a considerable period after tagging, whereas some fish >300 mm total length (TL) at tagging moved substantial distances within only a few weeks. Fish movement resulted in a significant ontogenetic habitat shift, from relatively protected shallow waters that support extensive meadows of seagrass, to southern, exposed, deeper waters and rocky reef. As a result of this systematic, directional displacement the age structures of the fishery catches also varied systematically throughout the two regions. In the northern areas these were simple and dominated by the 3+ age class, whereas in the south they were complex and consisted of the 2+ to 17+ age classes. Because the latter were the spawning populations, fish movement is an obligate step in the life history that ultimately results in supplementation of numbers on the spawning grounds. The consequences for fishery management are discussed.