HABITAT PARTITIONING BY WHITING SPECIES (SILLAGINIDAE) IN COASTAL WATERS

The ways in which the distributions of six species of whiting (Sillagi nidae) in the coastal marine waters of south-western Australia are rel ated to the type of substrate (bare sand vs. seagrass), degree of expo sure of habitat, water depth and body size have been investigated. Whi ting in nearshore waters (< 1.5 m) were sampled using a fine-meshed se ine net, while those in 'shallow' (5-15 m) and 'deep' (20-35 m) waters of the inner continental shelf were sampled with a trawl net. Shallow nearshore waters are shown to provide nursery habitats for five of th e six whiting species. In these waters, Sillaginodes punctata, Sillago burrus, Sillago schomburgkii and Sillago vittata mainly occur in prot ected areas,while Sillago bassensis predominantly occupies areas that are more exposed to wave and swell activity. The first three of these species also use estuaries as nursery areas. In nearshore waters, whit ing were captured almost exclusively over bare sand, rather than in in terpersed beds of the seagrass Posidonia spp., presumably reflecting t he fact that the dense canopies produced by the wide blades of Posidon ia spp. must inhibit penetration by the benthic whiting species. As O + S punctata increase in size, they tend to move offshore during the d ay and inshore at night. Many mature representatives of S. schomburgki i are present in nearshore areas, whereas the other four species move offshore into inner-shelf waters as they increase in length. Sillago b urrus and S. vittata remain in shallow inner-shelf waters, whereas the larger S. bassensis subsequently migrate into deeper inner-shelf wate rs. Large Sillago bassensis thus co-occurs with Sillago robusta, which is mainly found in those deeper waters, but does not reach as large a size. The larger S punctata occupy areas near reefs which could not b e sampled by trawl netting. There are thus interspecific differences i n (i) the times of recruitment of the O + age class into nearshore are as, (ii) the types of habitat occupied during juvenile and adult life, and (iii) the degree to which fish move into more offshore waters as they increase in length, and one species is restricted to deeper water s. The resultant partial segregation among habitats of the coastal wat ers of south-western Australia by different size groups of these relat ively abundant whiting species presumably reduces the potential for in tra- and interspecific competition amongst these species.