Seasonal and spatial distribution of fish larvae in waters over the North West Continental Shelf of Western Australia

Abstract

Distributional patterns of larval fishes in the waters of the North West Continental Shelf of Australia and the adjacent open ocean were examined in a series of stepped, oblique mid-water trawled samples to 100 m. These were taken from September 1982 to October 1983 at fixed sites distributed across 2 cross-shelf transects, the longer western transect extending to a water depth of 1000 m, the eastern transect extending to the shelf break at approximately 140 m. Larvae were, with a few exceptions, identified to family, and 104 taxa were used for subsequent analyses. The pooled larvae were least concentrated at the shelf break, becoming denser towards both the shore and towards the ocean. Mean concentration of larvae (all species pooled) varied with time at the 3 pairs of innermost sites. Concentrations increased in June at the 4 midshelf sites, but decreased at the inner shelf sites until August. Classificatory analyses, based on the presence or absence of each of the 104 taxa in all samples, distinguished a major discontinuity in faunal composition just inshore of the shelf break. Other differences in faunal structure (in order of size) pertained to type of sampling gear, depth of sampling site and time of year of sampling. Short-term differences (2 wk penods) were not detected, nor was any consistent longshore vanabillty. Charactenst~c of the deeper sites were those taxa found as adults on the continental slope and open ocean, whereas larvae of taxa that occupied the shelf as adults were characteristic of the shelf sites. The pattern of larval distributions were in general agreement with previous studies from the Pacific Coast of North America. Apart from a summer/winter change in both water conditions and larval taxon frequencies, little correlation was found between larval distribution or abundance and the hydrography of the area.