Large-scale spatial patterns of abundance within the assemblage of fish caught by Prawn Trawlers in Northern Australian waters

Abstract

Large-scale patterns of abundance within the by-catch of fish in the Northern Prawn Fishery of tropical Australia were investigated. Commercial catches were sampled from May 1988 to May 1989, between 128º and 138º E. Because sampling could not be stratified by fishing ground, depth, season and time of day, data on the abundances of 115 taxa of teleosts were restricted to: (1) 136 samples collected between 2000 and 0600 hours in depths of 18-49 m from four fishing grounds during August- November; and (2) 270 samples collected between 1800 and 0800 hours in depths of 18-76 m from seven fishing grounds throughout the period sampled. Pattern analyses indicated that samples clustered along geographic and bathymetric gradients, forming distinct western and eastern groups separated near 132º E, and shallow (<30 m) and deep (>30 m) water groups. Ten taxa had distributions centred east of 132º E, regardless of depth. A further 14 taxa had distributions centred east of 132º E and below 30 m. Nineteen taxa had distributions centred in the western region of the study area. Eight taxa occurred predominantly below 30 m throughout the area sampled, and 16 taxa had high catch per unit effort in shallow water (<30 m). Of the remaining species, seven were taken throughout the study area, and nine taxa were abundant on fishing grounds other than below 30 m in the Gulf of Carpentaria. Thirty-two taxa were rare. It was concluded that spatial patterns adequately described the abundance of taxa within the data, and that varying reports in the literature concerning the relative contribution of spatial and seasonal factors were dependent upon the scale of sampling.