Depth and the structure of assemblages of demersal fish: Experimental trawling along a temperate coast

Demersal fish were sampled at three depths (20-30, 40-50, 60-70 m), three times and at three locations off Sydney, Australia. The main faunal change in assemblages of demersal fish occurred with depth. Multivariate analyses revealed a strong gradient of change in the structure of assemblages from shallow to deep water. Assemblages also differed among sampling times and locations, but depth explained most of the variation. Samples separated into distinct depth-related groups, irrespective of time and location of sampling. The species primarily responsible for differences in structure of assemblages among depths were also primarily responsible for differences detected among locations and times. Importantly, the average difference in abundance of these species among depths was > 10 times greater than the equivalent differences among times and locations. Our study highlights considerable depth related variation associated with small changes in depth (10-20 m). This warns that small differences in depth have the potential to confound comparisons at larger spatial scales. These results support growing evidence that depth is one of the major' and more predictable sources of variation in demersal fish assemblages.