Abstract:
Fisheries legislation now calls for ecosystem approaches, incorporating large spatial and temporal scales, as well as single-species stock assessments. For implementing an ecosystem approach, improvements of our knowledge and understanding of the marine ecosystem, multispecies assemblages, and their trophic interactions are essential. Acoustic data may provide a significant source of quantitative and qualitative information about community ecology, distribution and species abundance. This study used opportunistic acoustic data collected during two Argo buoy deployment surveys between New Zealand and Chile to acoustically characterize the distribution patterns of pelagic fish abundance across the South Pacific Ocean. The area backscattering coefficient (sa) at 38 kHz in the upper 600 m was used as the measure of fish abundance. This backscatter was assumed to be dominated by mesopelagic fish. From the acoustic analysis, major vertical and horizontal patterns of pelagic fish distribution were identified. Vertical (depth) patterns included diel migration and layering. Key horizontal (spatial) patterns were coastal intensification and oceanic patchiness. These patterns were very consistent between transects, not showing any clear intrannual or interannual differences. Boosted Regression Trees (BRT) models were used to explain the relationship between sa and environmental (chlorophyll-a, temperature and sea surface height) and geographical variables (distance from coast and bottom depth). The key finding was that there was a strong correlation between primary productivity and distribution of acoustic backscatter. The incorporation of species composition information from biological sampling would improve the relevance of the results of this study. However, these large-scale estimates will still provide an opportunity to validate model predictions of mid-trophic functional groups.