Upper-slope fish communities on the Chatham Rise, New Zealand, 1992-99

Catch rate data for fish species caught in 200-800 in depths during summer trawl surveys of the Chatham Rise to the east of New Zealand in 1992-99 were explored to assess community structure and species associations and to identify changes that occurred within the time series of surveys. The community was dominated by hoki (Macruronus novaezelandiae Hector), a Southern Hemisphere merlucciid hake, which was more abundant than all other species combined and occurred at 97% of the 1048 trawl stations. Three groupings of fish species were identified on the basis of descriptive and cluster analyses, associated with the 200-350, 350-550, and 550-800 m depth ranges. Redundancy analysis showed that depth, latitude, and to a lesser extent longitude explained most of the variation in abundance and composition of catches. Spatial patterns in species diversity and fish density were investigated. Trends within the time series show a significant decline in the biomass of hoki and changes in biomass of several other abundant species. However, there was little change in diversity, density, or division into groupings. The changes in abundance may be related to increased fishing activity and/or other factors such as changes in catchability because of the rising bottom temperatures observed during the surveys.