CPUE indices used in Soviet krill fishery statistics from 1977 to 1992 and their possible utility for evaluation of relative changes in krill biomass
Key catch-per-unit-effort (CPUE) indices used in Soviet krill fishery statistics, as well as the main sources of data and primary data for their calculations, were examined. The main CPUE indices were catch per vessel-days fished (CVDF), catch per extended vessel-days fished (CEVDF) and catch per hour of trawling (CHT). A relatively high correlation between mean monthly values of CVDF and CEVDF, and CVDF and CHT was observed. However, the correlation between daily mean values was quite low as a result of significant fluctuations in haul duration due to the different fishing strategies used when targeting krill intended for different end products, i.e. human consumption, krill meal and frozen krill. Daily mean CVDF and CHT calculated for specific strategies were well correlated. Depending on the fishing strategy used, the number of hauls per day ranged from 1 to 15. The duration of hauls ranged from 0.1 to 16 hours. When a final product of higher quality was required, haul durations were shorter and, accordingly, the number of hauls per day was higher. The correlation between catch and tow time was considered to be an indicator of homogeneity in krill distribution. The utility of CPUE for direct evaluation of relative changes in quantitative parameters of krill distribution inside fishing grounds is assessed.