Application of the directed CPUE method to the multispecies bottom fisheries in the Emperor Seamounts region for the monitoring of stock status and fishing activity

Catch per Unit Effort (CPUE) is the most commonly used index in fisheries science to monitor stock status and the effect of management measures. To use CPUE to assess stock status in multispecies fisheries, we should consider how fishers direct their effort to each species. While CPUEs with different ways of adjustment were used in NPFC for several purposes including the use of surplus production model for North Pacific armorhead and splendid alfonsino, the validity of those values as an index of abundance in multispecies fisheries was not examined. In this study, we applied the method of directed CPUE proposed by Biseau (1998) to the multispecies bottom trawling in the Emperor seamounts region. We then compared directed CPUE with other CPUEs calculated by different methods. Both directed and nominal CPUE are useful for armorhead, but only directed CPUE is useful for alfonsino which targeted as an alternative of armorhead, because the amount of effort directed to alfonsino stock depends on the abundance of armorhead, instead of their own. This analysis suggests the shift of target from armorhead to alfonsino. Further examination of the validity and sufficiency of the directed CPUE method as the index of abundance is needed.