Relative abundance of troll-caught albacore

Trophia has been contracted by the New Zealand Ministry of Fisheries (MFish) to determine the relative abundance of juvenile albacore (Thunnus alalunga) in New Zealand fisheries waters. . The specific objectives for this project include:

1. To update standardised CPUE for the west coast albacore troll fishery using data up to the end of the 2007-08 fishing year.

Albacore caught by troll in New Zealand waters are juveniles of a single South Pacific Ocean stock that ranges from the equator to about 45oS. These small fish (about 5kg) are available to troll in the New Zealand EEZ mainly off the west coast of the North and South Islands during the summer months and provide some of the only information on recruitment to the wider South Pacific stock.

New Zealand has been undertaking annual catch sampling of the troll fishery since 1996/97 and while that programme is useful for evaluating relative strength of the three cohorts commonly present, it is less useful for estimating longer term trends in recruitment without reference to some measure of relative abundance.

Availability of juvenile albacore to this fishery will depend on many factors both within and outside of the New Zealand EEZ including wider ecological effects that would be confounded with year effect in any standardised analysis of abundance given the small spatial and temporal window in which the New Zealand data are collected.

Within the New Zealand EEZ, catch rates are determined by the ability of vessels to follow the migratory patterns of albacore, and their ability to expend effective effort, that is, coincident with the habitat preferences (temperature, depth etc.) of juvenile albacore. It is not surprising if there is little contrast in the environmental variables associated with fishing. As such, the emphasis of standardisation should be on accounting for any systematic changes in fishing practice that might mask the signal of availability of the stock to NZ waters.

This work is funded by MFish project ALB2008/02