2 Recruitment surveys

The abundance of several of New Zealand's important commercial species is likely to be recruitment driven. So the development of a recruitment index and its incorporation into a stock assessment model could reduce the uncertainty in the estimates of biomass and sustainable yields and might lead to a method of predicting fluctuations in abundance.

Because of this, a juvenile kahawai (Arripis trutta) recruitment index feasibility study was instigated [Gerring, P. K. & Bradford, E. 1998: Juvenile kahawai recruitment index feasibility study. NIWA Technical Report 36.38p ]. Juvenile kahawai were sampled in three areas of New Zealand between November 1996 and September 1997 to test the feasibility of deriving an annual recruitment index for this species.

Sites were chosen in each area based on ease of access, shallow beach gradient to allow deployment of the net, no obstructions (rocks, mangroves), and a fishable area ~100m long.

The net used for sampling was a beach-seine modified to catch 0+ aged kahawai. The net was 20m long by 2m deep with a 9mm mesh. Sites were sampled by beach seining with the net dragged parallel to the shore for ~100m. The net was then dragged ashore and the catch sorted by species.

Juvenile kahawai were counted and measured to the nearest 1mm (fork length), while other species were only counted. For large catches of kahawai, a random sample of at least 50 juveniles was measured. All sampling was done within 2 hours either side of spring high tide.