AN ESTIMATE OF ORANGE ROUGHY, HOPLOSTETHUS-ATLANTICUS, BIOMASS USING THE DAILY FECUNDITY REDUCTION METHOD

The recruited biomass of orange roughy, Hoplostethus atlanticus, was e stimated for the New Zealand mid-east coast orange roughy stock with t he daily fecundity reduction method (DFRM). These fish migrate to Ritc hie Bank and spawn between 850 and 900 m for about one month in winter . The biomass of spawning females was estimated by dividing mean daily planktonic egg production, N-o (eggs/day), by mean daily fecundity, D (eggs/kg per day). The stock biomass was then estimated by multiplyin g the spawning female biomass by the ratio of all recruited fish to fe males that would spawn that year, estimated with a wide-area trawl sur vey made over the stock area two months before the spawning season. Th e mean daily planktonic egg production was sampled near the peak of th e spawning season, by using a stratified-random plankton survey. Eggs were staged and aged after accounting for their thermal history as the y ascended the water column. Because young eggs were damaged by the ne t and older eggs were affected by advection out of the plankton survey area, relatively few egg stages were available for estimating N-0 (10 .9 x 10(9) eggs/day), and the estimate was somewhat imprecise (CV=0.46 ). Mean daily fecundity (787 eggs/(kg x day), CV=O.11) was estimated f rom the daily rate of decline in population fecundity per mature femal e weight (R-i). Fecundity per female weight was estimated from a trawl survey made in the spawning area during the spawning season and was c alculated as the mature eggs/kg of active spawners multiplied by the p roportion of active spawners in each trawl. Spawning female biomass wa s 14,000 t (CV=0.50), and stock biomass was 26,000 t (CV=0.50). Mean d aily fecundity was probably under-estimated because spent fish appeare d to migrate from the spawning area during the fecundity reduction mea surement period and reduce stock biomass to about 18,200 t. The DFRM b iomass estimate was of central importance in the introduction of great ly reduced total allowable catch levels in this fishery.