Estimation of the spawning stock biomass of blue grenadier (Macruronus novaezelandiae) off western Tasmania based upon the annual egg production method

Abstract

Egg production of the blue grenadier, Macruronus novaezelandiae, spawning stock off western Tasmania was surveyed from June through September in 1994 and 1995. In each year, daily egg production rates were calculated for each of three surveys and a normal curve was fitted to estimate annual egg production. The CV for the egg production estimate for the survey at the peak of the 1995 season was 14%. Daily egg mortality was not significant in any survey, but a value of 5% was assumed. The spawning biomass of blue grenadier on the west coast of Tasmania was estimated to be between 83 660 and 100 073 t in 1994 and between 59 727 and 71 376 t in 1995. Confidence limits of the estimates were obtained by a bootstrap where the highly skewed egg density data were resampled. Biomass was approximately inversely proportional to incubation time. Increases or decreases in mortality resulted in slightly smaller increases or decreases of biomass respectively. Uncertainty of the sex ratio between spawning and non-spawning periods caused the largest variation in biomass estimate (~30%). The proportion of the population that spawns each year remains a further uncertainty in the estimate of total stock biomass.