A daily egg production method estimate of snapper biomass in Hauraki Gulf, New Zealand

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Abstract
Snapper (Pagrus auratus, Sparidae) are an abundant finfish in waters off northern New Zealand and important in commercial and recreational fisheries, the largest stock being in the Hauraki Gulf. A daily egg production method (DEPM) survey was used to estimate Hauraki Gulf snapper biomass in Nov.–Dec. 1992, during the peak of the spawning season. Snapper are multiple spawners with indeterminate fecundity, and it is possible to estimate the proportion of females spawning each day in the population using trawl samples, making the DEPM appropriate for estimating biomass. Daily egg production was estimated using egg counts-at-age data from 298 plankton samples fitted to a maximum-likelihood model. Spawning proportion (the proportion of females with hydrated oocytes sampled before the onset of daily spawning), batch fecundity (counts of hydrated oocytes in ovaries) and sex ratio were estimated from trawl samples. Average fish weight was estimated from a mixture of longline market sampling, longline catch-at-sea sampling and trawl sampling. The estimated mean biomass (24 499 t, 95% confidence interval 17 994–48 650 t) was similar to that estimated from a tagging programme in the Hauraki Gulf one year after the DEPM survey. Possible sources of error and bias are discussed.

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