2 Marine Research Plankton Survey Database

Plankton surveys and primarily, egg surveys, are another tool to help fisheries scientists calculate biomass abundance [ Zeldis, J. 1993. The applicability of egg surveys for spawning stock biomass estimation of snapper, orange roughy and hoki in New Zealand. Bulletin of Marine Science 53(2) : 864-890 ]. The basic theory of population estimates made from egg surveys is well known:

"If one can estimate the total production of eggs or larvae of a stock, P, throughout a spawning season, and determine the mean fecundity, F, of a mature female and the proportion of females in a mature stock, K, then the total abundance of the mature stock

M=P/KF "

[Crossland, J. 1980. The number of snapper Chrysophrys auratus (Forster), in the Hauraki Gulf, New Zealand, based on egg surveys in 1974-75 and 1975-76. Fisheries Research Bulletin No. 22 40p.]

Plankton surveys in a sample area can be conducted in a variety of ways such as stratified, random, or grid stations and are often carried out in conjunction with trawl surveys. The nets used are varied in size and shape but all have very fine mesh They range from a large Engels midwater trawl for rock lobster pureluii, to small cylinder-code trawl nets, to vertically dropped plankton nets.

These surveys use a variety of egg production models (such as the: "daily egg production method" (DEPM); the "annual egg production method" (AEPM); and the "daily fecundity reduction method" (DFRM)) for fish stock biomass calculation.

The DEPM estimates the adult spawning biomass from the ratio of the daily production of planktonic eggs and the daily fecundity. The latter is calculated using daily spawning frequency, average batch fecundity, average female weight and sex ratio. The planktonic eggs are caught during a plankton survey and the spawning frequency, batch fecundity, and sex ratio are estimated using a trawl survey.

The DFRM estimates biomass of spawning females by dividing the daily planktonic egg production in the survey area by the weight-specific daily fecundity of females. Because it is a daily method, there is the advantage that the DFRM does not need to cover the entire spawning season.