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
[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.