Ja. Koslow et al., BIOMASS ASSESSMENT OF A DEEP-WATER FISH, THE ORANGE ROUGHY (HOPLOSTETHUS-ATLANTICUS), BASED ON AN EGG SURVEY, Marine and freshwater research, 46(5), 1995, pp. 819-830
An egg survey was carried out in 1992 to estimate the biomass of the o
range roughy stock that spawns off north-eastern Tasmania. Spawning oc
curs at depths of 700-1000 m around a single seamount from early July
through early August. Orange roughy have determinate fecundity, and th
eir fecundity and the depth distribution and development rate of the e
ggs are described elsewhere. A random stratified survey was designed f
or a limited area around the spawning site in order to sample the eggs
fully during their first day of development. The eggs were sampled wi
th vertical tows from 1000 m to the surface. There was no evidence of
significant mortality or other egg loss during the first day after spa
wning, when the eggs are mostly below 400 m. Subsequently, egg numbers
declined rapidly at an instantaneous loss rate of 0.036 h(-1), primar
ily through advection out of the survey area. The estimated biomass of
spawning fish was 27 445 t. From the proportion of non-spawning fish
in the population (29% of females and 10% of males) and estimated catc
h of spawners, total mid-season stock biomass in 1992 was estimated to
be 34 593 t with a coefficient of variation (CV) of 47%. A bootstrap
estimate of the variance indicated a somewhat lower CV of 41%. The poi
nt source of spawning activity appears to give rise to a highly patchy
egg distribution, which results in the high valiance of the biomass e
stimate. From the catch history of the fishery, virgin biomass of the
stock was estimated to be approximately 96 900 t, and the stock was at
28% of virgin biomass at the end of season. An acoustic survey of the
spawning orange roughy provided a similar estimate of stock size. The
acoustic survey had greatly reduced statistical confidence limits (CV
= 6%) but far greater non-statistical sources of uncertainty, i.e. th
e species composition of acoustic targets and the target strength of t
hese species.