100 
Fishery Bulletin 95( 1 ), 1997 
Figure 1 
Map of New Zealand showing spawning and feeding grounds of hoki, 
Macruronus novaezelandiae. 
biomass of western stock hoki present in winter to 
the recruited biomass of western stock hoki present 
in summer was 1:2.05 (Hurst and Schofield, 1995). 
Hurst and Schofield concluded that the total propor- 
tion of adult hoki that spawned in 1990 was between 
60% and 75%. 
The annual proportion of adult hoki that spawn was 
incorporated into the stock reduction analysis of hoki 
as a model parameter for the first time in 1992 (Sullivan 
and Cordue, 1992). A sensitivity analysis of the response 
of fishery indicators, such as stock and fishery risk to 
changes in various model parameters, found that they 
were particularly sensitive to the proportion of hoki 
that spawn in a given year (Sullivan and Cordue 1 ). In 
view of this sensitivity, a research program, to estimate 
more accurately the annual proportion of hoki that 
spawn and the maturity ogive of hoki at age in the 
western stock, was initiated. 
The spawning season for hoki begins in late June 
and can extend into mid-September (Sullivan et al. 2 ). 
Large fish tend to spawn earlier than 
smaller fish, and on the west coast, 
spawning extends northwards as the 
season progresses (Langley, 1993). Hoki 
are not caught in quantity in the vicin- 
ity of the west coast outside the spawn- 
ing season, and there is some evidence 
from commercial data and trawl survey 
data to suggest that hoki migrate to the 
west coast from the Southern Plateau 
during May-June (Ballara 3 ). Female 
hoki gain up to 40% of their total body 
weight as their ovaries ripen (Kuo and 
Tanaka, 1984), but for the remainder of 
the year, the ovaries are small, weigh- 
ing less than 1% of total body weight. 
Ovaries begin to ripen in April before 
female hoki migrate to their spawning 
grounds (van den Broek et al., 1981; Kuo 
and Tanaka, 1984). 
Evidence of spawning at other times 
of the year has not been reported (Kuo 
and Tanaka, 1984). Preliminary histo- 
logical work on hoki females collected 
throughout the spawning season indi- 
cates that hoki are either synchronous 
or group synchronous spawners. That 
is, their ovaries develop a single set of 
oocytes in a given season, and these 
ooocytes are released in a single event 
(synchronous) or over several spawning 
events (group synchronous) (West, 
1990). The same species in Tasmania 
develops a single set of oocytes in each 
season (Gunn et al., 1989). It is un- 
known whether the proportion of hoki that spawn in 
a given year is determined by environmental condi- 
tions or whether it relates to a shallow recruitment 
curve or even some nonannual endogenous rhythm. 
Our hypothesis was that if hoki develop their oo- 
cytes synchronously within each ovary, it should be 
possible to distinguish developing prespawners from 
nonspawners prior to the onset of the spawning sea- 
son and thereby estimate both the proportion of fish 
that would spawn and a maturity ogive based on his- 
tological characterization. 
In this study we collected monthly samples to moni- 
tor seasonal changes in the ovarian development of 
hoki prior to spawning. We also used random trawl 
surveys of the Southern Plateau in December and 
May in two consecutive years to determine 1) the 
3 Ballara. S. 1995. Natl. Inst. Water and Atmospheric Res., 
P.O. Box 14-901 Kilbirnie, Wellington, New Zealand. Personal 
commun. 
