Fishery Bulletin 95 ( 1 ), 1997 
I 10 
0.0 0.5 1.0 1.5 2.0 
Ratio of fish migrated to fish in survey area (x) 
Figure 7 
Estimated total proportion spawning (p) among age-7 + fish as a function 
of the migration ratio ( x ) in (A) 1992 and (B) 1993. 
ted curves are the functions at plus and minus two 
standard errors of this value. 
The solid vertical lines show the value of x with 
M = 0.25. The two dashed vertical lines shown are at 
plus or minus two standard errors of this value. 
Clearly the value of x is not at all well known. How- 
ever, the function is changing slowly over this range; 
therefore it is still possible to obtain a useful esti- 
mate of p. 
Discussion 
The number of studies that have attempted to mea- 
sure the level of nonspawning in adult fish and to 
determine its effect on population estimates used for 
stock assessment appears to be few. It is often as- 
sumed that although the steepness of the maturity 
ogive varies among species, it will always level out 
at or near 100% spawning (e.g. Hislop, 1984). Spe- 
cies documented to reach less than 100% spawning 
include orange roughy, Hoplostethus atlanticus, off 
southeast Australia at 55% (Bell et al., 1992), the 
brackish water burbot Lota lota (L) in the Baltic sea 
at 70% (Pulliainen and Korhonen, 1990), and the es- 
tuarine yellow-fin (surf) bream, Acanthopagrus aus- 
tralis at 50% (Pollock, 1984). Although the annual 
proportion of hoki that spawn is similar to that of 
these other species, the other studies did not adjust 
for population size or take migratory movements into 
account. Our estimates for hoki are close to the range 
indicated by Hurst and Schofield (1995) who did ad- 
just for population size. 
There were some potential sources of error that 
we could not measure. First, the number of undevel- 
oped fish surveyed on the Southern Plateau in May 
that were classified as nonspawners, which could 
have developed late and gone on to spawn, is un- 
known. This would lead to an underestimate of the 
proportion of prespawners. Second, if a number of 
fish remained undeveloped but migrated to the 
spawning ground anyway, the number of fish that 
