NOTE Davis and Farley: Size distribution of Thunnus maccoyn on their spawning ground 
385 
Table 2 
Percentage of spawning and nonspawning southern bluefin tuna caught at different bigeye indices (Pearson chi-square=24.1, 
«=326, df=4, PcO.OOl). 
BE index 
0.0-0. 2 
0. 2-0.4 
0.4-0. 6 
0.6-0. 8 
© 
bo 
o 
Total no. 
Spawning 
85.5 
71.4 
80.8 
56.4 
56.3 
227 
Nonspawning 
14.5 
28.6 
19.2 
43.6 
43.7 
99 
how these might change with fish size. This behav- 
ioral information is needed in order to interpret the 
patterns presented in our study and might best be 
achieved by pop-up satellite archival tagging. 
Because SBT aggregate by size and depth on 
the spawning ground, it is necessary to account for 
their distribution when determining the age and 
size structure of the spawning stock. This is espe- 
cially important when evaluating time series of size 
and age distributions in a fishery where there have 
been shifts in targeting between yellowfin and big- 
eye tuna. In the absence of reliable information on 
the depth of fishing, the most practical way of doing 
this in the Indonesian fishery would be to inversely 
weight the effort directed at the different levels of 
the BE index. The determination of spawning fre- 
quency should also take into account longline fishing 
strategies because it is likely that spawning frequen- 
cy is affected by fish size and because samples will 
be caught within or outside the spawning depth. 
If the increase in the proportion of SBT at the sur- 
face with size is due to spawning activity, then this 
feature will affect the contribution different size fish 
make to total annual egg production. A lower spawn- 
ing frequency, coupled with an exponential relation- 
ship between length and batch fecundity ( Farley and 
Davis 1998), would mean that individual small, but 
mature, fish make a relatively small contribution 
to total annual egg production. When making stock 
projections, it may therefore be more appropriate to 
adopt a parameter that reflects size at mean annual 
egg production rather than the currently accepted 
parameter of mean size at first maturity. Further 
histological research on the reproductive dynamics 
of small fish is required to better define these pa- 
rameters. Small fish were rarely caught when the 
histological work of Farley and Davis (1998) was 
carried out in 1992-95 but they have become more 
abundant in recent years ( Davis et al. 8 ) making such 
a study possible. 
H Davis, T. L. O., S. Bahar, N. Naamin, and J. H. Farley- 
1998. Catch monitoring of the fresh tuna caught by the 
Bali-based longline fishery. Commission for the Conser- 
vation of Southern Bluefin Tuna scientific meeting, 23—31 
July 1998, Shimizu, Japan, Rep. CCSBT/SC/9807/6, 17 p. 
CSIRO Marine Laboratories, PO Box 1538, Hobart, Tas- 
mania 7001 . Australia. 
100-j 
8(H 
60 — j 
40-1 
t 
v? 
ere 
20- 
C 
o 
0^ 
r 
o 
u_ 
o 
100-1 
CL 
80- 
60- 
40- 
20- 
Shallow 
ft 
Spawning 
Deep 
Nonspawning 
i i ; i i 
0.0-0. 2 0. 2-0.4 0.4-0. 6 0.6-0. 8 0.8-10 
Bigeye index 
Figure 4 
Proportion of spawners and nonspawners in landings by the big- 
eye tuna index. 
