NOTE Davis and Farley: Size distribution of Thunnus maccoyii on their spawning ground 
383 
Fork length (cm) 
Figure 1 
Length-frequency distribution (2-cm intervals) of southern bluefin tuna in landings by 
bigeye tuna index — a proxy for fishing depth. 
ter spawning. However, the two spent fish detected were in 
landings with a BE index >0.9. 
Discussion 
There is a systematic change in depth distribution with 
size over the whole size range of SBT caught on the spawn- 
ing ground. This pattern is clear, even though the BE 
index may only represent a crude approximation of depth. 
Deep longline catches are often contaminated by surface 
catches — 10% of bigeye tuna are caught when hooks are 
not at settled depths (Boggs, 1992). Also, both SBT (Gunn 
et ah ’; Davis and Stanley 6 ) and bigeye tuna (Holland et 
ah, 1990) might be caught outside their preferred depth as 
they regularly traverse the water column. 
5 Gunn, J. S.. T. Polacheck. T. L. Davis, M. Sherlock, and A. 
Betlehem. 1994. The application of archival tags to study the 
movement, behaviour and physiology of southern bluefin tuna, 
with comments on the transfer of the technology to groundfish 
research. ICES CM 1994/Mini: 21, 23 p. (Mimeo.] 
6 Davis, T. L. O., and C. A. Stanle. 2001 In prep. Vertical 
and horizontal movements of southern bluefin tuna, Thunnus 
maccoyii , in the Great Australian Bight observed bv ultrasonic 
telemetry. 
