Trends in mean length and maturity stage of toothfish were examined using observer data from the 1998, 1997 and 1996 toothfish seasons. Toothfish are distributed down the shelf slope in relation to their size, an approximately linear relationship existing between depth and fish length. A statistically significant pattern of changing mean length at depth with month was found in both 1997 and 1998 seasons, suggesting movement of animals both up and down slope at particular times of year. Months with a high mean length at depth are May and August, and with a low mean length at depth are April and July. Examination of maturity data suggests that in addition to a major spawning event in late July/August, there may be a small spawning event in April/May. The observed shifts in length at depth may be associated with movement of animals to preferred spawning depths. A general model would be that toothfish form spawning groupings at about 1000-1300m depth. Just prior to spawning, animals move (generally downslope) to this depth, dispersing immediately afterwards, mostly upslope. Behaviour appears to be different at Shag Rocks and South Georgia, but there is some evidence for exchange of animals immediately post spawning between the two areas. There may be two (or more) spawning periods during the winter, with the larger spawning period being late July and August.
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