Preliminary assessment of the South Georgia toothfish stock, 2007

1. The catch-at-length based CASAL model for toothfish at South Georgia is updated with data from the 2007 fishing season. The predicted spawning stock biomass and the yield is slightly higher than was estimated last year.
2. Improvements are made to the fit of tag data through (a) estimating a length-based ogive for tag-induced mortality, based on our 2005 survivorship experiment, in which smaller fish survive tagging better than large fish, and (b) by re-estimating from our tagging data that tag-induced growth retardation is also related to size, smaller fish suffering less growth retardation than larger fish, and that on average it is 1 year or more.
3. A new model is developed which uses estimates of catches at age from 1998 to 2006 (based on random sampling of the catch for age determination). Fits of all data (CPUE; catches at length for the early fishery; catches at age for the later fishery; tag recapture data) are all improved from earlier models, although some poor fits remain. The model estimates year class strength which corresponds, in some years, with estimates made from the South Georgia groundfish survey data.
4. Several of the requests of WG-FSA for developments of the South Georgia toothfish model have now been completed.
5. Note that the data used here are preliminary and lack some of the precaution previously included in assessments. We would recommend that only small changes to the current TAC be considered, and that the toothfish assessment is undertaken at periods of greater than one year.