CCAMLR

At its last meeting, WG-FSA noted a declining trend in the mean lengths of toothfish caught around South Georgia and Shag Rocks over the period 1995 – 1999. We re-analysed the mean lengths of fish taken on individual longline sets from which length frequencies were sampled by observers and found the same declines in mean lengths for the period 1997 – 1999 as previously noted by WG-FSA, but also that this trend was halted in 2000. A GLM analysis of these data shows that both area and depth are highly significant factors in explaining part of this variation over time, but not all of it. Taking depth and area into account there is less of an apparent decline in mean length over the period 1997-1999. The overall picture that emerges is one of variable mean lengths, both within and between seasons but with no consistent overall trend. The analysis also suggested that the exploitable population around South Georgia and Shag Rocks is quite spatially and temporally heterogeneous in terms of its length distribution. Separately, it is known that the effort distribution is also highly heterogeneous. The combined effects of these findings were examined in a set of preliminary analyses of length densities by area and depth zone. The results indicate that the annual distribution of effort by area and depth zone does have an important effect on the overall length-specific selectivity and that it is also likely that historical changes over years in the depth distribution of effort have led to different length-specific selectivity curves applying in different years. Combined length-specific selectivity curves were estimated for each of the years 1997-2000. Notable differences were found amongst these, but there was a consistent tendency for fish over 90 -100 cm to have a lower relative selectivity than smaller fish.

A series of longline sink-rate trials were conducted with and without weights, from an autoline fishing vessel working the New Zealand ling (Genypterus blacodes) longline fishery on the Chatham Rise, New Zealand during July and August 1998. The autoline equipment is designed to sink without weights, and non-weighted longline line sink data were collected first to provide baseline information. Further trials were conducted with weights added to the longline as in normal fishing operations. A robust attachment method for Time Depth Recorders was developed. A tori line was used at all times by the vessel, and the design was refined during the voyage. The aerial section of the tori line appeared to provide an effective deterrent to most seabirds. Statistical analyses of the data from the line sink rate trials indicate that the weighting regimes used (5 kg per 400 m) had no effect on line sink rate. However, direct observations at sea indicated that weights did have an effect on line sink for 20–40 m either side of the attached weights. Data on line sink rate and tori line coverage suggest that quicker line sink rates could help decrease the incidental mortality of seabirds during autoline fishing. Seabird incidental mortality rate for the voyage was 0.0093 seabirds per 1000 hooks set. Grey petrels (Procellaria cinerea) accounted for 90% of the observed incidental catch; of which 90% were foul hooked rather than having swallowed a baited hook. Fourteen species of large seabird and 5 species of small seabird were observed interacting with the vessel. The seabird community constantly changed in size, species composition, and relative proportion of each species present. A large proportion of the seabirds present at any one time were Cape pigeons (Daption capense). Seabird behaviour about the vessel varied with fishing activity. Four distinct community behaviours were noted: set behaviour, haul behaviour, steaming (no offal) behaviour, and steaming (offal) behaviour. A night vision scope was trialed and found to be of limited benefit because of ineffective range, and the mono-colour vision.