Longline sink rates of an autoline vessel, and notes on seabird interactions

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.