Longline sink rates on a bottom autoline vessel in New Zealand: draft

Longline sink rates were investigated using Time Depth Recorders on a bottom autoline vessel F.V. San Aotea in New Zealand. The objective of the project was to determine line sink rate, and the effect that adding weights to the line had on its sink rate. The vessel used Mustad autoline equipment that is designed to sink without weights, so non-weighted longline line sink rate data were collected initially to give an information baseline. Further trials were then conducted using added weights as would be used in normal fishing operations to test the effectiveness of weighting the longline as a method of accelerating line sink rate and thus avoiding incidental capture of seabirds. A new rapid attachment method for Time Depth Recorders was also developed and is documented. The study found the middle of an unweighted longline of this design sinks to 10m in a mean time of 63.0 seconds (n = 11, c.v. 16.7%), compared with the start of the longline which takes a mean time 31.1 seconds (n = 11, c.v. 30.4%) to reach 10m. The tori line aerial section covered the longline for a mean time of 26.3 seconds (n = 25, c.v.13.6%). The longline weighting trials indicate that the weighting regime used had no detectable effect on the overall line sink rate. However, observation indicated that the weighting regime did have quite noticeable effects on line sink rate for 20 - 40 m either side of the attached weight. Given the data collected on line sink times and tori line coverage it would seem that we need quicker sink rates to substantially decrease the incidental mortality of seabirds during auto line fishing.