Sink rate of baited hooks during deployment of a pelagic longline from a New Zealand fishing vessel

Abstract

The sink rate of baited hooks during the deployment of a pelagic longline was determined using time depth recorders. This study was undertaken to determine how long baited hooks are within the known diving ranges of seabirds. During the vessel's normal fishing operations the unweighted baited hooks reached a mean depth of 5.57 m, 30 s after being deployed. The tori line aerial section covered the longline for a mean time of 29.3 s. With the addition of a 60 g lead swivel, the mean baited hook depth attained more than doubled to 13.44 m, and a further trial using a lead core cord in the snood configuration showed a small but significant increase in hook depth over the unweighted control (7.27 m). An increase in wind speed caused the baited hooks to sink faster (0.54 m for each Beaufort unit increase in wind speed). There was also evidence that the apparent wind direction while the vessel is setting also has an effect on the depths realised, but swell height had no detectable effect. The temperature of the bait also significantly affected the hook depth: partially thawed baits sank faster than thawed baits. A 1°C rise in bait temperature reduced the depth by 0.19 m. During normal line setting on this vessel using unweighted branchlines and a tori line, a considerable proportion of the baited hooks are within the known diving range of sooty shearwaters (Puffinus griseus), white chinned petrels (Procellaria aequinoctialis), shy albatross (Thalassarche cauta), black browed albatross (T. melanophrys), grey headed albatross (T. chrysostoma), and light mantled sooty albatross (Phoebetria palpebrata). The addition of a 60 g weight removes the baited hooks from the recorded diving range of all of these species except sooty shearwaters. Investigating the behaviour of different gear configurations along with evaluating the effect that environmental conditions have on the deployment of a longline will greatly add to our understanding of why some seabirds are caught.

Keywords:

• seabirds
• time depth recorders
• pelagic
• tuna
• longline
• sink rates
• bycatch
• New Zealand