Longline sink rate verification

Download the publication

BCBC2020-11c: Longline sink rate verification (PDF, 1,574 K)

Summary

Sinking baited hooks to depth quickly, and underneath an effective tori line, is one of the most widely accepted and well-regarded methods of reducing seabird bycatch in both pelagic and demersal longline fisheries (ACAP 2019a and b).

Longline sink profiles constructed from data collected using Zebra Tech Wet Tag time depth recorders (TDRs) have shown that these particular recorders have limitations, particularly around recording the time that tags leave the vessel and the variable delay in recording after immersion in seawater. Conducting paired deployments with more accurate, but less ‘user friendly’, TDRs provides for assessment of the accuracy and suitability of Wet Tag data for estimating sink profiles and hook depths at given distances behind the vessel.

Hook sink profiles generated from TDR data need to be reliable, and comparable to bottle test data in order to reliably inform fishers of the depth of hooks at different distances astern. This data is particularly useful with respect to mitigation standards for demersal longliners.