Bottom longline sink rate testing
Introduction
This is the final report for MIT2021-03B Bottom longline sink rate testing. Published February 2024.Download the publication
Summary
The introduction of mitigation standards and subsequent changes to regulations require fishers to sink demersal longlines to a depth of five metres within the aerial extent of the tori line. This project involved recording sink profiles for a range of different gear configurations in the manual baiting ling, bluenose, hapuku, and bass demersal longline fishery.
Following review of current fishing practices, and consultation with stakeholders, a range of gear configurations were tested. Weight and float spacing and sequence were altered iteratively to identify various options for fishers to meet the regulations. Overall, 36 different configurations were tested with at least three repeats completed for 34 configurations.
Reducing weight spacing was found to be the easiest and most effective way of reducing sink times to depth. However, in the fisheries under consideration fishers tend to favour large weight spacings and multiple floats between weight to position hooks above the seabed. Increasing weight size was also a simple option but is limited by the amount of weight fishers were prepared to add to the line. Beyond 150 m weight spacing the use of modified floats was necessary to meet regulations. The modified floats had extra buoyancy but were attached to the mainline with a rope with a small weight adjacent to the mainline. These modified floats allowed for the longline to sink rapidly to a depth equivalent to the length of the rope, before the buoyancy of the float reduced the sink rate. Other options identified for reducing sink time to depth included increasing line tension, setting with the current, and using monofilament nylon as opposed to rope backbone.
Time depth recorder (TDR) positions three-quarters of the way after a weight generally sank slower than those midway between weights, unless weight spacing was relatively close. Tori line trials with a series of drag object on a thick rope demonstrated that 70 m aerial extent at 2.3 knots and 100 m at three knots was feasible. Options are presented which allow fishers to fish legally, including with large weight spacings and multiple floats between weights. Further testing is recommended to assess the practicality of the modifications suggested during a commercial fishing trip, and any influence on catch rates.
Publication information
Goad, D. & Olsen Z. 2023. Reducing sink times to depth in the small vessel manual baiting demersal longline fishery targeting species such as ling and bluenose. MIT2021-03B final report prepared by Vita Maris for the New Zealand Department of Conservation, Wellington. 22 p.