The accidental capture of sea birds associated with longline fisheries has been recognised as a factor in population decreases in certain sea bird species (Croxall and Prince, 1990; Weimerskirch et al, 1997). As New Zealand's waters are amongst those
used by longline fisheries, there is a government responsibility under the Fisheries Act 1996 and the Wildlife Act 1953 to manage accidental capture of sea birds. All New Zealand sea bird species, except one, are protected under the Wildlife Act. As New
Zealand is also a signatory to a number of international agreements, there are responsibilities internationally to address this issue.
Research undertaken, both in New Zealand and overseas, has discovered different mitigation measures and factors which affect accidental capture of sea birds (Brothers, 1991; Duckworth 1995). These measures include tori lines, night setting, thawed bait,
and bait throwers. Environmental factors affecting accidental capture of sea birds include moon phase, and fishing area (Duckworth, 1995). Brothers (1991) found that during the setting operation on pelagic longline vessels, most attempts by sea birds to
take the bait occur within the first 100m behind the vessel and rarely up to a maximum of 250m. It is during this period that baits are within the diving range of some sea bird species.
This report describes the development and testing of a device that sets the baits at a depth of three metres or more below the sea surface. The purpose of this device is to reduce the amount of time baits are within the diving range of sea birds.
The development of an underwater setting device (hereafter referred to as a USD) commenced in 1995, when Akroyd Walshe / Paul`s Fishing Kites were contracted to design and build a prototype. The results of this phase of the programme are reported in
Barnes and Walshe (1997).
This document reports on each stage of development and sea testing of the USD during 1997 and up until 1998.
TUNA LONGLINE FISHING IN NEW ZEALAND
A brief description of the method of fishing used in the tuna longline fishery is described in this section, to help readers understand how the USD is incorporated into the fishing operation.
The tuna longline fishery in New Zealand targets albacore, bigeye, yellowfin and southern bluefin tuna (SBT). Of these only SBT has a catch limit - 420 tonnes. The fleet is composed of approximately fifty domestic vessels and five chartered Japanese
vessels. (Ministry of Fisheries, 1997)
The fishing operation is a twenty four hour cycle. A 20 - 60 nautical mile (nm) mainline is usually stored on a hydraulically operated spool which is positioned in a variety of locations according to the deck layout of individual vessels. The line is
run through a series of pulleys or roller guides and the line set from the stern of the vessel.
Longliners vary in length from 10 - 50 metres. On some vessels the mainline is set by simply driving the vessel forward and the water resistance pulls the mainline from the mainline spool. Floats are interspersed at regular intervals along the length
of the longline. A length of rope between the float and the mainline controls the depth of the line. Other vessels use a device known as a line shooter. This pulls the line from the spool at a speed faster than the vessel is travelling through the water.
The line is set in a way that covers a range of depths as the slack line sags between the floats.
A line set without the assistance of a line shooter will set at very close to the same depth along its entire length, whereas for example a 20 nm line set with a shooter may cover a horizontal distance of only 15 nm but because of the line sagging between
the floats covers a much greater depth range. As the mainline leaves the vessel a snood line with a baited hook attached at one end is clipped onto the mainline and manually thrown overboard to the rear port side. The snood lines are usually between 10
and 20 metres in length. This line setting procedure continues for approximately three hours. Usually two or more crew are involved in the line setting process.
One of the crew is responsible for taking the correct hook from the snood box (which houses the hooked snoods when not in use), places the bait on the hook and throws the snood over the stern of the vessel. The other crew member takes the corresponding
clip and attaches it to the clip to the mainline after the baited hook has been thrown. Each line has anything from 800 to 2600 hooks present. The timing between traces varies between 7 and 9 seconds on the New Zealand vessels.
Radio buoys are attached at each end of the line and at it's centre. Buoys are placed along the line approximately every 12 hooks and each section between buoys is termed a basket. At the completion of this setting procedure, the line is left to soak
for approximately eight hours. It is then retrieved and the cycle is repeated.