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«P. Barnes and K.A.R. Walshe Published by Department of Conservation P.O. Box 10-420 Wellington, New Zealand 1 Science for Conservation presents the ...»

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Underwater setting methods to

minimise the accidental and

incidental capture of seabirds

by surface longliners

Report on a prototype device developed by

Akroyd Walshe Ltd


P. Barnes and K.A.R. Walshe

Published by

Department of Conservation

P.O. Box 10-420

Wellington, New Zealand


Science for Conservation presents the results of investigations by DoC staff, and by contracted science

providers outside the Department of Conservation. Publications in this series are internally and externally peer reviewed  October 1997, Department of Conservation ISSN 1173–2946 ISBN 0–478–01958–0 This publication originated from work done by P. Barnes and K.A.R. Walshe, Ackroyd Walshe Ltd., P.O. Box 28814, Auckland 5; funded by the New Zealand commercial fishing industry through the 1995/96 Conservation Sciences Levy as contract 95/3004, part of 1995/96 CSL Project CSL 2A. It was approved for publication by the Director, Science and Research Division, Science Technology & Information Services, Department of Conservation, Wellington.

Cataloguing-in-Publication data Barnes, P.

Underwater setting methods to minimise the accidental and incidental capture of seabirds by surface longliners : report on a prototype device developed by Akroyd Walshe Ltd / P. Barnes, and K.A.R.

Walshe. Wellington, N.Z. : Dept. of Conservation, 1997.

1 v. ; 30 cm. (Science for conservation, 1173–2946 ; 66.) ISBN 0478019580

1. Longlining (Fisheries)––Environmental aspects––New Zealand.

2. Bycatches (Fisheries) 3. Seabirds––New Zealand––Mortality.

I. Walshe, K. A. R. II. Title. III. Series: Science for conservation (Wellington, N.Z.) ; 66.

639.22 20 zbn97–101660 2




1. Introduction and objectives 5

2. Methods 7

2.1 First stage trials 7 2.1.1 U tube design 7 2.1.2 Fishing gear 7

2.2 Second stage trials 7 2.2.1 U tube design 7 2.2.2 Fishing gear 8

3. Results 8

3.1 First stage trial 8

3.2 Second stage trials 9

4. Discussion 9

4.1 Assessment of the U tube setting device 9

4.2 Modification and improvement to the U tube design 11

5. Conclusions 12

–  –  –

3 Abstract The accidental/incidental capture of seabirds by longline gear may be causing a significant decline in the numbers of some species — most notably albatross (Diomedea spp.) — in the Southern Oceans.

In response to the sea bird problem, the New Zealand Department of Conservation commissioned a programme funded by the fishing industry through the Conservation Services Levy in 1996 to develop a device to set baits underwater on commercial surface longliners.

Trials on two U tube devices developed by Akroyd Walshe Ltd are described. A device with the tube opening facing forward was unsuccessful in flushing bait to the setting depth; however, a backward facing U tube succeeded in flushing the bait on all trials to the setting depth of 1.5 m.

A second trial was undertaken with the backward facing U tube. On all trials the U tube successfully flushed the baits to the required 3 m depth.

The U tube is a simple mechanical device requiring only one additional item of equipment — a deckhose. The device requires minimal operator skill, and is easy to build and maintain.

The U tube has the potential to set baits at greater than 3 m. However, this capability has not been tested. Further study is required to test the device under commercial tuna vessel operating conditions and to assess the effectiveness of the device in thwarting seabirds from capturing the baits underwater.

1. Introduction and objectives When longlines are set sea birds may retrieve the baits on the surface or at shallow depths. Problems occur because bait loss effects fishing efficiency — bait loss rates of 70% have been recorded (Lokkeborg 1996), and because sea birds become snared by hooks and drown. Bird mortality caused by longlining has been suggested as the cause of decline in some albatross populations (Gates 1993).

When no measures are taken to keep seabirds away from the stern of vessels, most bait taking occurs within the first 100 m behind the ship (Brothers 1991).

At greater distances the bait is submerged to a depth beyond the reach of an albatross; although petrels, which can dive to a greater depth than albatrosses, can still retrieve baits which are subsequently taken off them at the surface by the larger albatrosses.

A number of measures are already used by surface longtliners to reduce accidental capture of seabirds. These include weighted side lines, bait throwers, thawed bait and night setting.

5 In December 1995 the New Zealand Department of Conservation requested tenders for a programme to develop underwater setting devices for surface longline vessels. This programme, which was funded by the fishing industry

through the Conservation Services Levy, had the following objectives:

1. Design devices for use on surface longliners that enable the bait to be set underwater at a depth of 3 m.

2. Build working prototypes.

3. Fit the devices to surface longline vessels, and undertake sea trials.

4. Refine the devices to overcome any difficulties identified during sea trials.

This report outlines a project funded to achieve these objectives.

Several concepts were considered by the authors resulting in five prototype

designs. The concepts considered were:

1. An enclosed tube carrying the backbone, snood and baits down to the release depth A design prototype was prepared but rejected because of potential entanglement problems between the backbone and snoods in the tube.

2. Venturi assisted water flow

Two variations were considered:

Firstly, the use of a funnel at the base of the longline tube. The funnel would be positioned on the upstream side (just above the lower end) of the longline tube. The water entering the funnel would accelerate into the longline tube and out the longline tube’s lower opening. This action would create a venturi action ‘sucking’ the water and baits down the longline tube.

The second variation considered was to introduce the venturi action at various points along the length of the longline tube. This would be achieved by perforating the longline tube’s upstream face at regular intervals throughout the 3 m depth range. These perforations would create ‘mini’ venturi systems throughout the length of the longline tube accelerating the snood and bait downwards and out the lower opening.

3. U Tube A third concept was to carry only the snood and bait down to the required depth. The longline backbone would stream out externally from the U tube.

The snood and bait would travel the length of the U tube and be released into the surrounding water before the snood came under tension from the backbone line streaming astern of the vessel.

Although five setting devices were considered only two U tube designs were trialled.

Advice on the concepts was sought from Professor Peter Jackson of the Auckland University School of Engineering. Although none of the concepts were rejected as potentially unfeasible, the U tube design was thought the most likely to succeed. It was decided to trial the U tube designs, and only if these were unsuccessful would the venturi concepts be trialed. Subsequent trials demonstrated that one of the U tube designs was highly effective. Consequently no trials were undertaken on the venturi concepts.


2. Methods Two stages of trials were undertaken. The first stage trials evaluated two types of U tube design at setting depths of 1.5 m. The second stage trial evaluated the most successful first stage trial design to a setting depth of 3 m.

2.1 FIRST STAGE TRIALS This trial was undertaken on a 10 m commercial snapper longline vessel Mark IV in Sand Spit Harbour north of Auckland. The vessel was a single hull fibreglass construction with a deck 1 m above water level.

The trials were undertaken at vessel speeds between 8 and 12 knots. These speeds were determined to be comparable with the speeds used for commercial tuna longline setting.

2.1.1 U tube design The setting devices comprised U tubes and paravanes but without hinging devices or a bait trough. The tube was attached to a cross bar that secured the upper end to the vessel’s stern. A paravane at the lower end of the tube ensured the tube remained stable at the required water depth.

A deck hose was attached to the upper end of the tube to flush the baits down to sea level.

One of the devices had the U tube slot facing upstream while the other U tube has the slot facing downstream. The devices were designed to release the baits at a depth of 1.5 m below sea level.

2.1.2 Fishing gear A 4 mm longline backbone was used; with 2 mm diameter, 25 m long monofilament tuna snoods fitted with 18/0 Tuna Circle and Terashima TH 3.2 hooks holding 30 – 60 count squid and large Sanmar bait.

2.2 SECOND STAGE TRIALS These trials were undertaken on a 13 m vessel MV Frae in the waters around Kawau Island in the Hauraki Gulf. The vessel is a twin hull steel construction with a deck 1.3 metres above water level. Trials were conducted at a range of speeds from 8 to 12 knots in calm seas.

–  –  –

8 Backward facing U tube The baits were flushed down the tube to sea level and carried underwater down the full length of the tube without obstruction. It was observed that, when the deck hose was not used and the vessel was underway, the water in the pipe at sea level was lower than the ambient water level. It would appear that there was a positive flow of water into and down the tube. Further observations by diving and underwater video record showed the bait and water in the tube was carried down the full length of the tube even at vessel speeds of less than a knot. In 87 trial releases of bait there were no snags as the bait moved down the tube.


Two hundred and three releases of bait were made down the tube, none of the releases snagged in the tube. On 27 releases the baits were timed from entry into the tube and release out of the 6 m long tube. On average the bait took 6.7 seconds to travel down the tube, the maximum time of release was just over 9 seconds.

To test the retention of the bait in the tube, the snood was stopped at various lengths as it travelled down the tube. At each point the bait remained in the tube, and (providing the operator held the snood close to the tube inlet) could be retracted up the tube without escaping. If the operator lifted the snood away from the tube, the line was captured by the water flow outside the tube and the snood and bait released to the surrounding water.


Both trial stages demonstrated the effectiveness of the backward facing U tube

setting device. The basic principles of the device are as follows:

1. Water flushes the baits to sea level down the U tube.

2. When the setting device is towed the sea water flows freely around the front of the tube. Because of the horizontally elliptical cross section of the angled tube, the water accelerates towards the open slot. If the tube were not slotted considerable drag (suction) would occur directly behind the tube.

The slot, however, provides an easier path to the water to escape.

3. The water enters the tube and adds to the velocity of the flushing water already in the tube. All water in the tube is constrained down the complete tube length and exits at a point at the bottom of the tube.

4. Towing the device faster through the water increases the water flow down the tube.

9 Because the trials were undertaken in relatively calm seas the tension of the setting device on the fishing vessel in rough seas has not as yet been tested.

However two simulations gave an indication of the performance of the setting device in a yawing and pitching sea.

While steaming at 12 knots the vessel made a number of turns to simulate a yawing sea. The setting device moved in the opposite direction to the turn but was still effective in releasing the baits from the base of the U tube.

The tension of setting device in a pitching sea was tested by pulling the tube upwards by hand as the vessel steamed at longline setting speed. As one operator could pull the U tube upwards there appeared to be little stress caused by a pitching action.

These simulations however do not reflect rough sea conditions where yawing and pitching actions occur simultaneously and with more rapidity and frequency than our simulations could produce.

As the vessel pitches in a sea, water may be forced up the exit point of the tube. The entry of such water might impede the downward flow of the bait and exit from the tube. The horizontal paravane however acts as a block to stop water being forced up the exit point of the tube.

The advantages of the U tube longline bait setting device can be summarised as


Limited operator skill required The device is a simple mechanical device requiring no special operating skills to manage the U tube. The lack of moving parts or supporting devices (such as hydraulic or electrical motors) makes the device very safe to operate.

Low construction cost and long operating life The U tube is a very simple construction, and uses low cost materials. Many fishers have welding skills and provided with the tubing and the diagrams in this report could manufacture the U tube in a home workshop. Running repairs at sea are likely to be minimal and well within the skill of most mechanically-minded fishers. The mechanical nature of the device and the simple construction and robust materials should ensure a long operating life.

There is little tension on the U tube in the water. Because of the cross section shape of the U tube the device creates little drag. The paravane creates only sufficient force to hold the U tube down and steady — during the trial the U tube can be lifted up by hand while the vessel steamed at 12 knots.

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