Publications
Prepared by
Biodiversity Group Environment Australia in consultation with the Threat Abatement Team
Environment Australia, 1998
ISBN 0 642 21420 4
The longline fishing practices and equipment described in Section 5 can be modified in a number of ways to reduce the likelihood of seabird by-catch. These modifications are termed mitigation measures. The measures focus on reducing by-catch during the critical period following release of the bait from the stern of the longline vessel until it has sunk out of reach of diving seabirds. Brothers (1991) estimates this period at approximately 10 seconds per hook in pelagic longline fisheries.
Effective mitigation of the threat relies on measures which:
The aim of this action is to develop a package of mitigation measures for each type of longline fishery operation which will minimise the seabird by-catch of that fishery.
The measures considered include existing measures which are known to be effective in reducing seabird by-catch and potential measures which are still under development.
Currently, there is limited or no data available on the level of seabird by-catch taken by demersal longline fisheries within the AFZ.
A number of mitigation measures are currently used by domestic and Japanese vessels in the AFZ. Bird scaring lines are mandatory equipment under Fisheries Management Regulation 19A of the Fisheries Management Act 1991. The Regulation requires all domestic vessels and Japanese vessels fishing south of 30°S to use a bird scaring line constructed in accordance with the regulation when setting their lines.
Other mitigation measures currently used on a voluntary basis in the AFZ include bait casting machines, night setting and bait thawing. The use of these measures can be attributed in part to the developmental work and education campaign being conducted by the Tasmanian Parks and Wildlife Service which highlights the conservation concerns and economic losses associated with seabird by-catch.
Some fishing industry and conservation groups in other parts of the world are developing mitigation measures. Australia and New Zealand are currently collaborating to develop economical underwater setting methods.
There are seven measures which have proven to be effective and a number of potential measures which are being developed. These measures are described below and summarised in tables 4 and 5.
Most seabirds caught on longlines are active during the day. Fishers can avoid catching birds by setting their lines at night (Harper 1987, Weimerskirch and Wilson 1992). This can result in a 60 – 96% reduction in seabird by-catch (Cherel et al. 1996, Alexander et al. 1997). This reduction in catch rate decreases around the time of a full moon. The deck lighting on the vessel can attract birds during night setting and should be minimised while ensuring the safety of the crew (Brothers 1991).
Weights can be added to the branchlines in pelagic longlines and the mainline in demersal longlines to hasten the sinking of baits, thereby reducing the time that a bait is available to birds. Weights in demersal longlines must be pushed overboard before the line tenses to avoid jerking the line to the surface and exposing the hooks (Brothers et al. 1995).
Seabirds congregate at natural feeding grounds and breeding sites at different times. These areas are often rich fishing grounds. Areas of high seabird by-catch can be closed to longline fishing on a temporary, seasonal or permanent basis to minimise the overlap between fishing operations and bird activity. For example, Croxall and Prince (1996) have identified the South Georgia shelf as an area of unacceptably high probability of seabird by-catch during March and May when the resident breeding albatrosses are consistently foraging in the area.
Seabirds sit on, or fly low over, the water behind the vessel when diving and attacking baits. A bird line suspended over the water above the area where the baits are being set deters birds from entering this area. The bird line consists of a main cord suspended over the stern of the vessel with a number of streamer cords attached which hang down over the water and move in an unpredictable way deterring seabirds from foraging on the baits. When constructed and set properly, bird lines can reduce mortality by 30 – 75% (Brothers 1991, Klaer and Polacheck 1995). The bird line is not uniformly effective in deterring all species. Skuas are bold foragers and will avoid the line by foraging near the back of the vessel (Brothers 1993). Bird scaring lines are currently the only mandatory mitigation measure in Australian waters.
One of the main problems with pelagic longlines is the sink rate of the baits. When baits are hand thrown from the vessel during setting they often get caught in the vessel's propeller turbulence which keeps them on the surface of the water. Bait casting machines provide for faster sinking of the bait by throwing the bait clear of the propeller turbulence (Brothers 1993). When used in conjunction with properly configured bird scaring lines, bait casting machines which contain a low arc of throw and facilities to vary the distance and side thrown, can achieve 40 - 80% reductions in seabird by-catch (Brothers 1993).
Seabirds are attracted to vessels by offal and discarded fish by-catch. This discharge typically occurs near the point of line hauling. Both the timing and location of discharge can be modified to make the vessel less attractive to birds during line setting and hauling (Cherel et al. 1996, Alexander et al. 1997).
Baits are stored frozen on board longline vessels. It is common practice to bait hooks with frozen baits and allow the bait to thaw in the water after setting. Frozen baits set in this manner float for longer than baits which are thawed prior to line setting (Brothers 1995). Reductions in by-catch of 50 – 70% have been suggested from use of thawed baits by Klaer and Polacheck (1995).
Most species of bait fish have swim bladders which are filled with air making them buoyant. These bladders decrease the sink rate of baits and should be punctured (Brothers 1995).
Seabirds often attack baits without being hooked (Brothers 1991). Smart hooks prevent birds from being caught by retracting the point of the hook until it has reached a safe depth. This measure is in the early stages of development.
As previously discussed, seabird by-catch occurs when baits are on or near the surface of the water during line setting or hauling. Underwater setting protects the baits during line setting by enclosing them in a chute or tube until they are outside the diving range of seabirds. This is a potential solution to the threat but also requires the greatest modification to vessels. Successful underwater setting methods must ensure that baits do not float to the surface after they have been set.
It is possible that during night setting deck lighting attracts birds and makes bait more visible, but the effect of deck lighting on seabird by-catch is yet to be determined. New vessel designs can consider seabird by-catch when designing lighting for their vessels. The safe operation of the vessel must be considered.
In addition to bird scaring lines described earlier there are a number of towed deterrents which could be used to reduce bird activity around the stern of the vessel during line setting. Buoys and other devices can be towed behind the vessels to disturb birds as they land to feed. This method is being advocated by fishers in North Pacific fisheries but its effectiveness has not been properly determined. Further international activities are underway to determine its efficacy.
Seabirds navigate using a geomagnetic compass. There have been some experiments conducted using magnetic fields to disturb the birds' compass to confuse them while in close proximity to the vessel. There has been no success in deterring seabirds using this method in tests (Brothers pers. comm.). The method also has potential Occupational Health and Safety considerations for the crew.
Sound is used to deter birds from airports and crops. A field study using equipment developed jointly by Japan Tuna and Blasting Technologies (a Japanese engineering company), assisted by the Tasmanian Parks and Wildlife Service through 1997 was unsuccessful. Responses by birds to the "scaring sound" were negligible.
A water cannon is used to prevent birds from entering the area astern of the vessel where the baits are sinking. This method has been used by Foreign longliners in the AFZ during winter 1997 with mixed results (AFMA Observer Reports).
In some fisheries fish or squid baits are replaced with lures which are potentially less attractive to seabirds. This measure shows potential for further development.
Live bait is used in some fisheries which can reduce by-catch rates.
To reduce the attractiveness of baits to birds and/or conceal them, baits could be dyed. The impact on fishing efficiency of this measure must be investigated.
| Measure | Stage of Develop-ment | Methods of Monitoring Use | Operational Use (Safety implications for crew) |
Relative Cost to Fishers in the AFZ |
Nature of Cost (fixed or ongoing) |
Impact of Catch per Unit Effort | Relative Effectiveness |
Impact on by-catch of non–seabird species |
| Night setting | Developed and tested | Observations | Safe provided lighting is adequate | High for domestic vessels | Ongoing | Reduced bait loss to birds | High | Increased by-catch of other species e.g. sharks |
| Line weighting | Partially developed | Observations | Caution required | Med | Fixed + Maintenance |
Unknown | High (if weight sufficient) |
Not known |
| Area closures | Developed and tested globally, but not for the AFZ | VMS, Aerial, Observations | – | High | Ongoing | Reduced access to stock | High | No by-catch in the closed area |
| Bird scaring lines | Developed and tested | Aerial, Observations |
Safe | Low | Fixed + Maintenance |
Reduced bait loss to birds | Med -High | None |
| Bait thawing and swim bladder puncturing |
Developed and partially tested | Observations | Safe | Low | Ongoing | Reduced bait loss to birds Increased setting preparation | Med | Not known |
| Bait casting machines | Developed and partially tested | Observations | Safe | Med | Fixed + Maintenance |
Reduced bait loss to birds Improved bait condition | Med (increased with use of bird scaring line) |
None |
| Offal discharge | Developed and partially tested | Observations | Safe | Low | Fixed | Reduced bait loss to birds | Low | Not known Impacts include artificial food provision |
| Measure | Stage of Develop-ment | Methods of Monitoring Use | Operational Use (Safety implications for crew) |
Relative Cost to fishers |
Nature of Cost (fixed or ongoing) |
Impact of Catch per Unit Effort | Relative Effectiveness |
Impact on by-catch of non–seabird species |
| Smart hooks | Not developed | Observations | Safe | Med | Initial equipment cost + replacement of lost equipment | Unknown | Unknown | Unknown |
| Underwater setting | Being developed outside Australia | Observations | Safe | Low - High depending on method | Fixed + Maintenance |
Reduced bait loss to birds Improved bait condition | High if baits are set deep enough so as to not resurface in turbulence |
Enables fishers to operate day or night and potentially reduces by-catch of other species |
| Deck lighting | Partially developed | Observations | Safety needs to be considered in planning | Low - Med | Fixed | Reduced bait loss to birds | Low (High in combination with night setting) | Reduced by-catch of species attracted to vessel by lights |
| Towed deterrents | Used in US fisheries | Observations | Potential gear conflict | Low | Fixed | Unknown | Unknown | Unknown |
| Magnetic deterrents | Tested | Observations | Unknown | Med | Fixed | Unknown | None | None |
| Sound deterrents | Limited testing | Observations | Unknown | Med | Fixed | Unknown | Very limited | Very limited |
| Water cannon | Partially developed | Observations | Wet crew | Med | Fixed | Unknown | Unknown | Unknown |
| Lures | Not developed | Observations | Safe | Med | Ongoing (considerable savings in bait costs) |
Unknown | Unknown | Unknown |
| Dyes | Not developed | Observations | Safe | Med | Ongoing | Unknown | Unknown | Unknown |