The efficacy of warp strike mitigation devices: Trials in the 2006 squid fishery

Recent research has shown that collisions with trawl warps are a source of seabird mortality in trawl fisheries. To minimise this mortality, a number of devices have been developed to prevent seabirds from being struck by the trawl warps, or from congregating at the stern of the vessels. Designs currently used in New Zealand include twin tori lines, bird bafflers and warp scarers. Legislation now requires that at least one of these devices is used by trawlers fishing within New Zealand waters.

Experimental trials of warp strike mitigation devices were carried out in the 2006 southern squid trawl fishery, involving 18 observed vessels. The trials were developed co-operatively by a group that included representatives from the fishing industry, the Ministry of Fisheries, the Department of Conservation and WWF-NZ. Tori lines, bird bafflers (the 4 boom variety) and warp scarers were compared using standardised seabird strike measurements: the number of birds that struck either the warps or the mitigation device during a 15 minute observation period. On the participating trips, different warp strike mitigation treatments were used on different tows according to a randomised experimental design.

Key results from the trial are:

• Tori lines were most effective at reducing seabird strikes on trawl warps. They reduced the warp strikes to between 5% and 20% of their frequency without mitigation.
• Both bird bafflers and warp scarers produced a significant reduction in the number of large bird strikes on the warps, to 35—90% of the level in the absence of mitigation.
• Bird bafflers and warp scares also reduced small bird strike rates on the warps, but this reduction was either not statistically significant (for bafflers) or only marginally significant (for warp scarers).
• Although there is limited data, the analysis suggests that the bird bafflers are more effective on vessels where the trawl block is closer to the waterline.
• Strikes on the mitigation devices themselves were generally less than 20% of the no mitigation strike rates on the trawl warps. The clear exception was the tori lines, which had a strike rate similar to that recorded on the trawl warps without mitigation.
• There is no information on whether seabird strikes on tori lines are as dangerous as those on trawl warps. Observer opinion on this was divided. Recorded comments indicate that at least some seabird interactions with tori lines are dangerous for seabirds. However, no dead or injured birds were retrieved from the tori lines, or any of the mitigation devices, during the experiment.
• The trials again highlighted the discharge of offal as the main factor influencing seabird strikes. Almost no strikes were recorded when there was no discharge, and strike rates were low when only sump water was discharged.
• There was considerable between vessel variability in strike rate. When no mitigation was being used, the vessels that had bafflers on board had a higher strike rate than other vessels in the trial.
• Five of the seven bafflers in the trial did not comply with the required specification as the dropper lines did not reach to with 0.5 m of the water surface. Some bafflers had dropper lines that were prone to tangling.
• The weights used on warp scarers typically did not allow them to be deployed sufficiently down the warp without entanglement. This often left the few meters of the warp closest to the water unprotected. The clips were not sufficiently robust, and did not pass splices etc. Some vessels had trouble deploying the warp scarers safely. The devices should prove more effective if these technical problems are solved.
• Warp scarers and tori lines must be well maintained to meet the required specification (i.e. replacing lost/damaged/dirty streamers etc.)