case study

Seabird third-wire interactions (US groundfish trawl fishery)

Archipelago was contracted by the National Marine Fisheries Service, Alaska Fisheries Science Center to test EM equipment as a means to monitor seabird interactions with trawl vessels’ sonar cable or “third-wire.”


Fishing vessels using trawl gear typically use a trawl sonar, mounted to the head rope, for information regarding how the net is deployed, where the net is in relation to the ocean bottom, and where the net is in relation to the target catch. The trawl sonar is often hard-wired to the ship through a cable typically known as the sonar cable or “third wire.” Seabirds attracted to offal and discards from the ship may either strike the hard-to-see cable while in flight, or get caught and tangled in the cable while they sit on the water due to the forward motion of the vessel.

Seabird mortality resulting from interactions with the third-wire has been documented, but observers deployed to these vessels Third Wire mitigationhave other critical duties important to fisheries management and science that preclude a comprehensive assessment of this issue. Monitoring the third-wire on several types of trawl vessels is the critical component to evaluating the threat to short-tailed albatross. Given the expected low frequency of interaction between third-wires and seabirds, the possible safety concerns for observers, and the straightforward monitoring required, this situation was considered an excellent candidate for field application of video monitoring.

Archipelago was contracted by the National Marine Fisheries Service, Alaska Fisheries Science Center to test EM equipment for possible use to examine seabird third-wire interactions. This pilot study field tested EM systems on the shore side delivery catcher vessels and groundfish factory trawlers conducting operations in the Bering Sea, U.S. Exclusive Economic Zone.

Third wire seabirdEM systems were deployed on five fishing vessels for fourteen fishing trips during a one-month period during the fall of 2002. Detailed analysis of about 200 hours of fishing imagery occurred, representing twenty catcher vessel fishing events and thirty-two groundfish factory trawler fishing events. Results from the study demonstrated that EM could effectively monitor seabird interactions with trawl third-wire cables. The EM system provided imagery of sufficient quality to detect the presence, abundance, and general behavior of seabirds during most daylight fishing events. EM-based imagery was also able to detect third-wire entanglements of seabirds although it was not possible at the time to determine the cause of these entanglements. In regard to monitoring seabird interactions with trawl third-wire, EM would be very suitable for monitoring the use and effectiveness of mitigation measures.

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