Mincing and mealing: A test of offal management strategies to reduce interactions between seabirds and trawl vessels

Recent work aimed at reducing the capture of seabirds on trawl warps has shown that warp strikes are associated with the discharge of waste while fishing. Offal management is therefore a key measure for reducing this incidental catch. An experiment to test the effect of two offal management strategies on the numbers of seabirds attending a fishing trawler was carried out during September — October, 2006. The two treatments were (1) sending all offal and waste to the meal plant, so that discharge was reduced to sump water, and (2) passing all waste through a mincer, with 25mm holes in the mincing plate, before discharging it. The two treatments were compared with a control, where all waste was directly discharged. To determine the response to the treatments, bird numbers were counted in a semicircular sweep of 40m radius, centered on the middle of the stern of the vessel and extending behind it. Birds were counted in five species groups (large albatross, small albatross, giant petrel, shearwaters and others, cape petrels) and three behavioural categories (flying, sitting and feeding). The trial covered 21 days, with 7 days of each of the two treatments and the control. During the experiment the vessel fished for hoki off the east coast of the South Island, and also on the Stewart Snares shelf. The mincing treatment did not significantly the numbers of birds within the count area, with the exception of feeding large albatross (Diomedea spp.). When the waste was mealed, numbers of feeding birds reduced markedly. The effect was most pronounced for the smaller albatross (Thalassarche spp.). Within this group, the numbers of feeding birds reduced to 5.3% of the number that were present when unprocessed waste was being discharged. A small number of seabird strike observations were made on the voyage. The vessel was using tori lines, and there were few warp strikes. The raw data suggested that strikes on the tori lines were reduced during each of the experimental treatments, however modeling suggests that this result is only significant for albatross during the mealed treatment.