Batching of waste to reduce seabird numbers behind trawl vessels

Seabirds are attracted to fishing vessels by the discard of waste from fishing. While this waste provides an available food source, seabirds foraging near vessels are at risk of either being caught or struck by the fishing gear. Within New Zealand waters, there were 212 seabirds observed killed by trawlers during the 2006–07 fishing year. Birds feeding on fishing waste behind trawlers may be struck by the trawl warps. The mortality from warp-strike can be reduced by using mitigation devices, such as tori lines, that deter birds from entering the region between the stern and the warps. While tori lines are partially effective, numbers of warp interactions are reduced to close to zero if no offal or discards are discharged. Strategies that reduce the discharge of fishing waste, such as converting it to fish meal and retaining it on board, or holding waste and discharging it while the vessel is not fishing, are expected to greatly reduce mortality. Many vessels hold waste in a container and discard it at intervals, a practice known as batching, in order to reduce interactions with seabirds. In this study it is determined whether increasing the interval between batched discharges of offal reduces the numbers of albatrosses and petrels behind a squid trawl vessel. The experiment was carried out on a trawler fishing for arrow squid (Nototodarus sloanii) in the Stewart-Snares and Auckland Islands regions, between February 5 and March 14, 2008. All fishing waste was held in a 4.5 m 3 capacity container, and then dumped at prescribed intervals. Batches were discharged at either 30 minute, 2 hour, 4 hour or 8 hour intervals. Seabirds were counted in 40m and 10m radius sweep zones extending behind the stern of the vessel. Observations were made at 5 minute intervals before, during, and after discharge events. Separate counts were made of large birds (principally albatross species), small birds (principally petrels) and Cape petrel (Daption capense). Cape petrel moved away from the area during the trip, and these data are not considered. A statistical model is fitted to the count data from the 40m sweep zone, using Bayesian methods. The experiment demonstrates a clear relationship between discharge and the number of birds close to the stern of the vessel. During discharge events, there was an increase in the total numbers of both large birds and small birds. There was also an increase in the proportion of birds …