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 m³ 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 40 m and 10 m 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 40 m 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 on the water, compared to in the air. The response of the birds to the individual discharge events was rapid, and when discharge ceased the numbers of birds on the water fell back to the level associated with sump water faster than could be resolved by the five minute observation interval.

In each of the different bird groups there were, on average, fewer birds present during discharge when there was a four or eight hour interval between discharges, compared to when there was a thirty minute interval. During the four and eight hour treatments, the best estimate was that bird numbers were reduced to between 56% and 89% of the number present when there was a 30 minute interval between batches. In all categories, the median number of birds decreased when the interval between batches increased from two to four hours. The same consistent decrease was not seen between the four hour and eight hour batch intervals. While there is a reduction in numbers as the batch interval increases to four hours, there does not appear to be any further benefit achieved by increasing the batch interval from four to eight hours.

During the trip, over 94% of the tows were less than 8 hours long. The eight hour storage capacity used for the experiment could be used to hold waste until the end of the tow, and discharge it when the vessel is not fishing. There could then be no interaction between the birds and the warps during the discharge. Rather than using the batching to reduce the numbers of birds behind the vessel, it could be used to eliminate discharge of offal during fishing.