Mincing, mealing and batching: waste management strategies aimed at reducing seabird interactions with trawl vessels

Experiments were carried out on two fishing trawlers to test whether mincing all waste to a paste before it was discharged reduced the number of seabirds around the vessels. The first trial was on a mid-water trawler targeting hoki (Macruronus novaezelandiae), and the experiment on this trip compared three treatments (1) mincing all waste (2) discharging unprocessed waste and (3) mealing all waste and reducing discharge to sump water. The second vessel was bottom-trawling for squid (Nototodarus sloanii). There was no meal plant on board and the mealed treatment was replaced with a batching treatment (4) where waste was held and discharged in batches. The response to the experimental treatments was determined by counting the number of birds within a 40m-radius semi-circular sweep zone behind the vessel stern. Results from the first experiment showed that mincing reduced the numbers of large albatross (Diomedea spp.) feeding around the vessel, but had no significant effect on other groups of seabirds. In contrast, mealing all waste had a marked effect on several of the bird groups. In particular, the abundance of small albatross (principally Thalassarche spp.) within the sweep area was reduced to 5% of the number that were there when unprocessed waste was discharged. On the second trial a smaller mincer was used. The vessel was bottom trawling, and there were problems with rocks going through the waste stream. The batched treatment was compromised by the limited ability of the vessel to hold waste, with only a 10 to 20 minute delay between discharge events being achieved. Despite low numbers of observations on the second trial, the minced treatment suggested a reduction in the numbers of all albatross within the observation area.