The risk of seabird bycatch in trawl fisheries is increased by high numbers of seabirds attending vessels to feed on fish waste discharged. We conducted an experimental test of whether mincing fish waste prior to its discharge from a factory trawler reduced the number of seabirds attending the vessel.
The trial was conducted on a mid-water trawler targeting hoki (Macruronus novaezelandiae) in New Zealand waters, and the experiment compared three treatments:
- discharging ‘unprocessed’ waste (fish offal and whole discards),
- mincing all waste to a small particle size before discharge, or,
- converting all waste to fishmeal and reducing discharge to sump water.
The response to the experimental treatments was determined using seabird abundance within a 40 m-radius semi-circular area centred on the vessel stern.
Mincing reduced the numbers of large albatrosses (Diomedea spp.) feeding astern of the vessel, but had no significant effect on other groups of seabirds.
In contrast, reducing discharge to sump water resulted in a significant reduction in numbers of all groups of seabirds. In particular, the abundance of the small albatross group (principally Thalassarche spp.), and some smaller procellarids (e.g. sooty shearwater, Puffinus griseus, and white-chinned petrel, Procellaria aequinoctialis), was reduced to less than five percent of the number that were within the sweep area when unprocessed discharge was released.
While mincing significantly reduced the abundance of large albatrosses at the vessel stern, relatively small numbers of these birds attend trawl vessels in New Zealand waters and associated bycatch rates are low. In contrast, reducing the quantity of fish waste discharge by mealing resulted in reduced abundances of a wide range of seabirds at the vessel.
Therefore, compared to mincing, we recommend fish waste retention as a more effective management strategy for reducing seabird bycatch.