December 2018
This is the final report on characterising discharge management in small-vessel trawl and longline fisheries.


Discharge is a major driver for seabirds to attend fishing vessels, increasing the risk of interactions and seabird captures. Discharge (material including unwanted whole fish, invertebrates, baits and fish processing waste) and its management has had extensive research effort, reflected in best-practise guidelines and regulatory frameworks, but most studies to date have involved large-vessel fisheries.

Discharge management practises and effects in smaller-vessel fisheries (<28m vessel length) remain relatively unknown. In New Zealand, discharging practises are highly variable. Despite lack of regulation and potential operational constraints, discharge is actively managed in parts of the smaller-vessel fleet. To reduce risk to seabirds it is important to understand discharge practises in smaller-vessel fisheries, what factors influence discharging practises, and how discharging practises influence seabird bycatch.

This work characterises discharging practises on observed trawl and longlining vessels smaller than 28m and explores whether and how discharging practises influence seabird bycatch events. Observer data showed that haul discharging was actively managed in 25–35% of longline operations, and discharge was always retained during setting. Trawlers rarely discharged material during hauling and actively managed discharge to reduce seabird risk in about 40% of trawl fishing reviewed. Most active management of discharging reported for bottom longline (BLL) operations involved offside discharging, or on the haulside in hauling breaks.

On surface longline (SLL) trips, discharge management primarily involved discharging in batches or in haul breaks, on both sides offside and haulside. Most trawl operations limited discharging to the tow stage, but about 15% also discharged during shooting. Discharge batching was documented more often for SLL than for BLL (18% cf. 7% of trips, respectively), and was documented for 11% of trawl trips. 

Discharge location was important for both bottom and surface-lining, with lower seabird capture rates with offside discharging than haulside discharging, and holding untaken baits during hauling also reduced capture rates. In observed trawl fishing, seabird captures rates were lowest when a bird baffler was used, and appeared lower with net cleaning, illustrating the combination approach required for effective seabird mitigation. However, discharge management practises were not consistent within fleets or between trips of the same vessel, so bird capture risks will also vary. 

Recommendations include a range of discharge management actions for liaison programmes to reinforce or progress with relevant fleets, and suggestions for next steps to advance discharge management work in smaller-vessel fisheries. Recommendations are also provided for enhancing data collection to improve understanding of the nature and extent of discharge management and protected species bycatch in New Zealand’s smaller-vessel fisheries.

Publication information

Rexer-Huber, K and Parker, GC. 2019. Characterising discharge management in small-vessel trawl and longline fisheries. Report to Conservation Services Programme. Parker Conservation, Dunedin. 43 p. 


Conservation Services Programme
Department of Conservation
PO Box 10-420
Wellington 6143


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