October 2020
This is the final report for MIT2019-04: Optimum batching interval for discharge management on vessels in the scampi fishery.


Batch discharge of fish waste is used as a seabird bycatch mitigation tool in fisheries, including the New Zealand scampi trawl fisheries. Fish waste is accumulated aboard the vessel, and then discharged as rapidly as possible. Batching aims to reduce seabirds feeding around fishing vessels, thereby reducing the risk of fatal seabird interactions with fishing gear. Following experimental studies in New Zealand and overseas, batching is considered a best-practice mitigation strategy in guidelines from the Agreement on the Conservation of Albatrosses and Petrels (ACAP). For this strategy, ACAP recommends a minimum interval of two hours between batches. The Deepwater Group’s Operational Guidelines for New Zealand scampi fisheries specify a shorter minimum interval of 30 minutes between batches, but have a particular focus on avoiding discharges around setting or hauling of the net. Most seabird captures in scampi fisheries are net captures.

Conservation Services Programme project MIT2019-04 was established to review existing observer data with the aim of determining if an ‘optimum’ batch discharge interval could be identified. Scampi fishing occurs in five key regional fisheries in New Zealand, using target bottom trawling. Like other crustacean trawl fisheries around the world, bycatch in the scampi fishery is high. Statutory data from the Ministry for Primary Industries’ electronic reporting regime, introduced gradually from 2018, allows a characterisation of the retained and discarded catch by the different regional scampi fisheries; the SCI 3 fishery on the western Chatham Rise has the highest rates of bycatch fish discards and the greatest proportion of catch retained in processed form. Numbers of seabirds observed around scampi fishing vessels do not show consistent differences between regions, although the composition of the seabird assemblage does vary regionally. Seabird captures have, however, varied with the highest estimates in the Chatham Rise and subantarctic scampi fisheries.

Detailed observational protocols on seabird attendance at vessels were developed for particular experimental work and required a dedicated observer. Routine data collection by fisheries observers currently only provides qualitative, trip-level information on vessel batching practices and is primarily focused on assessing vessels’ adherence to their Vessel Management Plans. The limited resolution of data on batching precludes a detailed investigation into the effects of variation in batch interval and batch discharge times on either seabird attendance or seabird captures. If fisheries managers require a more detailed understanding of how variation in batching parameters affects seabird attendance around scampi vessels, we suggest that an experimental approach would be more efficient than increasing the detail of observational data collection. Experimentation allows the covariates of interest to be modified while other covariates are held constant. In contrast, analyses of observational data have to address between-vessel variation in addition to temporal and spatial variation, and may detect little variation in batching practice.

In future, simple image-based data collection technologies could be developed to provide information on both batching practice and seabird attendance in place of intensive data collection by observers.

Publication information

Middleton, D and Abraham, E. 2020. Assessing management of fish waste discharge in the scampi fishery. Final report for MIT2019-04 prepared by Pisces Research Ltd for the Conservation Services Programme, Department of Conservation. 36 p.


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


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