October 2020
This is the final report for INT2019-03: Characterisation of marine mammal interactions with commercial fisheries and bycatch mitigation

Download the publication

INT2019-03: Characterisation of marine mammal interactions with commercial fisheries and bycatch mitigation (PDF, XX K)


Incidental captures of marine mammals in New Zealand waters have been documented for a range of commercial fisheries, including trawl, longline, set net, pots (and traps) and purse seine. The current study used existing information to characterise marine mammal interactions with these fisheries from 1992–93 to 2017–18, and reviewed potential mitigation techniques for reducing incidental captures in New Zealand waters. Additional analyses were conducted for species with significant numbers of captures or for which additional information was available; these species were common dolphin (Delphinus delphis), Hector’s and Māui dolphins (Cephalorhynchus hectori hectori and Cephalorhynchus hectori maui), New Zealand sea lion (Phocarctos hookeri) and New Zealand fur seal (Arctocephalus forsteri).

Across fisheries, trawl had the highest number of observed captures when aggregated by species and gear, followed by surface longline. Especially within trawl fisheries, there was often high variability in observed capture rates amongst regions and target species. Post-capture survival (whether an individual was recorded as alive upon release) was the highest for surface-longline fisheries, and almost all individuals caught in these fisheries were recorded as live releases. In contrast, post-capture survival was lowest for trawl fisheries, where almost all individuals observed caught were recorded as dead. The highest number of observed captures was pinnipeds, with New Zealand fur seal featuring the most frequently in observer records. Observed captures of this species were high in trawl fisheries and also in surface longline. New Zealand sea lion was almost exclusively caught in trawl. Common dolphin was the most frequently-recorded cetacean species in the observer data, followed by long-finned pilot whale and dusky dolphin.

The current characterisation also identified four specific associations between species and fisheries that resulted in significant numbers of captures: common dolphin and large vessel trawl fisheries targeting jack mackerel, Hector’s and Māui dolphins and set-net fisheries, New Zealand sea lion and trawl fisheries targeting squid, and New Zealand fur seal and trawl fisheries targeting southern blue whiting. Most of these fisheries have implemented mitigation measures over the study period, and reductions in observed and estimated captures were evident for all of these species in the time-series data. The ability to assess and estimate the extent of captures relies on comprehensive observations of fishing effort via the fisheries observer programme. High observer coverage is crucial for the recording of captures of rare species, for which even a low number of captures can have a significant impact on the population (e.g., Māui dolphin).

In addition, observer coverage needs to be adequately high to allow reliable capture estimates for species that are observed sufficiently often to inform a model in systematic bycatch assessments. The current analysis revealed that observer coverage was low overall across gears, although there were increases in some fisheries with high marine mammal capture rates in recent years. Observer coverage was particularly low in inshore trawl fisheries, which are characterised by high effort, while overlapping with the habitat of many coastal marine mammal species. This scarcity of observer data impedes a reliable assessment of interactions of marine mammal species with these fisheries. Mitigation efforts in New Zealand and elsewhere have focused on technical and other approaches to reduce or prevent incidental captures of marine mammals in commercial fisheries. Findings from this research highlight the challenges of testing the efficacy and effectiveness of different mitigation measures, and document the limitations of many approaches; they also show that successful mitigation techniques are often species- and fishery-specific.

Publication information

Tremblay-Boyer, L. and Berkenbusch, K. 2020. Characterisation of marine mammal interaction with fisheries and bycatch mitigation. Final report for INT2019-03 prepared by Dragonfly Data Science for the Conservation Services Programme, Department of Conservation. 66 p.


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


Back to top