Estimated captures of New Zealand fur seal, common dolphin, and turtles in New Zealand commercial fisheries, to 2017–18


Abraham, E. R., Tremblay-Boyer, L., & Berkenbusch, K. (2021). Estimated captures of New Zealand fur seal, common dolphin, and turtles in New Zealand commercial fisheries, to 2017–18. New Zealand Aquatic Environment and Biodiversity Report No. 258. 94 p.


Protected species such as seabirds and marine mammals frequently interact with commercial fishing operations, with some interactions leading to their capture and mortality. In New Zealand’s Exclusive Economic Zone, fisheries observers onboard commercial fishing vessels record these interactions to provide an independent record of protected species bycatch across different fisheries. Because observer coverage only extends to a proportion of the total fishing effort, bycatch assessments rely on the estimation of the total number of captures that would have been observed, if every vessel carried an observer. For protected species with sufficient numbers of observed captures, the estimation is based on the development of statistical models that incorporate observer and fishing effort data to derive the total number of captures in different fisheries.

This report documents the application of statistical models to provide total capture estimates of common dolphin (Delphinus delphi) in large-vessel mackerel trawl fisheries, New Zealand fur seal (Arctocephalus forsteri) in all trawl fisheries, and New Zealand fur seal and sea turtles (several species) in surface-longline fisheries.

For common dolphin, the estimation of total captures in trawl fisheries included data for the 16 fishing years between 2002–03 and 2017–18. Throughout this period, common dolphin were frequently observed caught in the large-vessel (90 m length and longer) mackerel trawl fishery off North Island’s west coast. Estimates of common dolphin captures were derived for this fishery. In 2017–18, there were no (0; 95% c.i.: 0 to 4) estimated common dolphin captures in the North Island large-vessel mackerel trawl fishery; the corresponding capture rate was 0.03 (95% c.i.: 0.00 to 0.24) common dolphin per 100 tows. Observer coverage in this fishery was high, at almost 90%.

Over the 16-year assessment period, there was a marked reduction (85%) in common dolphin capture rates in the large-vessel North Island west coast mackerel trawl fishery following the introduction of the Marine Mammals Operational Procedures. This code of conduct for mitigating marine mammal bycatch was introduced in the mackerel trawl fishery in October 2008; the lower common dolphin captures subsequent to its introduction likely indicate that these mitigation measures have been successful. Nevertheless, common dolphin are also captured in small-vessel fisheries, particularly in the northern South Island area (Taranaki to Golden Bay), but observer coverage in these fisheries is limited. It is possible that common dolphin captures in these small-vessel fisheries are considerably higher than in the mackerel trawl fishery, and increased observer coverage, in the Taranaki area in particular, would be necessary to reduce the uncertainty in estimated common dolphin captures in these trawl fisheries.

For New Zealand fur seal, capture estimates were based on observer data from trawl and surface-longline fisheries covering the 16-year period from 2002–03 to 2017–18. Updating the structure of the model for estimating fur seal captures in trawl fisheries led to improved characterisation of variability, which in turn resulted in a reduction in the uncertainty in the predictions. Across all trawl fisheries, there were an estimated 324 (95% c.i.: 233–462) fur seal captures in 2017–18, with an estimated capture rate of 0.44 (95% c.i.: 0.31–0.62) fur seals per 100 tows. The highest capture estimates were in hoki and southern blue whiting target fisheries. In hoki target fisheries, there were 190 (95% c.i.: 128–283) estimated fur seal captures in 2017–18, with a corresponding capture rate of 1.38 (95% c.i.: 0.93–2.05) fur seals per 100 tows. This target fishery had high capture estimates throughout the reporting period, particularly in the Cook Strait area. The southern blue whiting fishery had 100% observer coverage, so that the estimated captures were equal to observed captures. In this trawl fishery, there were 17 total fur seal captures in 2017–18, at a capture rate of 3.74 fur seals per 100 tows.

In surface-longline fisheries, there were an estimated 60 (95% c.i.: 32 to 96) New Zealand fur seal captures in 2017–18, at a capture rate of 0.026 (95% c.i.: 0.014 to 0.042) fur seals per 1000 hooks. The estimated captures were largely in fisheries targeting southern bluefin tuna.

Observers recorded four sea turtle captures in 2017–18, with all observed captures in surface-longline fisheries. These captures included the first record of loggerhead turtle (Caretta caretta) in New Zealand fisheries, in addition to one capture of green turtle (Chelonia mydas) and two captures of leatherback turtle (Dermochelys coriacea). Throughout the assessment period, observed turtle captures occurred exclusively in northern waters, around North Island and Kermadec Islands. The estimation of turtle captures focused on surface-longline fisheries, with observer data from 2002–03 to 2017–18. The model was updated to include area, chlorophyll, fishery, sea surface temperature, and vessel size covariates. In 2017–18, there were an estimated 53 (95% c.i.: 27 to 86) turtle captures, and the estimated capture rate was 0.023 (95% c.i.: 0.012 to 0.038) turtles per 1000 hooks. Estimated captures of sea turtle occurred mainly in bigeye tuna and swordfish target fisheries. The change in the model structure, particularly the inclusion of sea surface temperature as a covariate, increased the estimated number of captures relative to previous estimates.