Estimated capture of seabirds in New Zealand trawl and longline fisheries, 2002–03 to 2015–16

Interactions with commercial fisheries can lead to the incidental capture of non-target species, such as seabirds and marine mammals. In New Zealand waters, captures of seabirds and other protected species are recorded by government observers when they are on-board commercial fishing vessels. These data are used in bycatch assessments to estimate the number of seabirds that would be reported caught if every commercial vessel fishing within New Zealand’s Exclusive Economic Zone carried an observer.

This study presents the most recent bycatch assessment, including the 2015–16 fishing year. The assessment was based on statistical models that were used to obtain estimates of total seabird captures across all commercial trawl and longline fisheries. The time periods covered in this estimation were the 2002–03 to 2015–16 fishing years for trawl fisheries, and the 1998–99 to 2015–16 fishing years for longline fisheries.

The present assessment used a unified modelling framework to estimate incidental captures of seabirds for ten species and species groups: New Zealand white-capped albatross (Thalassarche steadi), Salvin’s albatross (Thalassarche salvini), Buller’s albatrosses (Thalassarche bulleri, combining both southern T. b. bulleri and northern T. b. platei subspecies), white-chinned petrel (Procellaria aequinoctialis), black petrel (Procellaria parkinsoni), grey petrel (Procellaria cinerea), sooty shearwater (Puffinus griseus), and flesh-footed shearwater (Puffinus carneipes).
Estimates were also derived for seabirds grouped as either “other albatrosses” or “other birds”.

The estimation revealed a total of 4517 (95% c.i.: 3760–5825) seabird captures in trawl and longline fisheries (c.i., credible interval, the 95th quantile range of the posterior distribution) in 2015–16. The total estimate included 1695 (95% c.i.: 1462–1998) seabird captures in trawl fisheries, 1982 (95% c.i.: 1343–3196) captures in bottom-longline fisheries, and 839 (95% c.i.: 632–1157) captures in surface-longline fisheries.

For individual seabird species included in the modelling, white-chinned petrel had the highest number of total estimated captures in 2015–16, with 1041 (95% c.i.: 577–2156) captures of this species. The second highest estimate was 496 (95% c.i.: 393–633) captures of New Zealand white-capped albatross, followed by 469 (95% c.i.: 299–751) captures of flesh-footed shearwater. Capture estimates for other species included 435 (95% c.i.: 291–679) captures of Salvin’s albatross, 360 (95% c.i.: 194–733) captures of black petrel,
358 (95% c.i.: 274–480) captures of Buller’s albatrosses, 266 (95% c.i.: 169–450) captures of sooty shearwater and 197 (95% c.i.: 72–517) captures of grey petrel. In addition to estimates for individual species, capture estimates for species groupings were 585 (95% c.i.: 404–866) captures of other birds and 306 (95% c.i.: 195–500) captures of other albatrosses.

There was a decrease in the total number of estimated captures for seven of the ten modelled species groups between 2002–03 and 2015–16. For three species, Salvin’s albatross, white-chinned petrel and grey petrel, there was no distinct decrease in total captures over the assessment period; only
white-chinned petrel had higher mean estimated captures in 2015–16 than in 2002–03. Across all trawl and longline fisheries, the mean number of seabird captures was lower in 2015–16 than in any year in the series. This decrease largely corresponded with decreases in fishing effort over the reporting period.

Large-vessel fisheries had sufficient captures to allow time-trends in capture rates to be estimated. In large-vessel squid trawl fisheries, there was an initial decrease in albatross captures after the introduction of mandatory warp mitigation before the 2006–07 fishing year, but capture rates showed an increase between 2013–14 and 2015–16. Capture rates of petrels in large-vessel squid trawl fisheries showed a distinct pattern of higher captures in alternate years. The reasons for this distinct fluctuation are unknown.

This analysis depends on observer coverage. Observer coverage remains low in small-vessel inshore fisheries where many seabird captures are estimated to occur. Increasing observer coverage in small-vessel fisheries would reduce uncertainty in bycatch estimates.