Estimated capture of seabirds in New Zealand trawl and longline fisheries, to 2016–17

Citation

Abraham, E. R., & Richard, Y. (2019). Estimated capture of seabirds in New Zealand trawl and longline fisheries, to 2016–17. New Zealand Aquatic Environment and Biodiversity Report 226. 85 p.

Summary

A wide range of seabird species are caught in New Zealand commercial fisheries.
Managing the impacts of fisheries on seabird populations requires understanding what species are being caught and in which fisheries the captures are occurring.
When government fisheries observers are onboard commercial fishing vessels, they record the seabird captures that occur, and these records may be used to estimate total seabird captures.

This study presents the most recent annual assessment of seabird captures, including the 2016–17 fishing year. The assessment used statistical models 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 2016–17 fishing years for trawl fisheries, and the 1998–99 to 2016–17 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".

There were a total of 4210 (95% c.i.: 3508–5296) estimated seabird captures in trawl and longline fisheries (c.i., credible interval, the 95th quantile range of the posterior distribution) in 2016–17. The total estimate included 1767 (95% c.i.: 1493–2145) seabird captures in trawl fisheries, 1869 (95% c.i.: 1286–2909) captures in bottom-longline fisheries, and 573 (95% c.i.: 426–789) captures in surface-longline fisheries.

White-chinned petrel had the highest number of total estimated captures in 2016–17, with 995 (95% c.i.: 557–1882) captures of this species. The second highest estimate was 448 (95% c.i.: 349–574) captures of New Zealand white-capped albatross, followed by 435 (95% c.i.: 293–670) captures of flesh-footed shearwater.
Capture estimates for other species included 451 (95% c.i.: 272–810) captures of Salvin's albatross, 286 (95% c.i.: 189–442) captures of black petrel, 226 (95% c.i.: 157–326) captures of Buller's albatrosses, 398 (95% c.i.: 276–615) captures of sooty shearwater and 176 (95% c.i.: 67–465) captures of grey petrel. In addition to estimates for individual species, there were 563 (95% c.i.: 376–859) captures of other birds and 228 (95% c.i.: 148–362) captures of other albatrosses.

Considering capture estimates over time, there was a decrease in the total number of estimated captures for seven of the ten modelled species groups between 2002–03 and 2016–17. This decrease largely corresponded with decreases in fishing effort over this period. 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 2016–17 than in 2002–03.

Among different fisheries, large-vessel fisheries had sufficient number of captures to examine temporal trends. In large-vessel squid trawl fisheries, there was an initial decrease in albatross captures after the introduction of mandatory warp mitigation in January 2006, but capture rates of albatrosses have not clearly decreased since the 2006–07 fishing year. 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.

When the model was updated, captures were estimated for all previous years. Comparison of estimates between the current and previous models for the 2015–16 fishing year showed some changes, including a decrease in the mean estimated captures of black petrel in small-vessel longline fisheries. Overall, however, there was agreement between the estimates from the two models.

The estimation of seabird bycatch relies on observer data. Between 2002–03 and 2016–17, around 50%, 60%, and 40% of small bottom-longline, surface-longline and trawl vessels, respectively, had no observers placed on them for any fishing during the 14 years. The observer coverage across these fisheries was also low (at around 2%, 6% and 2%, respectively). With a core of vessels that had no observer coverage, and with low observer coverage overall, bycatch in the small-vessel fleet may not be adequately represented in the observer data. Increasing observer coverage in small-vessel fisheries, and ensuring that observers are placed across the fleet so that all vessels have at least some observer coverage, would help to ensure that estimates based on observer data reliably reflect protected species bycatch across New Zealand's trawl and longline fisheries.