Summary of the capture of seabirds, marine mammals, and turtles in New Zealand commercial fisheries, 2002–03 to 2012–13

In New Zealand’s Exclusive Economic Zone (EEZ), government fisheries observers on-board commercial fishing vessels record the incidental captures of protected species, including seabirds, marine mammals and turtles. For fisheries with sufficient observer coverage, these data, combined with fishing effort data, allow the development of statistical models to estimate the total number of incidental captures. The current study updates previous bycatch assessments, providing total capture estimates of seabirds, marine mammals and turtles in trawl and longline fisheries for the period between 2002–03 and 2012–13 (with some models extending back to the 1995–96 fishing year).

There was a total of 4379 (95% credible interval (c.i.): 3654–5340) estimated seabird captures in all trawl and longline fisheries within the outer boundary of New Zealand’s EEZ in 2012–13. Most seabird captures occurred in trawl fisheries, with 2604 (95% c.i.: 2055–3465) estimated seabird captures, compared with bottom-longline and surface-longline fisheries that had 991 (95% c.i.: 666–1349) and 783 (95% c.i.: 567–1144) estimated seabird captures, respectively. Of the total capture estimate for 2012–13, 1658 (95% c.i.: 1355–2049) captures were of albatross, and 2721 (95% c.i.: 2079–3613) captures were of petrels and other seabirds. The estimation included captures in all trawl fisheries, and in small- and large-vessel surface-longline and bottom-longline fisheries (cut-off vessel lengths 45 and 34 m, respectively). Although small-vessel fisheries were associated with most of the seabird captures, observer coverage in these fisheries continued to be low, especially in small-vessel trawl fisheries.

Statistical models were also built to estimate total captures of seven seabird species or species groupings, including New Zealand white-capped albatross (Thalassarche steadi), Salvin’s albatross (Thalassarche salvini), southern Buller’s albatross (Thalassarche bulleri bulleri), white-chinned petrel (Procellaria aequinoctialis), sooty shearwater (Puffinus griseus), “other albatrosses”, and “other birds”. For New Zealand white-capped albatross, there was a total of 558 (95% c.i.: 435–724) estimated captures in 2012–13, compared with 487 (95% c.i.: 294–794) estimated captures of Salvin’s albatross, 259 (95% c.i.: 200–341) estimated captures of southern Buller’s albatross, 368 (95% c.i.: 242–576) estimated captures of sooty shearwater, and 586 (95% c.i.: 470–755) estimated captures of white-chinned petrel. For the two species groupings other albatrosses and other birds, capture estimates were 354 (95% c.i.: 219–598) and 1766 (95% c.i.: 1170–2654) captures, respectively.

For marine mammals, sufficient observer data allowed the estimation of captures of common dolphins, New Zealand fur seals, and New Zealand sea lions in different fisheries. Captures of common dolphins were estimated for the jack mackerel trawl fishery off the North Island’s west coast, as the majority of observed common dolphin captures occurred in this fishery (139 of a total 157 observed captures in all trawl fisheries between 1995–96 and 2012–13). In 2012–13, there were an estimated 15 (95% c.i.: 15–19) common dolphin captures in the large-vessel (90 m or longer) jack mackerel trawl fishery, and the estimated capture rate was 0.87 (95% c.i.: 0.87–1.11) dolphins per 100 tows. Both estimates were higher than those in the preceding fishing year, when there were seven (95% c.i.: 5–14) estimated common dolphin captures, with a capture rate of 0.43 (95% c.i.: 0.30–0.85) dolphins per 100 tows. Of the covariates included in the model, distance of the headline below the surface best explained common dolphin captures.

New Zealand fur seals were incidentally captured in a number of different trawl fisheries, involving a range of target species and fishing areas. Across all trawl fisheries, there were an estimated 398 (95% c.i.: 236–713) captures of fur seals in 2012–13, and the estimated capture rate in 2012–13 was 0.48 (95% c.i.: 0.28–0.85) fur seals per 100 tows. Most of the estimated captures were in the hoki trawl fishery, with 242 (95% c.i.: 114–534) estimated fur seal captures and a capture rate of 2.07 (95% c.i.: 0.98–4.57) fur seals per 100 tows. For other target fisheries, capture estimates in 2012–13 included trawls targeting middle-depth species (78 estimated captures; 95% c.i.: 29–189), southern blue whiting (26 estimated captures; 95% c.i.: 26–26), ling (15 estimated captures; 95% c.i.: 5–42), and hake (11 estimated captures; 95% c.i.: 8–21). Other trawl target fisheries had less than ten mean estimated captures in 2012–13, and included squid, mackerel, and scampi fisheries). Estimated fur seal captures in deepwater fisheries were close to zero (95% c.i.: 0–1). Low observer effort in inshore trawl fisheries (0.5% coverage in 2012–13) resulted in an uncertain capture estimate of 11 fur seal captures (95% c.i.: 0–49).

In addition to trawl fisheries, New Zealand fur seals were incidentally caught in surface-longline fisheries. In 2012–13, there were 112 (95% c.i.: 72–163) estimated captures of fur seals in surface-longline fisheries, and the corresponding estimated capture rate was 0.04 (95% c.i.: 0.03–0.06) fur seals per 1000 hooks.

The number of captures of New Zealand sea lions in subantarctic trawl fisheries was estimated for five combinations of target fishery and area, including the squid, scampi, and other trawl fisheries off Auckland Islands, the southern blue whiting trawl fishery off Campbell Island, and all trawl fisheries on the Stewart-Snares shelf. The total number of estimated captures across all strata was 33 (95% c.i.: 27–40) sea lions in 2012–13. This estimate was markedly higher than in the preceding fishing year, when there were an estimated 12 (95% c.i.: 5–21) sea lion captures. Most of the estimated sea lion captures were in the southern blue whiting fishery, with 21 (95% c.i.: 21–22) estimated captures and an estimated capture rate of 2.72 (95% c.i.: 2.72–2.85) sea lions per 100 tows. High observer coverage of 99.9% in the southern blue whiting fishery meant that the estimated captures were equal to the observed captures.

High numbers of sea lion captures in the Auckland Islands squid fishery over time resulted in the use of sea lion exclusion devices (SLEDs) that are fitted to trawl nets in this fishery. The use of SLEDs means that the number of sea lion that may have escaped the net and the post-escape survival of escapees are unknown. For this reason, the estimation of sea lion captures in this fishery includes the number of interactions, which is the number of sea lions that would have been caught had no SLEDs been used. The corresponding strike rate indicates the number of interactions per 100 tows. There were an estimated 54 (95% c.i.: 7–261) interactions in this fishery in 2012–13, and the strike rate was 5.26 (95% c.i.: 0.68– 25.41) interactions per 100 tows. The total number of interactions for all trawl fisheries was 83 (95% c.i.: 35–288) interactions, with a strike rate of 0.54 (95% c.i.: 0.23–1.87) interactions per 100 tows in 2012–13. The large uncertainty associated with these estimates highlights the difficulty of providing reliable estimates of the number of interactions and the strike rate. This uncertainty results from the data becoming increasingly biased towards tows that used SLEDs and a decrease in observed captures in recent years.

Observer data of incidental captures of sea turtles document 17 observed captures between 2002–03 and 2012–13. Most of these captures (15) were in surface-longline fisheries, with one observed capture each in inshore trawl fisheries and the snapper bottom-longline fishery. Based on the observer data, the capture estimate of sea turtles was 11 (95% c.i.: 5–20) individuals in 2012–13.