Estimated captures of New Zealand fur seal, New Zealand sea lion, common dolphin, and turtles in New Zealand commercial fisheries, 1995–96 to 2014–15

Citation

Abraham, E. R., & Berkenbusch, K. (2017). Estimated captures of New Zealand fur seal, New Zealand sea lion, common dolphin, and turtles in New Zealand commercial fisheries, 1995–96 to 2014–15. New Zealand Aquatic Environment and Biodiversity Report No. 188. 66 p. Retrieved from http://fs.fish.govt.nz/Page.aspx?pk=113&dk=24557

Summary

Commercial fisheries in New Zealand waters interact with non-target species, with some of these interactions resulting in incidental captures and mortality. Captures of protected species are documented by fisheries observers, providing an independent record of the number and identify of the species captured in New Zealand’s Exclusive Economic Zone (EEZ). For species with sufficient numbers of observed captures, the observer records and fishing effort data are used to develop statistical models to estimate the total number of captures across species and fisheries in New Zealand waters. This study presents total capture estimates of marine mammals and sea turtles in trawl and longline fisheries for the period between the 1995–96 and 2014–15 fishing years (a fishing year runs from 1 October to 30 September). For marine mammals, observer data allowed the estimation of captures of common dolphin, New Zealand fur seal, and New Zealand sea lion in trawl fisheries. In addition, estimates were made of New Zealand fur seal and sea turtles in surface-longline fisheries. Estimates of captures were made with hierarchical Generalised Linear Models (GLMs), fitted using Bayesian methods.

Observed captures of marine mammals during the 2014–15 fishing year included 176 fur seal captures (127 in trawl fisheries, 37 in surface-longline fisheries, and 12 in set-net fisheries), 24 common dolphin captures (21 in trawl fisheries, two in set-net fisheries, and one in surface-longline fisheries); and eight New Zealand sea lion captures in trawl fisheries. In addition, there were observed captures of two dusky dolphin in trawl fisheries, and one bottlenose dolphin in surface-longline fisheries. A green turtle was observed caught during trawl fishing, and a leatherback turtle was observed caught during surface longlining.

For common dolphin, capture estimates were made for the 20-year period between 1995–96 and 2014–15. Separate estimates of common dolphin captures were derived for the large-vessel (90 m length and longer) jack mackerel trawl fishery off North Island’s west coast, and for all other trawl fisheries in New Zealand waters. The jack mackerel trawl fishery has been characterised by high numbers of observed common dolphin captures throughout the reporting period. In the 2014–15 fishing year, there were an estimated 21 (95% c.i.: 19 to 28) common dolphin captures in this trawl fishery. Across all trawl fisheries within New Zealand’s EEZ, there were an estimated 104 (95% c.i.: 50 to 189) common dolphin captures. Included in these captures were 60 (95% c.i.: 14 to 135) estimated common dolphin captures in flatfish and inshore trawl fisheries in the Taranaki area (which extends from north of Taranaki Penin- sula to Farewell Spit, including Tasman Bay). We recommend increased observer coverage of inshore fisheries in the Taranaki area to reduce uncertainty in the estimates.

Estimates of New Zealand fur seal captures were made in trawl and surface-longline fisheries for the period from 2002–03 to 2014–15. Estimated fur seal captures in trawl fisheries peaked in 2004–05 at 1487 (95% c.i.: 964 to 2370) captures, and by 2014–15, estimated captures were reduced to 536 (95% c.i.: 332 to 969) fur seal. Hoki trawl fisheries had the highest captures, especially in the Cook Strait area, where there were an estimated 160 (95% c.i.: 36 to 527) captures during 2014–15. Between 2002–03 and 2014–15, the average observed capture rate on small hoki trawl vessels in the Cook Strait area was 22.33 fur seals per 100 tows. Observer coverage in this fishery has been limited, and to reduce the uncertainty in estimated New Zealand fur seal captures, we recommend increased observer coverage of small hoki trawl vessels operating in the Cook Strait area. In surface-longline fisheries, there were an estimated 116 (95% c.i.: 87 to 151) fur seal captures during 2014–15. Most (94.7%) of the fur seal observed caught in surface-longline fisheries were released alive.

Captures of New Zealand sea lion were estimated for 20 fishing years between 1995–96 and 2014–15. Across all trawl fisheries, there were 12 (95% c.i.: 8 to 17) estimated captures of New Zealand sea lion during the 2014–15 fishing year. There was high observer coverage in the fisheries that frequently catch sea lion (88.3% observer coverage of squid fishing in the Auckland Islands area, and 100% observer coverage of subantarctic southern blue whiting fishing), so estimated captures were close to the observed captures. In subantarctic squid and southern blue whiting fisheries, sea lion exclusion devices (SLEDs) are used to reduce the number of animals that are caught. There were 81 (95% c.i.: 27 to 281) estimated sea lion interactions, providing an estimate of the number of sea lion that would have been observed caught, if no SLEDs had been used. There is very high uncertainty in the number of interactions. There is little information available to inform the model, as the interaction rate (interactions per tow) may vary from year to year. We recommend that the model structure is simplified, for example, by assuming a constant base interaction rate over all years, or an interaction rate that is proportional to the sea lion population.

Estimates were also made of turtle captures in surface-longline fisheries between 2002–03 and 2014–15. In 2014–15, there were an estimated 13 (95% c.i.: 2 to 35) captures of turtles in surface-longline fisheries, an average capture rate over all New Zealand surface-longline fisheries of 0.005 (95% c.i.: 0.001 to 0.015) turtle captures per 1000 hooks.