Fur seal population estimate and bycatch analysis, Cook Strait hoki fishery

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POP2021-06 Assessment of New Zealand fur seal/kekeno bycatch by trawlers in the Cook Strait hoki fishery (PDF, 2,507 K)

Summary

The New Zealand fur seal / kekeno is the marine mammal most frequently caught in New Zealand commercial fisheries due to their large population size compared to other marine mammals, the spatial and temporal overlap between fur seal foraging areas and commercial fishing areas, and the likely attraction of fur seals to operating fishing vessels. This assessment focuses on bycatch from hoki trawling within the Cook Strait region, with the main following objectives: to identify fur seal breeding colonies and / or haul-outs within Cook Strait; to understand the interactions between fur seals and the hoki trawl fishery within this region; and to make recommendations that target knowledge gaps in order to help future mitigation actions that will reduce bycatch more effectively.

Boat-, land-based or helicopter surveys were carried out in the Cook Strait region, both in the North Island and South Island, in order to assess the status of seven breeding colonies / haul-outs, which included counts per age class. Bycatch data from hoki trawls were provided by the Ministry for Primary Industries for the calendar years 2012–2021, along with the database updated by government fisheries observers. For this period, we analysed the spatial and seasonal distribution of hoki trawls and Catch Per Unit Effort (CPUE). In addition, we estimated the probability of fur seal bycatch as a function of several factors, including fishing vessel (e.g. vessel length, tow speed), environmental (e.g. depth, distance from nearest colony / haul-out), and temporal characteristics (e.g. season, year), using a logistic regression framework. A test of proportions was used to infer whether a particular sex (female / male) and age class (juvenile / adult) was more likely to be caught by hoki trawls.

Only two breeding colonies around Cook Strait region were confirmed: Mātakitaki-a-Kupe / Cape Palliser and Ward Beach – Needles Point. A total of 2,016 hoki trawl tows were monitored by fisheries observers in Cook Strait in the period 2012–2021. From this, 112 tows had at least one fur seal caught, with a total of 157 individuals caught. Most hoki trawling with observers took place in autumn and winter, with a larger CPUE in winter and spring. Overall, the CPUE fluctuated annually, with a sharp increase noted in 2012–2013, 2015–2016 and 2020–2021. There is strong evidence for the probability of fur seal bycatch decreasing as vessel length increases, and increasing in winter and spring compared to autumn and summer. Regarding the demographic variables, most fur seals caught were adult males. The total estimate of the Cook Strait fur seal sub-population by direct count is approximately 1,800 individuals.

Recommendaitons include:

1. increasing fisheries observer coverage in spring hoki trawl trips due to higher levels of bycatch,
2. the establishment of a long-term monitoring programme to estimate abundance at the breeding colonies of Mātakitaki-a-Kupe / Cape Palliser and Needles Point, and
3. the assessment of other sources of human-induced mortality other than hoki trawl bycatch to understand the viability of the Cook Strait sub-population. Some suggestions are also provided for additional analyses that may be useful for assessing the interaction in more detail.

Publication information

Pavanato H, Schattschneider J, Childerhouse S, Briscoe D. 2023. Assessment of New Zealand fur seal / kekeno bycatch by trawlers in the Cook Strait hoki fishery. POP2021-06 final report prepared by Cawthron for Department of Conservation. 45 p. 

Additional presentation to CSP Technical Working Group by University of Otago on genetic analyses of fur seal bycatch found here.