Entanglement of cetaceans in pot/trap lines and set nets and a review of potential mitigation methods
Introduction
This is the final report for the MIT2016-02: Entanglement of cetaceans in pot/trap lines and set nets and a review of potential mitigation methods and replaces all previous and preliminary reports for it. Published September 2017.Download the publication
Summary
Between 1984 and 2017, there were 44 reported large whale entanglements in NZ waters, of which 39 were attributable to pot/trap and set net fisheries.
64% of large whale entanglements involved rock lobster and 'likely' rock lobster gear, 21% of entanglements involved set net gear and 15% of entanglements involved either rope from an unknown gear type, or the gear involved in the entanglement was unknown.
The outcome of entanglement events was variable. 29% of all documented entanglements were fully disentangled, with 10.5% partially disentangled. 10.5% of whales shed gear on their own (without intervention). Conversely, 18% of entanglements were linked to the death of the individual, either directly or indirectly, and the fate of 32% of entangled whales remained unknown.
The risk of entanglement to NZ populations of humpback and southern right whales is likely to be low. Risk to killer whales in NZ waters is likely higher, as is the risk to an individual animal once entangled.
The individual animals at greatest risk of entanglement are humpback whales on their northern migration along the east coast of the South Island. This timing and location coincides with a high level of commercial rock lobster fishery effort.
The recovery of whale populations is likely to lead to more frequent interactions with fisheries and heighten the need for adequate mitigation methods.
There are three main categories of mitigation employed to address the entanglement of large whales: acoustic deterrents; gear modifications; and management modifications.
Despite global efforts to mitigate the entanglement of large whales, few gear modifications have proven successful in reducing documented entanglement numbers. Acoustic deterrents have shown mixed results with large cetaceans, with most studies indicating no response by large whales.
Given the high economic value of the NZ commercial pot/trap and set net fisheries involved, as well as the current low documented incidence of entanglements, seasonal or temporal closures are not a viable mitigation tool. Seasonal, mandatory gear modifications focused on reducing the amount of slack rope in the water column is a more measured approach to reduce risk.
An advocacy campaign that targets fishers around the Kaikoura region and along the south-east coast of the South Island during the months of May-August may be effective. Disentanglement efforts will continue to be a vital.
Publication information
Laverick, S., Douglas, L., Childerhouse, S. and Burns, D. 2017. Entanglement of cetaceans in pot/trap lines and set nets and a review of potential mitigation methods. Report by Blue Planet Marine for the Conservation Services Programme, Department of Conservation, Wellington. 75 p.