Bycatch of protected species: Hector’s and Māui dolphins

  • Image, Hector and Maui dolphin bycatch.

    The Hector’s and Māui dolphins are subspecies of the small dolphin Cephalorhynchus hectori. These coastal dolphins are endemic to New Zealand (not found anywhere else). Māui dolphins are found on the west coast of the North Island, most often between Maunganui Bluff, north of Dargaville, and New Plymouth. Hector’s dolphins are mostly found around the South Island. Both subspecies are threatened with extinction.

    The Hector’s dolphin is classified as nationally endangered, while the Māui dolphin is nationally critical. Reporting the bycatch of protected species helps us understand the pressures our protected marine species face from fishing. Dolphins can become entangled in fishing gear used by both commercial and recreational fishers, with set nets posing a particularly high risk.

    We classified Bycatch of protected species: Hector’s and Māui dolphins as a case study.

    Key findings

    Between 1921 and 2015, entanglement in fishing gear accounted for up to 71.4 percent of the 301 Hector’s and Māui dolphin deaths for which a cause of death was determined.

    • There were different levels of certainty assigned to the cause of deaths from fishing gear: 26.6 percent were assessed as known entanglement, 11.0 percent were assessed as probable entanglement, 12.0 percent as possible entanglement, and 21.9 percent suspected entanglement.
    • Of the 174 deaths where the type of fishing gear was recorded, 86.2 percent were thought to be caused by commercial and recreational set nets.

    Figure 1

    Note:  Where possible, dead Hector’s or Māui dolphins are sent for necropsy to determine the cause of death. When a necropsy has been performed, the level of certainty that the cause of death has been identified is denoted as ‘known’, ‘probable’, or ‘possible’. When an animal has not been sent for a necropsy the cause of death is recorded as ‘suspected’.

    Figure 2

    Note: The Department of Conservation Incident Database for Hector’s and Māui dolphin deaths covers the period 1996–2015. The 327 dolphin deaths over this time were grouped into five-year periods. Unlike figure 1, the ’unknown’ cause of death category includes ’unknown trauma’ and ’human induced’ includes ’boat strike’. Data in this graph are heavily influenced by the variability and improvement in reporting of dolphin deaths over time. For example, the apparent increase over time in the number of dolphin deaths from natural causes may be due to better identification of cause of death.

    Figure 3

    Note: Cause of death for the 174 dolphins that died from entanglement in fishing gear, where the type of gear was identified. All four entanglement categories are included here: ‘known’, ‘probable’, ‘possible’, and ‘suspected’.

    Definition and methodology

    We report on two aspects of Hector’s and Māui dolphin deaths based on data extracted from Department of Conservation (DOC) Incident Database for 1921–2015:

    • the number of dolphin deaths by cause of death, including a comparison of deaths over 1996–2015.
    • the number of dolphin deaths from entanglement by type of fishing gear.

    Reporting and identifying the cause of Hector’s and Māui dolphin deaths have improved over the long time period covered by the DOC Incident Database.

    Where possible, dead Hector’s or Māui dolphins are sent for necropsy to determine the cause of death. When a necropsy has been performed, the level of certainty that the cause of death has been identified is denoted as ‘known’, ‘probable’, or ‘possible’. When an animal has not been sent for a necropsy the cause of death is recorded as ‘suspected’.

    The causes of death were: 

    • entanglement in fishing gear and drowning 
    • natural causes, for example from disease, predation, or extreme weather events 
    • human induced, including some historic cases of harpooning 
    • boat strike, where dolphins were hit by boats or boat propellers 
    • euthanasia, for example where a calf was found beached with no chance of survival 
    • unknown trauma, where the cause of death could not be identified beyond that the dolphin suffered trauma 
    • unknown, where a cause of death could not be definitively identified.

    We derive the number of dolphins that died after becoming entangled in fishing gear by adding deaths in all four entanglement categories: ‘known’, ‘probable’, ‘possible’, and ‘suspected’. Where possible the type of fishing gear involved is noted (‘cray pot line or floats’, ‘recreational set net’, ‘commercial set net’, ‘unknown set net’, ‘trawl net’, or ‘marine farm’).

    Overall, 54 percent of recorded Hector’s and Māui dolphin deaths have been attributed a cause of death.

    The statistics we present are rounded to one decimal place. This means that the figures for different levels of certainty for deaths from entanglement may not sum to total.

    Data quality

     Topic Classification   Relevance Accuracy 
    Resource use and management, and other human activities  Case study

     relevance-partial
    Partial

     accuracy-medium
    Medium

    See Data quality information for more detail.

    Archived pages

    See Bycatch of protected species: Hector’s and Māui’s dolphins (archived October 2016). 

    Updated 27 October 2016

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