dc.description.abstract |
Orca (Orcinus orca), also known as killer whales, are more widely recognised than other marine
mammals. Although they have been reported from all oceans of the world, including the seas
around New Zealand, information above anecdotal notes exists for only a few places. Orca are
an apex marine predator that exhibits cultural differences in diet, vocalisations, and behaviour,
between and within populations. This study was established to determine baseline information
on New Zealand orca and to provide recommendations for future management and conservation.
The conservation status of orca worldwide is poorly known, although two populations of the
Pacific North West Coast of North America have recently been classified as ‘Threatened’ and
‘Vulnerable’.
Photo identification was used to determine the population size, distribution around New Zealand
waters, as well as range use and association among individuals. The total New Zealand orca
population is small (range 65-167 animals, with 115 calculated alive in 1997). Resighting rates
were high, with 75 % (n = 88) of the animals seen on more than two occasions. The mean
number of sightings for the 117 photo-identified animals was 5.4, the mode was one sighting,
and the median 9 – 10 sightings. One orca was photographed over a 20 year period. Population
structure, frequency of association with others, and other social behaviours were used to
determine population demographics. The New Zealand orca population appears to be made up
of at least three sub-populations based on geographic distribution (North-Island-only, South-
Island-only and North+South-Island sub-populations). Preliminary mtDNA analysis supports
the hypothesis that some New Zealand orca do not mix. The mean Association Indices within
the North-Island-only and South-Island-only sub-populations are significantly greater than
within the North+South-Island sub-population. Those animals sharing food had higher
Association Indices than those who did not share food. Sex ratios appear similar within each
sub-population and calves were present in each, suggesting all sub-populations are breeding.
Feeding behaviour was observed to assess habitat use and differences between foraging
strategies and prey preferences. Twenty four different species of prey have been recorded in the
New Zealand orca diet. Of these, ten have not been recorded elsewhere. The prey consists of
four types; rays (the most common food type), sharks, fin-fish and cetaceans (pinnipeds have not
been identified as a prey source). Foraging strategies were different for each prey type, with
benthic foraging for rays in shallow waters the most diverse strategy used in New Zealand. Food
sharing was observed for all prey types. One of the three proposed New Zealand subpopulations
appears to be generalist or opportunistic foragers, feeding on all four prey types,
another sub-population slightly less so, feeding on three prey types, and the third sub-population
appears to be a more specialist forager, only recorded taking one prey type (cetaceans).
Potential threats to orca, in addition to small population size, such as bioaccumulation of toxic
chemicals, oil spills, boat strikes and shootings are considered and recommendations for
conservation and future management are offered.
Whether the three sub-divisions within the New Zealand orca population are reproductively
isolated and hence require separate management, and whether there is further sub-division within
the proposed North+South-Island sub-population, requires further study including genetic
analysis. Some level of ongoing monitoring is recommended to ensure that the population of
New Zealand orca does not decline.
In addition, records of stranding locations and details of strandings are appended. Twenty-four
live strandings occurred, involving 63 killer whales, of which 17 animals were successfully
refloated and two of these resighted. One was seen after three years (nine resightings) and the
other after four months (10 resightings). Refloating stranded orca is recommended. |
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