Compared to those elsewhere, New Zealand orcas have an unusual diet. Although like other orcas they consume a variety of food such as fish, squid, dolphins, sharks and seals, they are the only group known to eat stingrays, eagle rays and electric rays as a staple food.
Cruising the shallow waters in harbours such as Whangārei, Kaipara, Waitematā and Tauranga, orcas forage along the muddy or sandy floor, seizing rays by the head or tail tip in order to avoid the barb halfway along the tail. Sometimes they blow bubbles at them, possibly to frighten them into moving.
Eating such prey is dangerous. A young female orca was found dead in the Hauraki Gulf with two stingray spines inside her. She died either because of blood loss from the spines penetrating her body, or because she was poisoned by the spines.
Orcas eat larger prey than rays. Off Kaikōura in the South Island, 2-metre-long dusky dolphins gather in their hundreds to feed in the nutrient-rich waters, occasionally becoming the orca’s quarry. Seven separate incidents of orca attacks on dusky dolphin pods were observed over 11 days in November 1995.
Mako my day
In the Bay of Islands in 1998, an orca made a ferocious attack on a mako shark. Held by the tip of the tail, the shark tried to escape. It broke free to ‘hide’ under a boat, but the orca pursued it, bit it around the girth, and descended with the shark in its mouth, regrasping it near the tail. A second orca joined in the attack, biting the shark on the head, and together the two descended while eating their doomed victim.
By feeding close inshore, orcas run the risk of becoming stranded. About one orca is stranded in New Zealand each year; since records began more than 70 have been stranded, compared to only 12 in Australia. In 1955 at Paraparaumu north of Wellington, 17 came ashore – the largest recorded stranding of orcas at one time in New Zealand. To put them out of their misery they were shot. Today, strenuous efforts would be made to refloat them, and it is likely that many would have survived.
New Zealand orcas are well travelled and long lived. One female, first identified in 1977, has been seen at sites between the Bay of Islands and Kaikōura, and on the west coast at Kaipara Harbour. They can swim as far as 150 kilometres a day, and two males were logged making the return trip from Auckland to Kaikōura, a distance of 2,000 kilometres, in two months. Orcas are capable of accelerating to impressive bursts of speed, exceeding 30 kilometres an hour.
In countries such as Canada and Norway, studies have shown that captivity significantly shortens an orca’s life. By law, orcas cannot be taken captive in New Zealand, but they face other threats including pollution, being struck by boats, and harassment. Fishing lines have severed the tops of fins, and toxic chemicals may affect orcas, although this not been proven. There are well documented cases of orcas being struck by boats, which can be fatal. The only enemy of the orca is humans.