Systematic behavioural studies on common dolphins are extremely rare, not only in New Zealand but also in the rest of the world. Massey University scientist and New Zealand Geographic Trust research fund recipient, Karen Stockin, is conducting one of them. Her field work, which documents the behaviour of the popular marine mammal in the Hauraki Gulf, will soon feature in the international journal Marine Mammal Science, highlighting the importance of the Gulf as a feeding area for New Zealand common dolphins.

‘The problem is,’ says Stockin, ‘that outside of New Zealand and Mediterranean waters, common dolphins generally occur well offshore, out in continental shelf waters in large pelagic groups sometimes numbering in the hundreds.’

During three years of field work, Stockin tracked the dolphins around the Gulf, and along with behavioural data, recorded information describing their environment and group dynamics. She calculated the amount of time they devoted to specific activities, and found that common dolphins in the Gulf spend 47 per cent of their time feeding, significantly more than the 17 per cent previously reported for dolphins in the Bay of Plenty.

This matters because the amount of time the dolphins spend in a specific activity provides insight into the relationship between an area and an activity. ‘So the question becomes, is this habitat important for feeding? Resting? Breeding? Then we can assess whether human activities are influencing an animal’s normal behaviour and ways that those undesirable impacts may be mitigated.’

Stockin’s post mortem examinations to identify the stomach contents of the Gulf’’s common dolphins indicate their main diet consists largely of jack mackerel, arrow squid and pilchards, all commercially fished species. This makes them extremely vulnerable to commercial fishing.

‘We have a long history of bycatch in New Zealand—Hooker’s sea lions and the squid fisheries; Maui’s dolphins and set nets. Last December, 22 common dolphins were killed in a single trawl off the North Island’s west coast.’ Potentially, the common dolphin could be in decline in New Zealand waters, as it is in other populations around the world, but we just can’t tell as there are no population estimates available for this species in New Zealand waters. Stockin and colleagues at Massey University’s Coastal-Marine Research Group are currently using dorsal fin identification photographs to assess the size of the Hauraki Gulf population.

She says her next task will be to estimate the mortality rate of common dolphins here, and measure the effects that human activities are having on the population. The Trust will be detailing her progress in future issues.