"Until we understand the reasons behind the decline in the Bay of Islands, where commercial tourism levels are high, we need to look after the Great Barrier Island habitat that is used year-round by these large groups of bottlenose dolphins."
Miss Dwyer described the gulf area as a busy area with a lot of shipping traffic, which could pose a threat to the dolphins.
She said more information was also needed about the prey of two other species her study focused on - common dolphins and Bryde's whales.
There was a lack of data about the prey, with fisheries research tending to concentrate on larger and more commercially viable stocks.
"It's concerning that species such as pilchards are commercially fished, but yet we know hardly anything about their ecology," she said.
"If these prey stocks decline, we may also lose the dolphins and whales here."
Miss Dwyer carried out her 279-day study on a dedicated research boat - previous mapping exercises have relied on tourist vessels - from which she conducted visual surveys using occupancy modelling, which factored in the probability of spotting a dolphin or whale.
She found common dolphins and Bryde's whales all year round in the central northern regions of the inner Hauraki Gulf but that the whales' distribution varied considerably from year to year.
There were also differences in how each species used the inner gulf, with movement patterns likely driven by movements of prey.
Miss Dwyer hoped the data would be included in Sea Change: Tai Timu Tai Pari, a joint effort to create a marine spatial plan for the gulf due to be complete by September.
The bottlenose dolphin
• Classified as nationally endangered in New Zealand, where there are three main known populations - Doubtful Sound in Fiordland, between the Marlborough Sounds and Westport, and around the Bay of Islands area.
• Can reach lengths of nearly 4m and are dark or light grey on the back with white on the undersides.
• Are particularly susceptible to human impacts due to their coastal nature.
