Takāpu (Australasian Gannet) in the Hauraki Gulf

NIGEL ADAMS (School of Environmental and Animal Sciences, Unitec)

The coastal waters of the northern parts of New Zealand are the hunting ground for one of our most visible seabird predators of this ecosystem: the Tākapu or Australasian gannet (Morus serrator). After a short winter wandering more widely from their colonies, birds return to nest and breed.

The Hauraki Gulf , between the Mokohinau Islands to the north and the bottom of the Firth of Thames to the south, has a particularly high concentration of colonies. Every day during the summer, flotillas of gannets take off from their colonies in search of food. They spread out across the gulf, dramatically falling out of the sky to dive-bomb their prey.

Current Populations

With a stable or perhaps increasing total population, the species is not threatened. Accordingly, our work is seeks to understand key feeding interactions between seabird predators and their prey to inform ecosystem-based management decisions.

The proximity of the Hauraki Gulf to New Zealand’s biggest urban area, exacerbated by its partly confined nature, means this area faces severe challenges to its ecological functioning. The nature of these challenges is likely to be different between the inner and outer gulf and may account for differences in gannet population trends.

Counts of occupied nests at Mahuki Island, just off the coast of Aotea | Great Barrier, have increased substantially between 1980 and November 2017, when around 6,000 nests were counted. At the colony on Horuhoru Rock (off the northeast corner of Waiheke Island), occupied nests have decreased from 2,000 to about 1,000 over the same period(1).

Comparing Two Colonies in the Hauraki Gulf

The ability to find food is critical to meet the needs of an energy intensive lifestyle and feed chicks. Over the last four summers, we have been studying the diet and feeding biology of gannets at Mahuki Island and Horuhoru Rock, some 55 km south of Mahuki (Figure 1).

Our methods have included collecting and analysing regurgitation and faecal samples from gannets arriving at colonies after feeding at sea to determine prey type. We were able to recover highly digested food from stomachs of fish and squid ingested by gannets.

Prey Differences Between Colonies and Years

Gannets consume a range of surface shoaling fish and squid. The most consistent signal in the diet samples across all years and between colonies has been the persistence of Jack mackerel and anchovy. Other important species taken in the gulf include arrow squid, pilchard, saury, redbait and blue mackerel. The proportion of one species to the other varies between years, and the inner and outer gulf colonies reflecting variation in availability of potential prey species among years and between breeding sites.

In 2017, squid was particularly important for Mahuki birds followed by anchovy at Mahuki and pilchard at Horuhoru. In 2018, a reduction in squid was seen, replaced by Jack mackerel, redbait and saury at Mahuki and pilchard and anchovy at Horuhoru Rock. Jack mackerel was a frequently encountered prey species during 2019 at both colonies. The occurrence of species such as saury, flying fish and redbait in the diet of Mahuki birds is consistent with these birds being closer to more oceanic waters, with their associated fish species, than birds breeding at Horuhoru.

Where do Aotea and Waiheke Gannets Forage?

To understand where breeding gannets forage, we have been deploying GPS units attached to the birds’ tails at the time they make regular trips to sea to collect food for chicks. A preliminary study was conducted in January 2019, when 21 birds were tagged, followed by a more intensive study in December 2019, when 50 birds were tagged.

Gannets foraging trips are in the order of 20 to 30 hours long, with the birds capable of flights speeds of 50 to 60 km/h, placing the Mahuki colony and Horuhoru Rock colony within an hour’s flight of each other—and potential for a large degree of overlap in foraging area. Overlap was not however observed and likely explains some of the differences in the diets of these two populations. The January 2019 data indicated most Mahuki birds concentrated foraging trips along an axis running from the Hen and Chicken islands to the Mercury Islands off the western Coromandel. Horuhoru birds were restricted mainly to the inner Gulf but avoided mostly the shallower waters of the Firth of Thames. Our more extensive data set from December 2019 similarly indicated that birds from each colony avoided the waters immediately around their neighbouring colony, although some overlap was seen particularly between Cape Rodney and the Whangaparaoa Peninsula—an activity ‘hotspot’. Other hot spots included the coast of the northwestern Coromandel Peninsula (Horuhoru and Mahuki birds) and off the northern end of Aotea | Great Barrier (Mahuki birds).

While birds usually forage within 50 km or so of their breeding colony, birds do occasionally show more extended foraging trips. Birds from both colonies ventured into the Bay of Plenty.

Although the mechanism by which such separation of foraging areas is maintained is not well understood, it will have the effect of birds from one colony not foraging in areas potentially depleted of food by birds from the neighbouring colony. One implication of this pattern of Hauraki Gulf use is that Horuhoru gannets are sampling the inner gulf and aspects of their biology will be reflective of conditions there; and the Mahuki gannets of conditions in the outer gulf.

Future work on gannets could consider whether the location of these hotspots changes and how the foraging area of gannets from other colonies fits the patterns described here. Combined with data from other species, a picture is emerging of how the greater Hauraki Gulf is being utilised by its array of seabirds.

Notes:

Work described in this article was supported with funding from G.I.F.T. (Foundation North) to the Northern New Zealand Seabird Trust, Unitec and Birds New Zealand. The privilege of working at these sites was accorded by Ngāti Rehua and Ngati Manuhiri.

References:

1. Gaskin, C; Frost, P. & Friesen ,M. 2019. Indirect effects on seabirds in northern North Island. POP2017-06