The importance of the Aotea Marine Environment to the Gulf’s Marine Megafauna Community 

OLIVIA HAMILTON  (Recent PhD graduate, University of Auckland)

The Hauraki Gulf—Tīkapa Moana Te Moananui-ā-Toi— is a world-renowned biodiversity hotspot supporting a diverse marine megafauna community. The Gulf is famous for the Bryde’s whale and the more commonly encountered dolphin species. Yet, our only national marine park is home to one-fifth of seabird species, one-fifth of whales and dolphin species, various large shark and ray species, large migratory fishes, turtles, and seals. The Gulf’s long coastline and offshore islands, strong currents, and high primary productivity provide the ocean’s giants with critical habitat seasonally or year-round. 

The positioning of Aotea/Great Barrier Island in the outer Gulf – the region beyond Cape Rodney and Cape Colville, as the crow flies – puts it at the centre stage of the large-scale oceanographic processes that primarily influence the productivity of the Gulf. The productive waters support an abundance of zooplankton, fish, and squid, which support communities of seabirds, cetaceans, large predatory fishes and sharks, and rays(1)(2)(3)(4)(5)(6)(7)(8). Aotea is also at the heart of the circulation of these productive waters as it forms one side of the Cradock and Colville Channels and is directly influenced by oceanic water flowing through the Jellicoe Channel. The interaction between Aotea and surrounding landmasses such as Te Hauturu-ō-Toi/Little Barrier Island and the Coromandel peninsula, winds and currents, and persistent inputs of nutrient-rich water from the Firth of Thames creates productivity hotspots in the outer Gulf. While marine megafauna species have specific diets, they congregate in regions where food is abundant(9). A recent study identified an overlap between large marine predators and their prey, providing evidence that the Gulf is a critical feeding ground for sharks and cetaceans(10). This is what we see in the outer regions of the Gulf throughout the year, albeit changes to the assemblages of marine megafauna depending on the seasonal changes in food availability and prey type and ecology of species. 

Explaining the presence and prevalence of marine megafauna species throughout the year requires an understanding of the climatic and oceanographic processes that drive food availability in the Gulf. During winter and spring, westerly winds promote the upwelling of nutrient-rich waters from the deeper areas onto the shelf(6)(8)(11). The combination of wind-driven mixing and upwelling essentially fertilises the top layer of the water with nutrients that promote the growth of phytoplankton – tiny marine plants that form the basis of the entire food chain in the open waters of the Hauraki Gulf. The Gulf is core habitat to four populations of cetacean species – orca/kera wera (Orcinus orca), the ‘nationally critical’ Bryde’s whale (Balaenoptera edeni brydei), the ‘nationally endangered’ bottlenose dolphin/terehu (Tursiops truncatus)(23), and the common dolphin/popokanua (Delphinus delphis)(23) – the latter three commonly seen in the waters around Aotea year-round(12)(13). Habitat modelling indicates that we are likely to see these cetacean species more frequently with the onset of the period of intense upwelling and primary productivity(10). While bottlenose dolphins around the coast of Aotea will be a familiar sight to residents, Aotea has recently been identified as a hotspot for the North Island population(14). The Bay of Islands was once a stronghold of the North Island bottlenose dolphin population, but the numbers have dwindled over the last 20 years due to human pressures(24)(25), namely due to intense tourism(26)(27). As a result, the discovery that Aotea is a core feeding and breeding habitat for bottlenose dolphins will be vital to the conservation and management of this species due to its importance in sustaining the declining population. 

In summer, the marine environment in the outer Gulf is strongly influenced by the East Auckland Current (EAUC), an extension of the sub-tropical East Australian Current, which flows southeast down the northeast of the North Island, causing a sharp increase in the sea temperature. The presence of the EAUC brings with it a new assemblage of zooplankton and fish species as additional prey sources to large predators(15). High densities of fish were found on the east coast of Aotea in the warmer months during aerial surveys, including small planktivorous fish, such as jack mackerel/haature (Trachurus novaezelandiae) and pilchard/mohimohi (Sardinops sagax), and larger predatory fish, such as yellowtail kingfish/haku (Seriola lalandi), skipjack tuna (Katsuwonus pelamis), and kahawai/kōukauka (Arripis trutta)(16). Densities of zooplankton aggregations were also notably high from the Cradock Channel through to Colville Channel(13). The warmer months and periods of abundant food result in a seasonal influx of different species – one may be lucky enough to encounter whale sharks (Rhincodon typus) and devil rays (Mobula japonica) on the east coast of Aotea(17)(18)(19).

Several other cetacean species pass through the waters around the outer Gulf. Blue whales (B. musculus intermedia), fin whales (B. physalus), sei whales (B. borealis), minke whales/pakake (B. bonarensis and B. acutorostrata), beaked whales (Ziphiidae), humpback whales/paikea (Megaptera novaeangliae), southern right whales/tohorā (Eubalaena australis), long-finned pilot whales/ūpokohue (Globicephala melas), and false killer whales (Pseudorca crassidens), occasionally use this area either as part of their migration corridor or part of their overall range(4). The Gulf is also a seabird hotspot, with 20% of migratory and residential global seabird species occupying the numerous islands within the embayment(2)(5). Twenty-seven seabird species breed in the HGMP, mainly on offshore islands. Aotea and Te Hauturu-ō-Toi are the regionally endemic black petrel/tāiko (Procellaria parkinsoni) breeding sites – meaning that the entire global population breeds on these two islands. In addition, many of the surrounding islands and island groups in the outer Gulf support most of the breeding seabirds, such as the Mokohinau Islands, which are home to one of the richest seabird breeding communities in Aotearoa. The outer Gulf is, therefore, an important feeding ground for breeding seabirds, which are often constricted to foraging close to their breeding colonies. At-sea distribution maps of seabirds indicate that the waters around Aotea are core habitat for a range of seabirds, including the flesh-footed shearwater/toanui (Puffinus carneipes) and the Buller’s shearwater/rako (Puffinus bulleri), Australasian gannet/tākapu (Morus serrator), New Zealand storm petrel/takahikare (Fregetta maoriana), and the white-fronted tern/tara (Sterna striata)(28). Fish-eating seabirds, such as petrels and gannets, and plankton-eating shearwaters are often seen as part of multi-species feeding aggregations with Bryde’s whales and common dolphins, with the species composition dependent on the prey being schooling fish or zooplankton(29). The Pulse of the Gulf project, led by Professor Rochelle Constantine from the University of Auckland, is leveraging the power of drones and artificial intelligence to learn more about the dynamics of these feeding aggregations to inform conservation and management decisions. 

While most megafauna research has focused on the more commonly sighted cetacean species in the inner Gulf, we are seeing more megafauna research in the outer Gulf and around Aotea. For example, the oceanic manta ray (Mobula birostris) was previously considered an occasional visitor to the Gulf. However, the data suggest a more significant and persistent population than previously thought, thanks to the wonderful work of the Manta Watch Aotearoa New Zealand (MWANZ). A combination of dedicated surveys and citizen science input has revealed a foraging hotspot on the Pacific side of Te Hauturu-ō-Toi and Aotea and the regular occurrence of individuals in ‘cruising mode’ along the northern coast of Aotea. Furthermore, large-scale aerial surveys of the inner and outer Hauraki Gulf identified offshore regions of Aotea as crucial seasonal habitats for the highly migratory mako shark/mako (Isurus oxyrinchus) and blue shark/taha pounamu (Prionace glauca)(10). The same study pinpointed the coastal waters of Aotea as important habitat for the smooth hammerhead shark/mangōpare (Sphyrna zygaena) and bronze whaler shark/horopekapeka (Carcharhinus brachyurus), who move into the Gulf to feed and pup over summer. In addition, the Far-Out Ocean Research Collective is making strides in our understanding of oceanic and more elusive species, such as the false killer whale, who frequent the Gulf in warmer months and form large feeding aggregations with oceanic bottlenose(20) and may be sighted both seaward and landward of Aotea (Jochen Zaeschmar, pers comm). 

While the Gulf is classified as a marine megafauna hotspot, there has been a marked reduction in biodiversity since human settlement. There has been a 94% decline in the biomass of marine mammals(21). For example, although once abundant in the Gulf, fur seals were almost entirely extirpated from the Gulf by 1800. The development of commercial fisheries from 1950 to the present day has resulted in a substantial reduction in the biomass of sharks and fish and caused changes to the food web structure(22). While marine mammals and seabirds are protected species, there are currently no designated protected areas for these species in the Gulf. There is virtually no safeguard to the large migratory sharks that frequent the Gulf during the warmer months, such as the smooth hammerhead and mako shark – both listed in Appendix II of the Convention on International Trade in Endangered Species (CITES). 

Activities such as farming, construction, and urban development, have caused substantial changes to the catchments of the Gulf, and many fish stocks have been depleted through commercial fisheries(1)(30). Due to the wide-ranging nature of marine megafauna species and the diversity of habitats they occupy, the ocean’s giants are vulnerable to a wide array of land and sea-borne human pressures. As a result, an ecosystem-based management approach that considers food-chain links, species interactions, and the cumulative impact of human pressures is crucial for protecting the megafauna community. Marine megafauna are important ocean predators and are vital components of all ecosystems, and any hope of maintaining a viable pelagic ecosystem in the Gulf depends on their persistence. 

References

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