The largest marine inlet in the southeast of the South Island, Otago Harbour offers significant sheltered habitat for a range of species, from birds to bottom feeders.
‘‘You have to go up to Banks Peninsula or down to Stewart Island to find an inlet comparable to Otago Harbour,'' internationally respected ecologist Keith Probert says.
• Our harbour - the heart of a city
A former head of the University of Otago marine science department, Associate Professor Probert notes about 90 species of fish have been recorded in the harbour.
‘‘There is an obvious southern element (e.g., southern pigfish, girdled wrasse), as well as a seasonal element (more warehou, moki, greenbone in summer).
‘‘The harbour is mostly quite shallow and there are extensive sediment flats exposed at low tide (about 30% of the harbour area during spring tides). The sandflats at Aramoana are especially well known for their diversity of salt-tolerant plants. The Aramoana Ecological Area is nationally important.
‘‘One of the more conspicuous animals of the intertidal sandflats is the cockle. There are dense beds, with densities of several hundred cockles per square metre being common, and the total stock in the harbour is one of the largest in New Zealand (if not the largest).
‘‘The cockles are hugely important as filter feeders and provide ‘ecosystem services' by filtering and cleaning the water. They no doubt play a vital role in the ecology of the harbour in regulating the amount of plant plankton in the water.
‘‘But there is an abundance of other species living in the harbour sediments; some 400 macroscopic species have been identified.''
The sediment flats are important as feeding grounds for birds and the seagrass beds there provide an important nursery habitat for fish (juvenile sole and flounder species), shellfish species, as well as being a source of organic detritus for the harbour ‘‘food web''.
Bounded on one side by the Aramoana mole and on the other by Taiaroa Head, the harbour's opening to the sea is only a 400m gap, through which the ocean tide is the driving force in hydraulic processes.
The mole was built in the 1800s to increased the tidal flow through the harbour entrance, reducing the need to dredge in that area.
Prof Smith: ‘‘It works pretty well, but the rest of the harbour requires dredging all the time to keep it deep enough for shipping.''
The harbour is divided into roughly two equally sized areas (upper and lower) by peninsulas at Port Chalmers and Portobello and the two islands at its halfway point, Quarantine Island and Goat Island.
This geology has a significant effect on each area: With a composition similar to open coastal water, the lower harbour (from Port Chalmers/Portobello to Taiaroa Head) is flushed with a relatively short ‘‘residence'' time of between 12-24 hours; in contrast, the upper harbour (from Port Chalmers/Portobello to the harbour basin) requires some 27 tidal cycles (or 14 days) to flush, generally showing decreasing salinity towards the mouth of the Water of Leith.
Prof Probert says scientists are particularly interested in the gradation in environmental factors from the lower to upper harbour.
‘‘As you move along the harbour, the degree of shelter increases, bottom sediments become finer, and temperature and salinity become somewhat more variable in the upper harbour.
‘‘There is a general natural decrease in biodiversity as you move towards the upper harbour as a result.
‘‘The increased residence time of water in the upper harbour also makes the upper harbour more susceptible to potential contaminants. Although the overall amounts of freshwater in the harbour are usually small, during heavy rain stormwater drains can have quite a strong influence on the inner harbour.''
Professor Abby Smith, of the university's marine science department, notes sediments in the harbour vary from muddy near the Water of Leith and Dunedin to sandy near the harbour entrance, providing habitat for creatures such as cockles and worms.
‘‘In the channels, the bottom comprises hard ‘pavements' of large shell fragments and rocks. In the middle of the harbour, sediments may be as much as 50m thick.''
Flocking to the harbour
Some years back, John Darby was asked to compile a list of bird species found around the main New Zealand harbours and their coastlines.
He was not altogether surprised that the waters around Otago Harbour recorded a greater variety of seabird species than any other harbour in New Zealand.
‘‘On a day-to-day basis, shags dominate the harbour scene and can be found perched on wharf piles, jetties, channel markers, boats and almost anywhere they can get a foothold,'' he notes.
‘‘There is a large colony of Otago shags that nest at Taiaroa Head. Recent research suggests that these are a subspecies of the closely related Stewart Island shag (new preferred name Foveaux shag). The other two species of shag most commonly seen within the harbour are the spotted shag and the little shag.''
Darby points out that spotted shags, as with the Otago shag, are cliff-nesting species. Though these can be found nesting on the outer coast of the peninsula, they find the harbour a safe fishing ground and skeins of this species can frequently be seen making their way in either direction in or out of the harbour.
‘‘My favourite is the little shag - a difficult species to identify at times, because of its variable plumage from drab brown as a juvenile to various stages of being pied as it matures.
"It is the shag most likely to be seen perching on phone and power lines. In contrast to the other two species of shag, they more often will nest in trees and thus avoid the abundance of terrestrial predators that decimate so many of our ground-nesting birds.''
Taiaroa Head comes within the compass of Otago Harbour and the presence of the northern royal albatross as a breeder makes Otago unique in the world, Darby says, adding a large colony of little blue penguins reside just inside the Heads.
‘‘Once upon a time, there was a small colony of yellow-eyed penguins tucked a further 400m or so into the harbour.
‘‘Bar-tailed godwits are a migratory species and can be seen in their hundreds in late September to March on the sand flats surrounding Aramoana. Of the gulls, red-billed predominate and black-backed are never far away. An abundance of wading birds, including the pied oystercatcher frequent sandy shore lines and mid-harbour mudflats.''
Uncommon and rare species of birds are the joy of bird spotters, but by definition such birds are unlikely to be seen by the everyday visitor to the harbour precincts.
‘‘Nevertheless, watching out for rare visiting tattlers, plovers and godwits is a pleasurable occupation enjoyed by many.
‘‘One of the rarest birds that may still winter over in Andersons Bay inlet is the southern crested grebe. The one I regularly saw kept company with mallard ducks and was a constant reminder of the conundrum of how freshwater species so easily adapted to a saline environment and vice versa when the need arose.''
