MUDDY FEET - Auckland Regional Council

34
5 Mangroves: The Cornerstone of a Dynamic
Coastal Environment
by Bill Brownell
(with contributions from Marie Buchler, Ria Brejaart, Bruce Burns and Steve Clark)
5.1 History
Writing in 1769, Captain Cook comments on his discovery of the “River Thames” (meaning
the current Firth of Thames and the Waihou River combined), “About the entrance to the
narrow [Waihou] part of the River the land is mostly cover’d with Mangroves and other
Shrubs, but further in are immence (sic) woods of as stout lofty timber as is to be found
perhaps in any other part of the world: in many places the woods grow close upon the very
banks of the River but where it doth not the land is Marchey (sic) …” (Beaglehole 1968).
Mangroves were seldom mentioned in subsequent accounts, such as the Hauraki Minute
Books of the Maori Land Court records (Phillips 2000) and the history of the Hauraki Plains
(Tye 1974), mainly because they were not a dominant vegetation type in the area at the time,
and were considered by both Maori and Europeans to be of little value.
The aerial photography series of the southern Firth of Thames coast and Hauraki Plains from
1944 indicate mangroves growing only at the mouths of the Waihou and Piako rivers. In
1952, mangroves extended out slightly more into the tide from the river mouths (including the
Waitakaruru), and they already had achieved quite dense coverage along the banks of the
lower 2-3 kilometres of the Piako. A smattering of them were evident in small groupings on
the coast extending away from the river mouths, with most of the intervening coastline still
showing as apparently shelly/sandy beaches with no mangroves. By 1963 the whole coast
from the Miranda Stream right around to the Waihou had limited coverage of fringing
mangroves, with increasingly extended groupings around the river mouths. Subsequent aerial
photographic records (1977, 1983, 1992, 1993, 1996 and 2002) provide a chronology of
mangrove advancement in the upper Firth of Thames over the past 40 years. According to
veteran local fishermen, the greatest rate of advancement has occurred over the past 3 years
(2002-2004), mainly since the last aerial photos were taken in February 2002 (R. Smith and
Peter Thorburn, personal communications 2004).
5.2 Ecology
The mangroves, or manawa, of the Firth of Thames, and indeed throughout their range in
northern New Zealand, are of a single species originally called Avicennia marina var.
resinifera, more recently referred to as A. marina var. australasica (Duke 1991, Young and
Harvey 1996 and Burns 1998), and now simply identified as A. marina in both New Zealand
and southern Australia (A-M Schwarz, personal communication 2004)
Mangroves have been in New Zealand for at least 9,800 years, and most likely arrived here
considerably before that in the form of propagules drifting over from Australia in the middle
tertiary period (Crisp, et al. 1990). A. marina is the most cold tolerant of all the numerous
mangrove species found mainly between the tropics of Cancer and Capricorn around the
world, and currently exists at a latitude of 38 o 54’ on the south coast of Australia. The
southern margin of the Firth of Thames and associated tidal rivers sustains (at 37 o 14’) one of
the most southerly occurrences of mangroves in New Zealand, and by far the most abundant
population found near the current limits of its range. There are also substantial mangrove
populations at nearby Tauranga and Ohiwa Harbours southeast of the Firth of Thames.
EcoQuest Education Foundation EQRS/1 12/04