Porirua Harbour and Catchment History
The progressive effects of post European development dominate the history and bio-physical condition of the Porirua Harbour catchment and its water systems.
The name Porirua is derived from the maori parirua meaning twin flowings of the tide. The two water systems of the harbour (the Pauatahanui Inlet and the Onepoto Arm) were once eco systems that supported a bountiful supply of fish such as sole and cod, eels and sting rays, seals, penguins, and shell fish such as cockles and pipis. Not surprisingly, Maori settled in the Porirua harbour and shore system from at least 1450AD. In addition to the marine species, the harbours were surrounded by rich forests that were the source of many birds including moa. Flax was abundant in the swamps.
From the 1820s Europeans began to settle in Porirua. From the 1850s onwards, major impacts on the harbour system were caused by forest clearance propelled initially by an increasing demand for timber. Totara, matai, rimu, kahikatea, maire and hinau were particularly valued. Forest clearance proceeded rapidly so that within some 40 years, lowland Porirua was transformed from a mostly forested into a mostly pastoral landscape. Interestingly, there is more vegetation around the harbour system now than there was at the end of the 19th Century.
With the disappearance of the forests went many seed distributing birds and shore nesting ones, and geckos and skinks that also helped distribute some seeds. Marine mammals, mutton birds and penguins also disappeared.
Initially, pastoral land provided wool, but the advent of refrigeration made the meat trade viable and led to clearance of the steeper, inland parts of the region. The progressive clearance for pasture resulted in a massive increase in sediment, which started filling the harbours at a rate of 2 4mm/year from a pre European background inflow of 1mm/yr.
The next big effect was urban development and settlement – this increased sediment movement and deposition. Combine this with the effects of roads, railways and reclamations the shoreline and the tidal prism (the amount of tidal water that could move in and out of the harbour system) were dramatically altered . Sediment rates increased substantially so that by the mid 70s the average rate was between 6 and 9mm/yr. In parts of the Pauatahanui Inlet it increased to 10-15mm/yr. If continued, these rates would result in the Inlet being infilled and becoming a swamp in 145 – 195 years and the Onepoto Arm in 290 390 years. (Gibb, 2009, 2011).
In addition to sediment, urban development added chemical and biological contaminants and nutrients, together with toxins from urban run off. Agricultural chemicals and industrial run off in the post second world war era added further pollution which is now embedded in harbour sediments and affects its shell fish and fish stocks.
Fortunately this unfortunate legacy is now being addressed and PHACCT has an important role in helping the community and councils take action to make positive change to the catchment and harbour eco-systems.
For more information on Poriua, it’s heritage and the history of it’s harbours natural heritage and ecology, click any of the links below:
Porirua City Council