Mangrove invasion threatens Ahuriri Estuary

Date:  24 February 2020

Ahuriri Estuary is an incredibly important ecosystem in the region. It provides habitat for endangered species like the kuaka (eastern bar-tailed godwit), spoonbills, dotterels and occasionally terns and herons.

The introduction of mangroves could devastate this habitat, reducing space for foraging and roosting along with the biodiversity that attracts these species.

The mangrove seedlings were discovered on separate occasions by locals Mike Hockey and Hans Rook, who removed the invasive weeds and immediately contacted DOC.

“We’re alarmed by increased reports of mangroves being found in the estuary by vigilant members of the community,” says DOC Biodiversity Senior Ranger, Denise Fastier. “The reckless introduction of these plants to Ahuriri Estuary could be catastrophic for the delicate estuarine ecosystem that supports so many threatened species of wading birds.”

In some areas, mangroves can offer some environmental positives, but in Ahuriri, they have not historically been a part of the environment. If they were to establish, Fastier says, they would choke the estuary and upset the balance irrevocably.

Anyone found planting mangrove seedlings in Ahuriri Estuary could find themselves facing prosecution under the Conservation Act, 1987.

Anna Madarasz-Smith, Hawke’s Bay Regional Council’s, Team Leader Principal Scientist Marine and Coasts says, “Mangroves are only naturalised in areas north of East Cape. They have many great qualities including binding sediments to reduce erosion, habitat for fish and invertebrates and filtering water to improve water quality.

“However, they can also grow in excess where sediment coming in from the catchment is changing estuarine habitats, and some areas are having to remove them. Hawke’s Bay is not within mangroves’ natural range and we advise strongly against trying to introduce them to estuaries in this area.”

Even where mangroves are part of the environment, density has rapidly expanded to the point of congestion. Northland local authorities have responded with mangrove management projects. These focus on restoring coastal access, firm sand flats, shellfish beds and improving biodiversity by removing mangroves.

If you see mangroves in Ahuriri Estuary, please contact DOC HOT (0800 362 468)

Background information

• Scientific name for mangroves: Avicennia marina subsp. australasica.
• Mangroves respond to higher nitrogen levels by growing faster and producing more viable seed.
• Ahuriri Estuary is part reserve, part wildlife refuge, with different aspects managed by DOC, HBRC and NCC.
• Seedling planters could face prosecution under the Conservation Act, 1987, Section 51S(1) (g) and (h).

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