Thursday, February 4

Rarotonga’s wetlands

Kia orana from Rarotonga.
You could be excused for not realising that 2 February is World Wetlands Day because there are an awful lot of ‘world days’ and this is one of the more recent ones (the first was in 1997).
The word ‘wetlands’ brings to mind mangrove swamps and bogs and we have neither of these in the Cook Islands but apparently on various southern group islands we do have freshwater lakes, swamps and marshes, mountain streams and also one tidal salt marsh.
The latter is at Ngatangiia so to mark the day I decided to pay a visit.
(The following information comes from the Wetlands International website compiled by Anna Tiraa, then of the Cook Islands Conservation Service; and the Cook Islands Biodiversity Database)
Ngatangiia Harbour is a natural harbour on the east coast of Rarotonga, with extensive intertidal silt and sand flats and the estuaries of two streams, Avana Stream (the largest stream on Rarotonga) and Turangi Stream. The muddy delta of Avana Stream filled much of the harbour after forest clearance early in the century. The shallow grassy marshes which occur in the intertidal zone around the harbour are the only saline marshes in the Cook Islands (bottom right in the photograph).

Ngatangiia harbour showing intertidal salt marsh at bottom right.
There has been extensive dredging in the harbour north of the Avana Stream mouth, and the lagoon is reported to be considerably degraded.
Birds such as the reef heron and the wandering tattler (in the video) feed in the area on small crustacea and a large number of fiddler crabs make their home there.
These crabs are common in the southern group islands
They feed outside their burrows when the tide uncovers ground during daylight hours. Waving displays, combat, and courtship usually occur for 2 or 3 days around spring tide. The large females usually build chimneys at this time and males approach the chimneys (sometimes waving). Male combat is ritualised and does not involve injury.
Well, you can certainly see the waving and combat but I think it was a bit too early for the chimneys. A photograph in the CI Biodiversity Database shows they can be quite tall so it must take a while to build them.
The wandering tattlers are summer visitor. They breed in Alaska and then fly down here and stay from September until about April. You can often see them feeding along the shoreline on the island’s beaches.
This small area of the Avana lagoon is the only tidal salt marsh in the entire Cook Islands and since it’s under water at high tide it should all be Crown land so not available for reclamation and building but the area seems to have shrunk over the last twenty-five years or so.
It is probably an important breeding and nursery area for certain marine and freshwater species so let’s hope it can survive and help us do the same.