_Advance of the Mangrove-Borders of Low Islands on the North Coast of Vanua Levu between 1840 and 1895._
Thukini, or Gibson Island of Wilkes 700 to 800 yards Nangano, or Piner’s Island of Wilkes 300 to 400 " Nandongo, or Nuvera of Wilkes 500 " Talailau (two new islands) 400 to 900 " Nukunuku or Clark’s Island of Wilkes } Not much change. Thakavi, or Day’s Island of Wilkes }
It will be noticed that the islands of the Talailau Reef are not marked in the chart of 1840; they are both low mangrove islands, the largest being slightly under a mile long and the smallest a little under half a mile. In Nukuira Island, the Vatou of Wilkes, there has been a decrease of about two-thirds of a mile during this period. The difference between Thukini in 1840 and in 1895 is very noticeable. In the time of Wilkes the mangroves only occupied about one-third of the reef-patch. Now they occupy about two-thirds, the area of the reef-patch remaining much about the same. Taking the minus and plus values of all the islands here measured, the average rate of the advance of the mangrove-margins during this half-century may be placed at about 250 yards in the case of these reef-islands, which would amount to a mile in 400 years.
It is probable that a long island like Ndongo, which is about four miles in length, has been formed by the union of smaller mangrove islands. Therefore, taking half its maximum breadth of a mile as a guide, it would at this average rate of growth require two centuries for its formation. But since the extension of the mangroves depends on the growth of the reef-patch, which takes place on the average at a much slower rate, it follows that this can only be a minimum limit for the age of this island. We can only assume that if the reef-patch had suddenly appeared 200 years ago, Ndongo Island could by this time have acquired its present dimensions. It does not follow that the mangrove border has been continuously advancing. A hundred years ago there may have been a state of equilibrium between the growth of the mangrove and the reef-patch, which does not now exist. All we can say of some of these low islands is that the mangroves have been rapidly extending their margins during the last half century, and that the normal adjustment between reef-growth and mangrove-growth, which must have once existed, does not now prevail.