Mapping barriers to fish migration

WCS Fiji field officers recently completed a mission to find hanging culverts in Vanua Levu. Hanging culverts are a major barrier to migration of endemic fish from the oceans to upstream areas. Knowledge of hanging culvert locations will help us evaluate threats to biodiversity in the rivers and creeks of the important corridor between Mt. Kasi in the east, and Mt. Navotuvotu in the west. This will help to focus upcoming biodiversity survey effort on the least obstructed rivers which are likely to still be home to healthy populations of endemic fish.

After a 4am start in Suva, work began in as soon as Waisea and I got off the ferry at Nabouwalu, working our way eastwards from there towards Wailevu East in Cakaudrove. Every dusty, bumpy road or track was explored in search of the troublesome culverts. The survey took a total of 5 days to complete, with approximately 280 culverts being mapped, photographed and described. We identified about one third of these as the type of hanging culvert which blocks fish migration routes.

About 98% of Fiji’s freshwater fauna make contact with the sea to complete their lifecycle. So at certain periods in their lives they migrate down to the sea, and ultimately will need to return upstream to complete their lifecycle. These fish are skilled at navigating natural obstacles on their upstream migrations, but unfortunately culverts which hang away from the river surface present an impossible obstacle even for the most intrepid climbers.

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In recent freshwater surveys in Fiji, the presence of hanging culverts correlated with low species diversity and abundance relative to similar areas without hanging culverts, even in areas where the river was surrounded by intact forest. This implicates the hanging culverts as a possible cause of the decline in fish diversity and abundance.

It can be concluded from the culvert survey that most major catchments which have bridges do not have culverts, whilst small creeks do have culverts. This was a promising sign since most of these major catchments therefore run right up to the headwaters without migration barriers. There were also good indications of intact forest within some areas of Wainunu and Dama districts.

This project is kindly supported by the Critical Ecosystem Partnership Fund (CEPF), a joint program of l’Agence Française de Développement, Conservation International, the Global Environment Facility, the Government of Japan, the John D. and Catherine T. MacArthur Foundation, and the World Bank.

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