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Lake Wanam Management Project

Dave Wilson

Concerns were raised about the survival of the Lake Wanam rainbow in 1994 - 5 when adventurer Heiko Bleher had noticed significant change in the fish populations of Lake Wanam. Over a period of 3 years Heiko observed large increases in the tilapia population of the lake and a corresponding dwindling of Glossolepis wanamensis. Indeed in 1995 he was only able to collect 2 old male specimens of G wanamensis.

  

Lake Wanam - PNG
image - Heiko Bleher

Lake Wanam is situated in the picturesque Markham Valley, inland from Lae. The Lake has a surface area of approximately 3.7km2 with a maximum depth of 20metres. It is surrounded by many steep grass covered hills and is fringed by large areas of Nymphaea sp (water lily) and Nelumbo sp (lotus).

The lake is home to two rainbowfish, Glossolepis wanamensis (the Lake Wanam rainbowfish) and Chilatherina fasciata (barred rainbowfish). Other species occurring in the lake are Oxyeleotris fimbriatus, Ophieleotris aporos?, Glossogobius sp Redigobius sp, Glossamia gjellerupi (mouth almighty).

The survey team also identified the following non-native feral fish - Oreochromis mossambica (tilapia), Cyprinus carpio (carp) and Gambusia affinis(mosquitofish)

During the 1998 ANGFA Convention in Brisbane, discussions were held with Heiko about the situation at Lake Wanam and it was decided that further survey work would need to be undertaken. It was felt that the endemic Lake Wanam rainbowfish was in such low numbers as to cause concern for its future in the wild.

Matt Vincent and Gary Slater from the Melbourne Zoo travelled to PNG to discuss the problem with Peter Clarke, Director of the Rainforest Habitat in Lae. These discussions led to the setting up of a tripartite agreement between ANGFA, Melbourne Zoo and the Rainforest Habitat. These three bodies formed the Lake Wanam Management Project on 21 December 1998.

An extensive survey of Lake Wanam was undertaken in June of 1999. In depth survey reports can be found in Fishes of Sahul 13/3 pp621 – 638. This survey turned up some very interesting observations. First and foremost was the confirmation obtained from natives that had assisted Heiko Bleher of the fact that hardly any G. wanamensis had been collected during Heikos last visit. Just as important as this information was the collection of large numbers of G. wanamensis by the survey teams. Yet of all those fish collected only 2 juvenile specimens were noted. The remainder of the wanams were approximately 2 years of age. What is happening with respect to the breeding or growth of the fry? The survey team noted small groups of fry but was concerned by the low numbers within these groups (approx 20 specimens).

Also of great concern to the 1999 survey team was the lack of Chilatherina fasciata. Despite extensive sampling of areas known to contain hundreds of specimens in the past not a single fish was collected.

The survey has raised a number of questions.

  • Where are all the wanamensis 2 - 3 years old? Where are the juveniles and fry?
  • What has happened to Chilatherina fasciata?
  • Why has the tilapia population decreased so rapidly?
  • Why has the human population of Lake Wanam increased so much recently? Are Sepik people migrating to the area?

In the early 80’s Gabensis Village had a population of 4-500 people. Today that population is 17-1800 and growing rapidly. It is now the largest village in Moroke Province. In 1995 Heiko observed only 1 family living at the lake. The 1999 survey found many fishing camps and small huts about the shoreline of the Lake and much evidence of long line gill netting using 4 –5" mesh. The weekly catch is estimated at 900 tilapia average size 800gms – 1kg.

It was thought that during the severe drought of 1997 that hundreds of thousands if not millions of fish died when the water level of the lake fell 1.5 metres. Apparently dead fish were floating everywhere and thick on the shore. G. wanamensis are known to withstand high temperatures very well and perhaps this event left a niche big enough for the population to recover temporarily.

Sixty specimens of the Lake Wanam rainbowfish were removed from the lake to the Rainforest Habitat. Recent reports confirm that this captive population is breeding satisfactorily. Captive populations in Australia and Germany still survive on a very limited gene pool in desperate need of recruitment. It is felt that the lake population is still under heavy pressure and considered by the survey group to be highly vulnerable.