Water bird numbers at the world famous Menindee Lakes, near the site of this year's massive fish kill, are in long-term decline, amid an ongoing failure to manage water levels to match weather fluctuations, leading ecologist Richard Kingsford says.

Bird numbers at the lakes in far-western NSW peaked at about 140,000 in 1985, according to surveys taken annually since 1983. For each good wet year since that record, the bird count has been falling.

Pelicans over Menindee Lakes in 2017: Numbers are on a long-term declining trend. Credit:Richard Kingsford, UNSW

Water populations "are a bit like a tennis ball...but the bounce is just getting lower and lower" during good years, said Professor Kingsford, who is director of the Centre for Ecosystem Science at the University of NSW.

The lakes "are one of the hotspots for water bird in eastern Australia, and they've been declining for some time".