 Globally , pelican populations are adversely affected by four main factors : declining supplies of fish through overfishing or water pollution , destruction of habitat , direct effects of human activity such as disturbance at nesting colonies , hunting and culling , entanglement in fishing lines and hooks , and lastly the presence of pollutants such as DDT and <unk> . Most species ' populations are more or less stable , although three are classified by the IUCN as being at risk . All species breed readily in zoos , which is potentially useful for conservation management . 
