That 2001 prohibition of lead shot in Spanish wetlands considered of international environmental importance -- and the consequent use of steel shot by hunters -- has started to bear fruit, a report in the journal Environment International says.
Lead shot accumulating in wetlands, with more than 100 per square yard in many areas, remains in sediment for decades, and poses a risk to waterfowl who may ingest it, the researchers said.
When the shot is eaten, it is retained in the gizzard and is worn down in the stomach, allowing lead to reaches the animal's tissues, they said.
"The birds start to develop neurological problems, they cannot move and they also suffer from anemia," Rafael Mateo Soria of the Hunting Resources Research Institute said.
"Normally, if they ingest lead, they die with notable emaciation after days or even weeks after starting to ingest the shot.
In species such as the mallard, 30 percent caught at the start of the 1990s in the Ebro delta in northeast Spain had ingested lead shot, a figure that has now dropped to 15 percent, he said.
"The most important part of our work is that it shows that, despite it's still covering a partial area, the change of material from lead to steel shot has reduced waterfowl poisoning and the contamination of hunted meat," said Mateo, a co-author of the study.