SENDAI-Alien infiltrators are to blame for the decimation of native waterfowl numbers in one of Japan's wetland preserves, researchers report.

The researchers blame the decade-long drop on voracious black bass, a non-native species of fish that sport fishermen likely introduced into the lakes and rivers.

Black bass are notorious for gobbling every fish in sight-leaving nothing for the native water birds.

``The main reason for the drop in aquatic birds is that the black bass are eating up all the small fry that the birds feed on,'' said Tetsuo Shimada, a researcher at the Miyagi Prefectural Izunuma-Uchinuma Environmental Foundation.

The group has counted the birds inhabiting the Izunuma Uchinuma marshland in northern Miyagi Prefecture several times a month for 10 years.

The marshland is one of 13 sites in Japan registered with the Ramsar Convention, an international treaty governing the conservation of wetlands as waterfowl habitats.

The researchers found the average number of birds at the marsh had dropped sharply between 1994 and '95 and 2000 and '01. The group was to present its findings at a symposium on Saturday in Kashimadai in the prefecture.

According to the study, the deep-diving little grebe (kaitsuburi) and merganser (mikoaisa) populations have been nearly wiped out, falling 93 percent and 69 percent, respectively, in the same five years. Little egrets (kosagi), which feed on surface-skimming fish, also fell 87 percent.

In the April-August nesting season, an average of 18.7 little grebes were spotted in 1994-95, while only 1.6 were seen during the same period in 2000-01.

The study also noted fewer numbers of other bird species that don't feed on fish, although the damage was not as severe.

Numbers of European pochards (hoshihajiro), tufted ducks (kinkurohajiro) and common coots (oban) dropped an average of 31 percent.

Since 1996, fishermen have reported netting only about one-third the number of small native fish, such as broadstriped bitterlings (tanago) and topmouth gobies (motsugo), than they used to catch. Researchers blame the black bass explosion.

The survey found that, while no fishermen reported catching black bass in the marshland in 1994, by this year, up to 42 people a day have reeled in the fish.

The black bass is an extremely fecund species that is native to North America.

With black bass sport fishing growing in popularity, researchers are also worried that fishermen are disturbing the nesting grounds of little grebes in the marshland.(IHT/Asahi: November 20,2004)