Watery grave

日期:2018-01-05 03:08:35 作者:蔺钍 阅读:

By Oliver Klaffke WHEN a river floods, up to 90 per cent of the young fish die, German scientists have found. This is nearly twice the natural annual mortality rate. Christian Wolter of the Institute of Freshwater Ecology and Inland Fisheries in Berlin and his team have been monitoring fish in the river Oder on the border between Germany and Poland since it broke its banks last year, in its biggest flood this century. They found that while no fish populations were permanently damaged, the number of young fish of some species decreased dramatically. Between 80 and 90 per cent of the roach and common bream that hatched last year were washed out to sea or died of starvation in the flood, compared with a natural mortality rate over a year of about 50 per cent. “Both species are well adapted to a wide range of water speeds, but the current in the mainstream Oder was too much for them,” says Wolter. The fish could not find enough food as they fought the currents. The flood washed away sediments that hold invertebrates on which several species depend. In addition, some young fish that had hatched in streams failed to find their way up the river because the flood gates were shut. But some fish benefited from the flood. Chub, dace and pikeperch, which live in parts of the river where the current is stronger, thrived because the high water removed other species with which they compete for food. “[Food shortage] affected all species, but those that tolerate higher water velocity were also able to forage a wider range of habitats in the flooding river,” says Wolter. “Their [feeding] and reproductive behaviour is adapted to flooding.” The researchers even found a species not seen in the lower Oder before—the whitefin gudgeon. They believe the flood created new habitats for it. The team concludes that the regular flooding of lowlands helps a number of fish species to survive. Spring floodings create shallow waters that provide ideal breeding conditions for several species. Most river fish are adapted to spring floods and reproduce early in the season, relying on currents to wash the fry out to flooded areas, says Wolter. He suggests that instead of repairing the dykes in the Oder National Park, where people were not at risk, the area should have been allowed to flood naturally. This, he argues,