Aves, p�le ornithologique de Natagora

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Aves, 40/1-4 | 2003 | 100-104

  Study on the quality of streams used for fishing by a Black Stork (Ciconia nigra) pair.
Mahieu, M. & Jadoul, G.

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This study was initiated after a Black Stork pair was caught and equiped with Argos-satellte transmitters. The precise locatios of the birds were recorded on 1/25.000 scale maps to delimit their territory. Then, the strams where birds used to forage for food were discovered and analysed. Physical and chemical analyses of the water were conducted first and then a biological analysis and electric fishing were performed. Mains results. The femal territory is composed of two circular areas, the first covering an area of 15,9 km2 and the second of 19,63 km2. The male territory is spread on a 38,5 km2 area. Sixteen different streams were studied. Most of them are narrow (from 4à centimeters to 1 meter) and shallow. The shallowness of the frequented streams is probably linked to the fishing method of the storks - they nedd to see their prey. The bottom is often constituted of gravel with few rocks and blocks. Vegetation is variable from place to place. Physical and chemical analyses show a general good quality of water for almost every stram. Nitrous elements are in low concentration in most cases. Hardness analyses show that all waters are soft or really soft. Biological analyses show a good quality of streams with average pollution levels from none to very low; Fishing relsuts showed that Cottus gobio and Salma truta are the most common species found in the streams frequented by storks. Their low tolerance to pollution and their abundance indicate again the generally good quality of those habitats. Nevertheless, afew streams or parts of them differed from this conclusion. Two parts of streams, close to the end of a village, present high phosphate concentrations. This pollution with phosphate ions is due to laundry waters in the village sewage system. On the other hand, analysis conducted in the immediate surroundings of a freeway didn’t show any sign of water pollution. In two cases, we faced another problem in terms of diversity, following low fishing results. Two streams belong to a fishing society that pour fish raised in fish farms and therefore imbalance the populations. Trout are predators of other species. On the other hand, this abundance of fish attracts a lot of Black Storks. Some low fishing results were obtained in streams where water is more acid, when the vegetation is almost non-existent or when the bottom of the river is covered with mud. A stream was also re-profiled and we think that following this work, storks will desert it in future years.

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