Ecological monitoring of fish assemblages downstream of a hydroelectric dam in French Guiana (South America)

Dam impacts on the downstream section of dammed rivers are known to be important for fish because of changes in the physical and chemical river characteristics. However, the available data seem to be insufficient to draw general conclusions regarding the transformation of fish communities. A hydroelectric dam was built on the Sinnamary River in French Guiana and closed in January 1994. From December 1991 to December 1996 fish collections were made from the downstream section. The results show a rapid decrease in fish abundance in the middle downstream course immediately after dam closure. This low abundance was observed until the end of the filling phase, when the flow was maintained at a minimum level. Afterwards, abundance returned to a level comparable with that prior to dam closure. Fish collection in different zones showed that the decrease in abundance was probably due to the escape of fish to shelter areas. Another marked effect was the reduction of sample diversity due to a decrease in species richness, as well as in the regularity of the distribution of species’ relative abundance. Examination of biological traits of species suggests that under these conditions feeding strategies could have been the most important factor affecting the abundance of populations. Copyright © 1999 John Wiley & Sons, Ltd.