Passage of Rhone River fish through irrigation pumps in the Camargue: First results.

Irrigation pumps are the main aquatic connexions between the Isle of Camarg ue and any other freshwater system. Fish passage was monitored in 2 sites a mong the 15 pumping stations of the Fumemorte catchment (68 km(2)) during a n entire rice cultivation period, from April to October 1997. Fish can survive the passage through the pumps. They were mainly small-size d (2-5 cm). However, large fish (up to 46 cm) can pass alive through the pu mps. Estimation of fish quantity entering through the Sambuc pump was 14,70 0 individuals and 98,000 for the whole Fumemorte basin. Twenty four species were caught in our samples. Dominant species were roach, eel, gudgeon and white bream. The species composition of the pumped fish was compared with the fish assem blages of other habitats in the Camargue, in order to assess the significan ce of this immigration in the functioning of fish populations in the Fumemo rte drainage basin. Pumped fish assemblage was characterized by the relativ e abundance of Rhone River species such as the nase, the chub, the barbel, the roach, the gudgeon and the shad. A sampling site in an irrigation canal located 10 km far from the pumping station was shown to bear a fish assemb lage which was relatively close to the pumped one. Fish assemblage from ano ther sampling site located in the Fumemorte drainage canal differed from th ose of the irrigation system by the relative rarity or absence of some Rhon e River species such as the nase, the chub or the barbel, and the relative abundance of species of brackish waters which enter the drainage system by the Vaccares lagoon, such as the three-spine stickleback, the common goby o r the sand smelt. In a seasonally-flooded marsh connected occasionally both to the irrigation system and to the drainage system, the fish assemblage a ppeared to be a subset of the drainage canal assemblage, in which species s uch as the roach, the gudgeon and the pike-perch were lacking. These result s suggest the following roles of immigration through pumping in the functio ning of fish populations in the Camargue : (1) pumping allows exotic specie s to enter the Camargue waters, (2) some of the pumped fish species do not find suitable habitat conditions in the Camargue and their populations in t he Camargue are only maintained by immigration, (3) fishes in seasonally-fl ooded marshes come more from the drainage system than from the irrigation o ne, and (4) for the pumped fish species having abundant populations in the Camargue, the importance of this immigration in the functioning of their po pulations in the Camargue remains difficult to assess.