A study of the structure and typology of the fish populations in three ancient arms of the Garonne river was performed during the period 1988 to 1990. Fish were sampled using two complementary methods, regular abundance sampling using electro-fishing of spot samples (E.P.A.) and using nets. The samples obtained through E.P.A make up the bulk of the data used in this study. The fish populations in the three ancient arms, which are generally similar, are dominated by the cyprinids. These ancient arms, all having direct access to the river, explain the important interactions and frequent migrations of populations from ancient arm to river and vice versa. The species in these ancient arms are comprised of different size classes each one corresponding to the different stages of development of the fish. Relationships between the presence of each species at a certain life stage and seasonal changes in environmental characteristics were studied. A correspondence analysis between species interactions helped to identify three spatial groups (i.e., corresponding to habitats) of fish populations. The bitterlings as well as cyprinid juveniles prefer the shallow littoral zones with fine sand-mud substrates near aquatic vegetation. Zones with semi-submerged aquatic plants offer a favorable habitat for the small carnivorous species such as the sunfish and juveniles of perch and black-bass. Moderately sloped ancient arm banks and zones of deeper water along walled banks of gravel substrates favour adult populations. Abundances of species from these three spatial groups depend upon the seasonal changes. However, three important tendencies had been identified, these are : summer and autumn recruitment of juveniles in the ancient arms, winter abundance of immature cyprinids and the preponderance of adults in spring.
