First documented case of anadromy in a population of introduced rainbow trout in Patagonia, Argentina

The examination of population-specific adaptations of introduced salmonids to the wide range of environments found in Patagonia (southern South Americ a) can help unveil some of the genetic and environmental contributions to l ife history variation. The rainbow trout Oncorhynchus mykiss introduced int o Argentina originated from a few parental stocks. Although some of these s tocks were anadromous, all of the dozens of established populations describ ed until now have been freshwater resident. In this paper we provide the fi rst documentation of the presence of an anadromous run of rainbow trout in the Santa Cruz River, the second largest river of Argentinean Patagonia. Mi crosatellite analysis revealed that anadromous and resident fish from the S anta Cruz River are genetically indistinguishable, probably representing al ternative life histories within the same population. Both wild types are ve ry different from the fish of Danish origin that were reared in a local hat chery, suggesting that they are descended more directly from California sto cks or that they have been affected by strong drift or selection. Marine gr owth and freshwater residence are comparable to those of California steelhe ad. River entrance peaks in early fall. Population age structure and scale pattern analysis indicate that fish enter the river at age 3 but that most do not spawn until their next river entry as 4-year-olds. An unusual aspect of Santa Cruz anadromous fish is that they are long-lived and highly itero parous. For instance, 20% of the fish analyzed had experienced as many as f ive spawning events.