The spawning migration of anadromous rainbow trout in the Santa Cruz River, Patagonia (Argentina) through radio-tracking

Adult anadromous rainbow trout (Oncorhynchus mykiss) from the Santa Cruz River (Patagonia, Argentina) were implanted with radio-transmitters in May 2000 as they ascended the river from the ocean to locate spawning areas, assess the range and timing of the spawning migration, and determine the composition of the spawning populations. Radio-tagged fish revealed geographic and temporal migration patterns and previously unidentified spawning areas. Telemetry data indicated that anadromous fish dwell and spawn in the main stem river. Range of the total detected upstream migration varied between 2-280 km. Spawning begins by mid winter and peaks in the spring, matching the raise in water temperature, which ranged from 4°C at the beginning of the spawning season to 10°C at the end of the season in December. This pattern is similar to that of trout indigenous to the Pacific Northwest, USA, in which a raise in the water temperatures stimulates the onset of spawning. The arrangement of spatial locations of anadromous fish also indicated that spawning occurs in different sections along the main stem, giving some credence to the idea that subdivision at the intra-population level may exist. These results provide strong support to the idea that anadromous fish are at present more closely related to resident fish occurring in the upper Santa Cruz River, while resident fish from the lower reaches of the river may well constitute a separate spawning aggregation