Trophic relationships of exotic anadromous salmonids in the Southern Patagonian Shelf as inferred from stable isotopes

On the basis of their thermal preferences, salmon introduced in South America may be able to distribute over the whole Southern Patagonian Shelf Ecosystem (SPSE). Stable isotopes (δ¹³C and δ¹⁵N) used to analyze their trophic relationships show that the SPSE is dominated by zooplanktivorous species (15 of 37) and its community is distributed over six trophic levels, with salmonids occupying levels 4.2 to 4.9. A dual isotopes mixing model to estimate diet proportions of salmonids (validated with stomach content analyses) showed that Chinook salmon (TL 4.9) and brown trout (TL 4.7) feed largely on sprats (96% and 79 % predicted by the model) and were clustered together with intermediate size fish and cephalopod predators (TL 4.6-5) a diet similar to that in their native ranges (North Pacific and North Atlantic Oceans). Anadromous rainbow trout was clustered together with zooplanktivorous species (TL 3.2-4.4) and the model predicted a diet dominated by planktonic crustaceans (55%), very different from the diet dominated by fish and squids characteristic of steelhead in the North Pacific Ocean. Diet predictions based on stable isotopes for Chinook salmon were undistinguishable from direct assessments of stomach contents of fish captured by bottom trawlers, and overlapped widely with the diet of Magellanic penguins. The lack of reports of anadromous rainbow trout on bottom trawlers suggests this species has a shallower distribution. Stable isotope marks for rainbow trout are very similar to those of rockhopper penguins, known to feed on planktonic crustaceans, fish larvae and juveniles.