The short answer
- Mostly aquatic invertebrates: in 396 trout stomachs from the Palena River there were mayflies, stoneflies, caddis, two-winged flies, beetles, snails and decapod crustaceans.
- Adult brown trout leaned on snails (25% of prey items) and caddis (21%); adult rainbows spread their diet more evenly.
- Fish were rare: the only one found was the small native puye, in a few adult trout.
- Where pancora crabs live, they can dominate: 66–67% of the dry weight of identifiable stomach contents in river trout sampled in Chile.
- The bigger the trout, the wider the menu: rainbows went from six prey types as fry to thirteen as adults.
What did 396 trout stomachs reveal?
The most complete trout diet study in Chilean Patagonia comes from the Palena River. Between February 2012 and June 2013, researchers from Universidad Austral de Chile, University College Dublin and Universidad San Sebastián sampled 912 trout in 16 stretches of the river and its tributaries, through all four seasons, and examined the stomachs of 396 of them. This is what the adult fish had eaten:
| Prey group | Adult rainbow | Adult brown |
|---|---|---|
| Snails (Basommatophora) | 14.9% | 25.0% |
| Caddisflies (Trichoptera) | 13.1% | 21.3% |
| Two-winged flies (Diptera) | 18.9% | 18.8% |
| Stoneflies (Plecoptera) | 11.7% | 18.8% |
| Mayflies (Ephemeroptera) | 11.3% | 15.0% |
| Beetles (Coleoptera) | 14.9% | 1.3% |
| Decapod crustaceans (Decapoda) | 14.0% | 0.0% |
| True bugs (Hemiptera) | 1.4% | 0.0% |
Share of the invertebrate prey items recorded in adult stomachs. Source: Bravo, Whelan & Silva (2021), table 5.
Adult browns took more snails and caddis; adult rainbows took more beetles, two-winged flies and decapods. Young trout eat differently: mayflies were 30% of the prey items of rainbow fry and 39% of brown fry. The authors also measured the result of all that eating: consistently high body condition, “a clear indication that the fish were thriving in an environment rich with food.”
Why is the riverbed the real pantry?
Around Coyhaique, a survey of 14 sites in the Aysén River basin identified 59 kinds of bottom-dwelling invertebrates, and 83% of them were the immature stages of insects. The richest groups were stoneflies (19 taxa), mayflies (14) and caddisflies (10). In the January riffle samples from rivers we fish, the Mañihuales held 1,340 midge larvae per square meter, the Ñirehuao 385 mayfly nymphs of the genus Meridialaris, and the Emperador Guillermo the richest mix of the whole basin, 27 taxa. Each big insect group has its own field note: mayflies, stoneflies and caddis.
Do trout really eat that many snails?
In the Palena, snails were the largest single prey group for adult brown trout, a quarter of the prey items, and 15% for adult rainbows. Around Coyhaique, the snail Chilina patagonica was the most abundant invertebrate in the Simpson River samples, at 126 per square meter. Snails never hatch: they are the food that is still there on days when nothing is coming off the water.
Is the pancora crab behind the big Chilean trout?
Pancoras are South American freshwater crabs of the genus Aegla. In 1971 they were collected in Chile and Argentina and shipped to California to judge whether they were worth introducing there as trout forage, and the study examined stomachs and scales of Chilean trout. In river-dwelling rainbows, Aegla made up 66% of the dry weight of the identifiable stomach contents; in river-dwelling browns, 67%. Trout from waters with crabs grew fast, and the author concluded that the abundant crab populations “probably contribute significantly to the large size attained by Chilean trout.” In lake trout, fish mattered more than crabs.
The crab is no saint. In the same study it ate aquatic insects and fish, and in an experiment it crawled through 26 cm of gravel into simulated trout redds and consumed 76% of the embryos and alevins, which is why California rejected importing it. Around Coyhaique, the basin survey recorded Aegla alacalufi, and in the Palena decapod crustaceans, the group that includes pancoras, were 14% of the prey items of adult rainbows. A dark, weighted fly fished slowly along the bottom imitates crabs and snails at once.
How much do trout here eat fish?
Less than most anglers assume. In the Palena study the only fish in the stomachs was the native puye (Galaxias maculatus), a small fish usually under 8 cm (3 in), and it turned up in very few adult trout. Elsewhere in southern Chile, researchers found that small trout eat mostly invertebrates and larger ones turn more piscivorous, that brown trout eat fish more often than rainbows, and that piscivory was highest where native fish were densest. The streamer has its place, mostly for big browns, and a small, slim pattern is the one that matches the puye.
Where do terrestrial insects fit in?
In a forested stream of the Patagonian Andes in Argentina, rainbow trout ate about 40 kinds of invertebrates, yet 15 prey items made up 82–92% of the diet, and land insects were only 5% of prey numbers and 4% of biomass across the year. Most of the biomass came from aquatic insects that feed on fallen leaves, led by the big stonefly Klapopteryx kuscheli and crane fly larvae. That is the honest frame for summer: the weeks of the cantaria beetle and the hoppers are when big trout look up and the dry-fly fishing peaks, but a trout's year is built underwater.
What does it mean for your fly box?
Flies are included in both of our programs, so read this as the logic the guide's box is built on:
- Nymphs in the sizes of the mayflies, stoneflies and caddis that fill the riverbed.
- Dark, weighted patterns fished slowly on the bottom for crabs and snails.
- Small, slim streamers for big browns and the puye they hunt.
- Foam terrestrials for the summer weeks when trout look up.
Rods and lines are covered in our packing list.
Sources
- Bravo, S., Whelan, K. & Silva, M.T. (2021). Assessment of trout populations inhabiting the Palena River, southern Chile. Latin American Journal of Aquatic Research 49(1): 29–39. Read the study
- Burns, J.W. (1972). The distribution and life history of South American freshwater crabs (Aegla) and their role in trout streams and lakes. Transactions of the American Fisheries Society 101(4): 595–607. Read the study
- Oyanedel, A., Valdovinos, C., Azócar, M., Moya, C., Mancilla, G., Pedreros, P. & Figueroa, R. (2008). Spatial distribution patterns of benthic macroinvertebrates in the Aysén River basin (Chilean Patagonia). Gayana 72(2): 241–257. Read the study
- Arismendi, I., González, J., Soto, D. & Penaluna, B. (2012). Piscivory and diet overlap between two non-native fishes in southern Chilean streams. Austral Ecology 37(3): 346–354. Read the study
- Buria, L.M., Albariño, R.J., Modenutti, B.E. & Balseiro, E.G. (2009). Temporal variations in the diet of the exotic rainbow trout (Oncorhynchus mykiss) in an Andean-Patagonian canopied stream. Revista Chilena de Historia Natural 82: 3–15. Read the study
