The short answer
- Chile has 57 described mayfly species; 56% live only in Chile, and the family Leptophlebiidae holds 36 of them.
- In January samples, Meridialaris nymphs were the most numerous mayfly in all four rivers we fish that were surveyed, with up to 643 per square meter.
- Small Baetidae mayflies, such as Andesiops and Baetis, share the same riffles.
- A Meridialaris species studied in a Patagonian mountain stream produced two generations a year, emerging in spring and summer.
- Guides in Aysén report the most consistent hatches in January and February, matched with dries in sizes 12 to 18.
Which mayflies live in Coyhaique's rivers?
Mayflies in Chile are a southern story. Of the 57 species described for the country, 56% are found nowhere else and another 33% are shared only with Argentina. The family Leptophlebiidae dominates, with 36 of those species.
In January 2006, scientists from Universidad de Concepción and the Patagonian Ecosystems Research Center (CIEP) in Coyhaique sampled the riverbed at 14 sites of the Aysén basin, including four rivers we fish. In all four, the most numerous mayfly was Meridialaris, a leptophlebiid:
| River | Meridialaris nymphs per m² | All mayfly nymphs per m² |
|---|---|---|
| Emperador Guillermo | 643 | 759 |
| Ñirehuao | 385 | 429 |
| Mañihuales | 230 | 240 |
| Simpson | 36 | 50 |
Riffle samples on cobble bottoms, January 2006. A square meter is about 11 square feet. One survey and one visit per river: read the order of magnitude, not the last digit. Source: Oyanedel et al. (2008), table II.
Small mayflies of the family Baetidae, including Andesiops, Baetis and Deceptiviosa, turned up in the Emperador Guillermo, the Ñirehuao and the Mañihuales. The Simpson held the fewest mayflies of the four.
When do mayflies hatch around Coyhaique?
The closest life-history study followed Meridialaris chiloeensis in a forested mountain stream of the Patagonian Andes in Argentina. It produced two generations a year, a winter one and a summer one, with emergences in spring and summer, and its nymphs were more abundant in the low-water period than in high flows.
On the water, guides who fish the Aysén Region place the most consistent mayfly hatches in January and February, consistent with that summer generation. Late in the season the windows shorten: in March and April mayflies tend to come off around midday, in the warmest hours, as our guide to Coyhaique's rivers explains. No scientific study has yet mapped mayfly emergence week by week in Aysén, so treat any hatch calendar, including this one, as a pattern rather than a promise.
How do trout feed on mayflies?
Mayflies matter most to young trout. In the Palena River, in northern Chilean Patagonia, they were 30% of the prey items of rainbow fry and 39% of brown fry, falling to 11–15% in adults, whose menu widens with size (see what trout eat here). In a forested Patagonian stream, the grazing insects that scrape algae off stones, Meridialaris chiloeensis among them, were selected consistently by rainbow trout: eaten in a larger share than their share of the riverbed, which the authors read as extra vulnerability to predation.
How do you match them?
Flies are included in both of our programs, but this is the logic of the mayfly box. Guides in Aysén use standard dries, Parachute Adams and Comparadun styles in sizes 12 to 18, and small beadhead nymphs such as the Pheasant Tail in 14 to 16. For long leaders and drag-free drifts on clear, slow water, see our spring creek guide. The other two pillars of the riverbed have their own notes: stoneflies and caddis.
Sources
- Camousseight, A. (2006). Estado de conocimiento de los Ephemeroptera de Chile [Current state of knowledge of Ephemeroptera of Chile]. Gayana 70(1): 50–56. Read the source
- 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 source
- Hollmann, M.E.T. & Miserendino, M.L. (2006). Habitat preference and life history of the mayflies Metamonius anceps (Nesameletidae) and Meridialaris chiloeensis (Leptophlebiidae) in a Patagonian mountain stream. Annales de Limnologie 42(4): 233–240. Read the source
- 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 source
- 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 source
- Heinonen, T. Matching the hatch in Aysén, Patagonia. Kalakamu field guide (a guide's field observations, not a scientific study). Read the source
