Unexpected trophic diversity in the endemic fish Orestias chungarensis in a high-altitude freshwater ecosystem, Lake Chungará (4520 m), northern Chile

Orestias chungarensis Vila & Pinto, 1986 is a small-bodied (max fork length = 120 mm) cyprinodontiform fish with a very restricted global distribution. The species is limited to a single, small (283 km²), high-altitude (4520 m) catchment located in the Altiplano of northern Chile. Until the late 20th century, O. chungarensis was the only fish species inhabiting both Lake Chungará and its main afferent river, the River Chungará. The introduction of rainbow trout [Oncorhynchus mykiss (Walbaum, 1792)] at this time raised concerns for the long-term conservation of Orestias. By 2017, O. chungarensis were no longer present in the River Chungará but remain relatively numerous in Lake Chungará. Although O. chungarensis are of elevated conservation concern, little is known regarding their ecology, and the few studies conducted have relied on individuals captured from shallow littoral habitats. Here, we captured O. chungarensis from different lake habitats and analysed multi-tissue stable isotopes (δ¹³C, δ¹⁵N, δ³⁴S) and stomach contents to characterise their trophic ecology. We also used geometric morphometrics to analyse any putative habitat-associated variation in body shape. O. chungarensis showed very wide variation in their stable isotope values (range: δ¹³C = -15.1 to -8.0‰; δ¹⁵N = 8.9-14.1‰; δ³⁴S = -10.5-1.7‰). A k-means cluster analysis indicated that individuals could be best classified into two groups in stable isotope space. A discriminant function analysis supported the separation of the sampled population into two groups (jack-knifed classification success = 98%). Individuals belonging to either a putative littoral group (¹³C-enriched, ¹⁵N-depleted and ³⁴S-depleted) or a group associated with pelagic-derived materials (¹³C-depleted, ¹⁵N-enriched, ³⁴S-enriched), which likely fed offshore or in deeper waters. Stomach contents results showed that O. chungarensis from the two putative groups had consumed similar prey prior to capture, feeding mainly on benthic macroinvertebrates (amphipods, chironomid larvae and pupae and gastropods). Mixing models analysis showed a broadly similar diet between groups, but the scale of contribution to the assimilated diet differed between groups. Comparisons of stable isotope niche size and overlap showed limited niche overlap, providing more evidence for differential foraging patterns. The dichotomy between the results from stable isotope and stomach content analysis suggests that O. chungarensis individuals forage on taxonomically similar diets, but their prey are fuelled from materials derived from different lake habitats (littoral and open-water). Given the remarkable plasticity found in the genus, our results could reflect the existence of a previously unrecognised ecotype.