Orestias (fish)

Four Orestias (possibly O. luteus) and two Trichomycterus catfish caught in Lake Titicaca, the center of Orestias richness

Orestias is restricted to freshwater habitats at an altitude of 2,800–4,600 m (9,200–15,100 ft) in the Andes of central and southern Peru, western Bolivia, and northeastern Chile.[2] Although freshwater habitats at low and middle altitudes in the Andes are relatively rich in fish, few are found in the highest, with Orestias and the catfish Astroblepus and Trichomycterus being the primary—in many places only—native genera.[4][9] Of the 45 recognized species of Orestias, the majority are found in Lake Titicaca (including 23 endemics to its basin), with the remaining from other Altiplano lakes, rivers or springs.[2] In addition to differences in feeding, the many species in Lake Titicaca segregate by habitat preference, with some living among totora reeds, some among submerged macrophytes, some at the bottom in water too deep for macrophytes (deeper than 10 m or 33 ft), and some pelagically in the open water.[10] Orestias species found elsewhere than Lake Titicaca often have very small ranges.[2]

Relatively little is known about the behavior of Orestias.[4]

They feed on small crustaceans (such as amphipods and cladocera), aquatic insects and larvae (such as chironomids), snails, small fish, fish eggs, algae, macrophyte seeds and plant detritus.[3][11][12] There are significant variations depending on exact species, especially in Lake Titicaca where there is a degree of niche differentiation, including some that mainly feed on zooplanktonic organisms, two (O. albus and O. cuvieri) that are particularly willing to take other fish, primarily smaller Orestias (fish eggs are regularly consumed by a wider range of species), several that mainly feed on small bottom- or plant-living organisms, and some that are generalists that will feed on a wide range of things,[3][4][10][12] sometimes varying depending on season.[11]

Females tend to grow larger than males. Although generally rather dull-coloured, breeding males may become partially yellow or orange. The eggs, up to a few hundred, are placed in shallow water among vegetation. Orestias appear to reach maturity when around one year old.[4]

Many species of the genus became rare in recent decades due to predation by, and competition with introduced fish species (especially rainbow trout and Argentinian silverside), pollution, overfishing and other human activities in their habitats.[13][14] In the 1960s, an expedition to Lake Titicaca led by Jacques Cousteau reported seeing many dead Orestias and when studied they were found to be infected by a disease introduced with the trout.[15]

The two largest species in the genus, the Titicaca orestias (O. cuvieri) at up to 22 cm (8.7 in) in standard length and 27 cm (10.6 in) in total length, and O. pentlandii up to 20 cm (7.9 in) and 23.5 cm (9.3 in) respectively,[5][6] have fared the worst. The Titicaca orestias was last seen in 1939 and is almost certainly extinct.[13] O. pentlandii is from the same lake and it may have disappeared in its native form.[14] It was still regarded as "only" vulnerable by the IUCN in 2009, but in 2014 the species was regarded as critically endangered in a review of Lake Titicaca fish.[13] In the 1990s, some were introduced to a small highland lake in Peru in an attempt of safeguarding the species, but over time they have changed (possibly due to genetic drift, inbreeding depression, or their new and different habitat) and now appear very different from the original O. pentlandii of Lake Titicaca.[16] The remaining Orestias of Lake Titicaca are also threatened to various degrees,[13][14] as are the three species in Lake Junin and its vicinity,[9] which in turn may threaten the Lake Junin giant frog that feeds on the fish and invertebrates.[17] Many Orestias found elsewhere are vulnerable because of their highly restricted ranges,[6] like O. ascotanensis where the entire habitat covers 18 km² (7 sq mi) of Salar de Ascotán and O. chungarensis found only in Lake Chungará.[18][19]

Despite their moderate to small size, some species are important to local fisheries, but they are overfished, there are few regulations and limited enforcement of the rules that do exist.[14][16] In Lake Titicaca, the moderately sized O. agassizii and O. luteus (complexes) represent more than 90% of the total catches of native fish species,[7] but others like the small open-water ispi (O. forgeti and O. ispi) are also frequently caught.[20] The natives have now been greatly surpassed by fisheries for the non-native introduced trout and Argentinian silverside.[14] Because of pollution, studies have revealed levels of metals in Orestias of Lake Titicaca that exceed the internationally recommended safety thresholds for human consumption.[21] O. agassizii and O. luteus are also farmed and spawned in captivity; both to supply the food market and for release to support their vulnerable wild populations.[7][22]

Three Orestias species showing some of the variations in shape in this genus

There are currently 45 recognized species in this genus,[5] divided depending on their relationships into four species complexes.[6] The genus is estimated to be 5 million years old, with a significant diversification occurring within the last one million years. Lüssen (2003) researched the phylogeny of several species including mtDNA sequence data; because hybridization is known to occur (Villwock 1964, Parenti 1984), mtDNA data cannot be relied upon on its own to resolve the evolution of this genus. For example, O. agassizii and O. luteus reach a similar maximum standard length of c. 17 cm (6.5 in),[21] but otherwise they are quite different. Despite this, hybrids between them are known.[7]

• Lüssen, Arne (2003): Zur Systematik, Phylogenie und Biogeographie chilenischer Arten der Gattung Orestias VALENCIENNES, 1839. Doctoral thesis, University of Hamburg, Germany. [in German] PDF fulltext
• Villwock, W. (1964): Vermeintliche Artbastarde in der Gattung Orestias (Pisces, Cyprinodontidae). Mitteilungen aus dem Hamburgischen Zoologischen Museum und Institut, Kosswig-Festschrift: 285-291. [Article in German]
• Villwock, W. (1972): Gefahren für die endemische Fischfauna durch Einbürgerungsversuche und Akklimatisation von Fremdfischen am Beispiel des Titicacas-Sees (Peru/Bolivien) und des Lanao-Sees (Mindanao/Philippinen). Verhandlungen des Internationalen Vereins für Limnologie 18: 1227-1234. [Article in German]