Oncorhynchus mykiss (Walbaum, 1792)
Common names: Rainbow trout (English); Trout (Sinhala)
Synonyms: Salmo mykiss Walbaum, 1792; Salmo gaidneri Richardson, 1836; Oncorhynchus kamloops Jordan, 1892;
Oncorhynchus mykiss nelsoni (Evermann, 1908); Salmo nelsoni Evermann, 1908; Salmo irideus argentatus Bajkov, 1927; Salmo kamloops whitehousei Dymond, 1931
Identification Characters: D III-IV.10-12; A III-IV.8-12; C 19
The rainbow trout can be distinguished from other freshwater fishes of Sri Lanka by its elongate, moderately compressed body and by the presence of a jaw with teeth. Its colour is grey dorsally and silvery on the sides. A faint pink stripe is present on both sides of the body. Its entire body is spotted, including the head and the fins.
Total length: Ranges between 45-60 cm, but can reach a maximum of 120 cm.
Morphologically similar species:
There are no known native or introduced species that are morphologically similar to the rainbow trout in Sri Lanka. However, the Brown trout (Salmo trutta, Linnaeus, 1758), which was introduced to the island in 1880 and 1882, is somewhat similar to this species but it has not been recorded within the island since then (Pethiyagoda, 1991; and Welcome, 1988).
History of introduction:
The Rainbow trout was introduced to streams in the higher elevations of the Central Hills in Sri Lanka in 1889 (Kottelat & Whitten, 1996) as a sport fish. A hatchery in Nuwara Eliya initially replaced the introduced stocks when they were depleted, but it is no longer functioning (Pethiyagoda, 1991).
Rainbow trout are native to the northern Pacific coastal belt of eastern Siberia, and to the Pacific north-western coasts of USA and Canada (Froese & Pauly, 2014).
In Sri Lanka, populations have only established themselves in a few streams in the montane regions between 1700-2000m above sea level (Pethiyagoda, 1991), for example, Belihul Oya and Agra Oya in the Horton Plains and Pattipola.
Dispersal and reproduction (breeding and dispersal):
Rainbow trout females excavate small depressions and deposit eggs. The demersal eggs sink to the bottom and the male externally fertilizes the eggs. Eggs incubate in the bottom of the depression and hatch within four to seven weeks (Gall & Crandell, 1992; and Pethiyagoda, 1991).
Impact on native species and habitats:
The actual impact of this species on the native aquatic fauna has not been studied in Sri Lanka. However, in other countries it has been responsible for driving many native species into extinction or endangerment, such as the Californian golden trout and the humpback chub in the Grand Canyon (Gawrylewski, 2004).
Direct exploitation/destruction of native species:
It is suspected that the decline of the endemic freshwater shrimp Lancaris singhalensis from Horton plains was caused by the introduced rainbow trout through predation (Pethiyagoda, 1991).
Current uses: Rainbow trout is a sport fish and are highly sought after by anglers. However angling/fishing is prohibited within Horton Plains National Park.
Natural threats (predators): Otters are their main natural predators in the area.
Prevention and control: Nothing has been reported on preventive or control measures for this species.