Salmo caspius, Kessler, 1877

Salmo caspius View in CoL View Figure

Common name. Caspian trout.

Diagnosis. Resident populations distinguished from Lake Sevan trout by: ● flank brown, grey, yellowish or green- ish (silvery without orange spots in anadromous form) / ● black spots usually restricted to back and upper part of flank, rarely on midlateral flank, then spots usually grey or pale-brown / ● parr marks distinct up to about 350 mm SL / ○ flank with many red spots in individuals smaller than 350 mm SL. Anadromous individuals larger than 350 mm SL distinguished from Lake Sevan trout by: ● flank with very small, irregularly shaped, black blotches (vs. many, very large, roundish spots) / ● caudal truncate (vs. emarginate). Size up to 1300 mm SL in anadromous form, usually up to 300 mm SL in resident form.

Distribution View Figure . Caspian, Lake Namak, and Lake Urmia basins. Kura and Aras drainages north to Volga and Ural. Introduced in Western Kavir, and Karun ( Iran). Records not shown on map as details are lacking.

Habitat. At sea, along coast to 50 m depths. Locally resident in lakes. Migrates to mountain streams. Resident part of pop- ulations in streams and headwaters with fast currents, cold, clear water, and stony or gravelly bottoms. Spawning sites usually in upper reaches of fast-flowing rivers and streams. Biology. Anadromous, lacustrine, and resident forms. First spawn at 3–5 years, females 1 year later than males. Spawns October–December. Before construction of dams and river regulation, two migration waves, first in summer– autumn, peaking in October, with adults spawning in same autumn and returning to sea in December; second in late autumn–winter, peaking in December–February, with adults spawning in following autumn after about 10–12 months in river. Eggs hatch in 6–8 weeks. Anadromous individuals in Terek drainage probably spawn once in life, but some females from other populations spawn up to 4 times. Parrs spend 2–4 years, sometimes more, in rivers and then migrate to sea. Commonly spends up to 4 years at sea before returning to rivers. Parr and resident adults feed on a wide variety of aquatic and terrestril invertebrates. Anadromous and large lacustrine individuals feed mainly on fish but also on large crustaceans.

Conservation status. LC.

Remarks. The northern range of S. caspius remains unclear, as S. trutta appears to have naturally invaded the upper Volga. The diversity of trout in Caspian basin has been greatly confused by the description of S. ardahanensis , S. araxensis , and S. murathani from Türkiye and the restriction of S. caspius and S. ciscaucasicus to anadromous populations. We strongly disagree with this approach. In all well-studied cases of Salmonidae , anadromous and resident individuals are conspecific, and this should have been the null hypothesis to be rejected by data. That an anadromous trout is (was) reproductively isolated from sympatric resident trout should have been demonstrated, as the opposite is the only demonstrated situation in Salmonidae . Reversing the burden of proof leads to a situation where each population could, in principle, be described as a separate species—and others are asked to prove the opposite. Although sympatric anadromous and resident Salmo species cannot be completely excluded, such a finding would be the first and only case in Salmonidae in riverine ecosystems. We await further studies on Caspian trout to resolve the case, but in the meantime, we have to accept the different species as valid. Salmo ciscaucasicus from the European Caucasus is probably a synonym of S. caspius .

Further reading. Berg 1949b (biology, morphology, distribution; as S. t. caspius ); Dorofeyeva 1967 (morphology, taxonomy); Belyaeva et al. 1989 (biology, fisheries); Danilov-Danilyan 2001 (conservation); Turan et al. 2020 (Cytochrome b phylogeny); Hashemzadeh Segherloo et al. 2021 (genomic data); Turan et al. 2022b (restriction to anadromous populations).