Salmonidae, Jarocki or Schinz, 1822

Freyhof, JÖrg, Yoğurtçuoğlu, Baran, Jouladeh-Roudbar, Arash & Kaya, Cüneyt, 2025, Handbook of Freshwater Fishes of West Asia, De Gruyter : 621-627

Family Salmonidae

Trout

Salmonids are medium- to large-sized fishes that inhabit a wide range of habitats with clear, cold, and well-oxygenated water. They are native to the northern hemisphere but have been widely introduced for aquaculture, commercial, and recreational fisheries in cold waters worldwide. They have a very diverse and fascinating evolutionary biology, grow to a commercial size, and have good-tasting flesh with few bones. Nevertheless, few groups of fish have been so heavily impacted by human activities. The most significant challenge for salmonid conservation is stocking non-native salmonid species, or populations, for recreational and commercial purposes. In Europe, salmonid stocking has resulted in the genetic pollution of many native populations. Recent molecular studies suggest that genetic pollution of trout is limited in West Asia, as mostly rainbow trout are stocked. However, the over-exploitation of water sources, river regulation (dams, weirs), poaching, and climate change pose significant threats to many trout populations in the region.

Until the late 20 th century, virtually all trout native to the West Palearctic were identified as Salmo trutta . Many species described in the 19 th century are now treated as valid, and many new species have been described in the 21 st century, especially from Türkiye. Recent genomic studies indicate that trout populations in the Mediterranean and Ponto-Caspian regions diverged relatively recently, within a timeframe of 0.5–2.5 million years ago. This resulted in minimal to no mitochondrial DNA divergence. Standard molecular approaches, such as cytochrome oxidase 1 barcoding, have been unsuccessful in recognising the diversity of trout in West Asia. This is because all populations have relatively recently separated and, therefore, are not comparable with the evolutionary age of other species of fishes in the region. Species richness, high rates of phenotypic morphological and ecological diversification, and a very low genetic divergence between species make studies on the evolution of trout attractive and challenging. It is evident that recent gene flow and isolation of the populations of trout, particularly in the Mediterranean and Persian Gulf basins, have resulted in the evolution of small morphological differences, which has led to the recognition of a high number of species. Furthermore, natural hybridisation between different trout lineages has probably played an important role in the origin of several populations. This phenomenon requires further investigation into the species of West Asia. In conclusion, the available genomic evidence supports the view that trout are distributed in many isolated populations, which can be distinguished by details in colour and morphology. Should trout be classified as a multi-species taxon or better recognised as populations of one species ( Salmo trutta ) with small differences? This is a debated question and we are curious to follow the research on this in the coming years.

Some authors treat Coregonidae and Thymallidae as subfamilies, a view that is not adopted here. Further reading. Stearley & Smith 1993 (systematics); Stearns & Hendry 2004 (evolutionary biology); Geiger et al. 2014 (barcoding); Hashemzadeh Segherloo et al. 2021 (genomic data).

Open Access. © 2025 JÖrg Freyhof, Baran Yoğurtçuoğlu, Arash Jouladeh-Roudbar and Cüneyt Kaya, published by De Gruyter. the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

https://doi.org/10.1515/9783111677811-031

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Oncorhynchus mykiss ; Litani drainage, Lebanon; 160 mm SL.

Oncorhynchus mykiss lake forms may undertake long migrations to spawning Common name. Rainbow trout. streams. Females build redds in gravel bottom of moder-

Diagnosis. Distinguished from species of Salmo by: ● wide, ate to fast-flowing streams or rivers. Breeding populations pink to red stripe from head to caudal base (except in will not be established if peak juvenile emergence coinsea-run form) / ● caudal with black spots at least on upper cides with flood season and cold summer temperatures, lobe, usually on whole fin. Size up to 1000 mm SL. and if temperatures do not fall below 13°C. In agonistic

Distribution. Widespread or stocked from fish farms, behaviour, O. mykiss fry dominate Salmo fry of similar especially in Türkiye and Iran. Unclear where established. size, and juvenile and adult Salmo dominate similar size Native to Kamchatka and south to lower Amur drainage and rainbow trout. Feeds on a wide range of aquatic and terres- Pacific basin of North America south to northern Mexico. trial invertebrates and small fish. At sea, feeds on fish and

Habitat. Stocked in cold lakes, rivers, and streams. Usually cephalopods. Feeding is negligible below 5°C and above not stocked in water with summer temperatures above 25°C 25°C water temperature.

or in ponds with very low oxygen concentrations. Spawns Conservation status. Non-native; introduced for recreaon gravel in fast-flowing water. tional fisheries and as escapees from fish farms.

Biology. Anadromous, lacustrine, and resident forms. Further reading. Smith & Stearley 1989 (systematics, Often spawns first time at 1 year, 3–5 years in north, usually nomenclature); Behnke 1992 (diversity); Stearley & Smith in February–May when temperatures rise above 10–15°C. 1993 (phylogeny); Zelinsky & Makhrov 2001 (chromosomes); Farmed trout mature August–April, depending on strain. Fausch et al. 2001 (invasion success); Yoğurtçuoğlu et al. Makes short spawning migrations. Anadromous and 2021c ( Türkiye).

Too many species of trout? Twenty-five trout species native to West Asia have been identified, with one non-native species also present. Most of these species have been described based on narrow morphometric and meristic differences and small details in colour and colour pattern. Sympatric trout with different life histories have been treated as distinct species without clear evidence that they are not simply life-history forms. The majority of species have been described by a single population, and none of the species is well-supported by molecular characters, as they are all very young. It is acknowledged that trout in West Asia live in numerous, often small populations, and genomic data demonstrate that these are isolated from each other. This is also the case in other fish species. If we apply the same approach to other fish species, we could describe hundreds or thousands of cyprinids, minnows, loaches, and others isolated for several thousand years. This raises the question of whether we are constrained by the theoretical challenge of distinguishing between populations and species. Many additional trout species will likely be described, as many isolated populations remain unnamed. Alternatively, could we find ways to tackle the number of trout species by distinguishing between species and isolated populations with some private characters? Unfortunately, or happily, there is a lack of consensus on how to distinguish, describe and recognise trout species as valid. This allows discussions, diversity of opinions, and pluralism, which are all important for scientific progress. Nonetheless, it is important to remember that a species description is only a hypothesis regarding the relative and consistent distinctiveness of a set of individuals. The descriptions are influenced by several factors, including the prevailing attitudes and the authors’ perspectives. Further reading. Hashemzadeh Segherloo et al. 2021 (genomic data).

In high-altitude streams, such as in the Pontic mountains, Salmo are often the only fish species.

3b - Flank silvery or grey; 103–118 scales along lateral line counted until end of hypural complex; 24–28 scales rows between lateral line and dorsal origin.

………………4

4a - Red spots on flank larger than pupil; 3–7 black spots on opercle; no black spots on top of head; 1–2 dark-brown or grey bands on posterior part of flank in individuals larger than about 230 mm SL.

……………… S. baliki

4b - Red spots on flank smaller than pupil; 8–17 black spots on opercle; 5–18 black spots on top of head; 4 dark-brown or grey bands on flank in individuals larger than about 230 mm SL.

……………… S. okumusi

5a - Few black spots on back and upper part of flank; number of black spots not increasing with size.

……………… S. euphrataeus

5b - Numerous black spots on back, middle and upper part of flank; number of black spots increasing with size.

………………6

6a. - Adipose depth 8–10 % SL in male; lateral line with 117–125 scales counted until end of hypural complex; flank brownish in life.

……………… S. trutta (based on introduced population)

6b. - Adipose depth 3–8 % SL in male; lateral line with 109–116 scales counted until end of hypural complex; flank silvery in life.

……………… S. fahrettini

Trout on speciation pathways? Trout populations are found in numerous lake basins. In many cases, there are resident trout in the lake tributaries and trout in the lake itself. In larger lakes with sufficient food resources, individual lake trout may grow larger than stream trout, become silvery, slender, lose the red spots, and become piscivorous or planktivorous. Trout populations in lakes adapt to pelagic life by undergoing changes similar to those observed in anadromous trout. As anadromous-resident trout, large lake trout typically join their siblings at spawning sites. Despite these similarities, the two trout forms are not distinct species. They represent different life-history forms of a single population. However, life-history strategies are subject to distinct selective challenges. Assortative mating would allow them to adapt better to their particular environments, whether in the lake or the stream. Furthermore, as all male trout prefer to mate with large females, large male lake trout can monopolise female lake trout, leading to assortative mating over time. This process should lead to the reproductive isolation of lake trout from resident trout. However, fully reproductively isolated trout species within one lake-stream system are rarely documented. The lake-stream system’s instability may interrupt the speciation process too often. Fluctuating environmental conditions and unbalanced ratios between lake and resident trout at the spawning places lead to hybridisation. This process acts against the isolation of both “kinds” of trout. The permanent activity of small male trout, which engage in sneaking, may inhibit the ability of large male lake trout to monopolise the attention of large females. If trout species cannot engage in assortative mating due to sneaking behaviour, spatial and temporal segregation is the only mechanism that allows them to reach reproductive isolation and speciate. This pattern is observed in sympatric trout species, with each species spawning at different times and locations. In West Asia, the only documented example of multi-species lake trout is Lake Sevan in Armenia.

Salmo abanticus ; aquaculture, Lake Abant, Türkiye; ~ 225 mm SL.