Salmonidae, Jarocki or Schinz, 1822

Family Salmonidae View in CoL

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).

Key to genera of Salmonidae in West Asia

1a - Lateral-line scales very elliptical, reduced to little more than nerve tube, smaller than scales in adjacent rows, with no or little overlap with scales in front or behind. ……………… Salvelinus

1b - Lateral-line scales roughly circular, as large as or larger than scales in adjacent rows, overlapping with scales in front and behind. ………………2

2a - Caudal with black spots at least on upper lobe, usually on whole fin; flank with a pinkish to red midlateral stripe or band. ……………… Oncorhynchus

2b - Caudal plain; flank without red midlateral stripe or band. ……………… Salmo