First description of redds of the only non-introgressed population of a critically endangered salmonid in a stream vulnerable to droughts

Location, dimensions, substrate and depth of the redds of the only non-introgressed population of the critically endangered Mediterranean trout Salmo cettii Rafinesque 1810 were investigated over 2 days, in January–February 2021, in a reach located in the headwaters of Tellesimo stream, Sicily. The habitat type (pool, riffle and glide) was identified for all the 13 redds observed; lengths, widths, depths and substrate size were measured in a sample of them. Eighty-five percent of the redds were located in pools and 15% in glides; no redd was found in riffles. The median total redd length was 80 cm (range: 43–580 cm; n = 11), and the median total area was 0.20 m² (range: 0.01–2.09 m²; n = 11). The median size of dominant substrate in pot was 0.65 cm (range: 0.20–2 cm) and in tailspill 1.25 cm (range: 0.50–2.00 cm). Redds were found at median water depth of 27.5 cm (range: 12–98 cm). These results represent the first description of the characteristics of the spawning nests of this autochthonous salmonid.