Morphometric and ecological assessment of freshwater fish across an altitudinal gradient within the Zat watershed (Morocco)

The phenotypic flexibility and adaptability of freshwater fish can lead to morphological and structural changes in response to environmental variations. Documenting these responses are of paramount importance to understanding the vulnerability of these species to environmental perturbations. The Zat basin (Morocco) is facing interacting impacts from both anthropogenic stress and climate change, we examined how geographic and physicochemical variations across an altitudinal gradient impact fish. Surveyed fishes originated from two high-elevation (> 1300 m) and two low-elevation (< 850 m) sites. We found the Zat to be home to four endemic species of barbels, Luciobarbus ksibi, Carasobarbus fritschii, Atlantor reinii, and Luciobarbus magniatlantis. We explored correlations between several meristic and morphometric variables using geometric morphometric and scaled mass index (SMI) analyses. Analyses of water physicochemical quality revealed its improvement upstream, favoring the isolation of L. ksibi and revealing its adaptation to high-altitudes conditions and its resistance to low-altitude pollution. L. ksibi showed a reduction in the distance between their dorsal and pectoral fins, a higher value for eye diameter, a longer preorbital distance, and a shorter preorbital distance as they moved upstream, while C. fritschii showed an increased eye diameter and maximum body depth downstream. At the same time, the disappearance of L. magniatlantis downstream highlighted its sensitivity to decreasing environmental quality. The body condition of the fish was also influenced by altitude and pollution, where L. ksibi displays the highest SMI at low elevation (20.19 ± 2.27), whereas the lowest SMI is recorded downstream (8.03 ± 0.75). These observations suggest potential adaptations of L. ksibi relative to other fish to different environmental conditions along the altitudinal gradient, in response to thermal and chemical variations. These results highlight the sensitivity of barbels to altitude and pollution effects, paving the way for future studies in the field of ecological conservation.