 Stretches of both rivers have been designated as SSSIs because they are of special interest primarily for important populations of otter "Lutra lutra", bullhead "Cottus gobio", river lamprey "Lampetra fluviatilis" and brook lamprey "Lampetra planeri".They are also of special interest for sea lamprey Petromyzon marinus; for the range of river habitats including beds of submerged aquatic plants often dominated by water-crowfoot "Ranunculus" spp., as well as a variety of associated riverside habitats.The Cleddau rivers are also a Special Area of Conservation designated for the European bullhead ("Cottus gobio"), European river lamprey ("Lampetra fluviatilis"), Brook lamprey ("Lampetra planeri"), Otter ("Lutra lutra"), Sea lamprey ("Petromyzon marinus"); rivers with floating vegetation often dominated by water-crowfoot (watercourses of plain to montane levels with the "Ranunculion fluitantis" and "Callitricho-Batrachion"vegetation); active raised bogs; and Alder woodlands on floodplains (Alluvial forests with "Alnus glutinosa" and "Fraxinus excelsior" (Alno-Padion, Alnion incanae, Salicion albae)).