A fish is an aquatic vertebrate with scales, fins, and gills. About 32,000 fish species are known, divided into jawless fishes (agnathans), cartilaginous fishes (chondrichthyans), and bony fishes (osteichthyans). Fishes occur wherever water of reasonable integrity exists, from sea depths of 8000 m to mountain streams and lakes at 5000 m altitude. Marine fishes constitute 58% of all species, freshwater species 41%, with 1% of fishes moving regularly between sea and fresh water. Tropical areas have the highest diversity. Fish biodiversity is threatened by many human activities, but habitat modification, overharvest, and introduced species are particularly injurious.

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