The flora and fauna of a river is determined by the availability of sufficient oxygen and the variety of essential chemicals for river life. The flora is influenced by the trophic status of the water, i.e. soft acidic water, hard calcareous, or something in between, and these conditions will typically be influenced by the nature of the substrate underlying the river. The flow of the river and water bed material also influences where plants can grow and where the fauna is most abundant and diverse. Rivers also host a number of high profile animals, such as otters and migratory fish, e.g., Atlantic salmon, sea trout, sea and river lampreys, and Twaite and Allis shad, and the distribution and abundance of these species in a river system can be a measure of a rivers health. The abundance and diversity of invertebrates such as mayflies, stoneflies and caddis flies can also indicate the health off a river as these species require well-oxygenated water to survive.