Why are some rivers good for sea trout while others are not?

Sea trout probably occur in every river where there is access to and from the sea.  Their relative abundance in each seems to be linked with a balance of advantages and disadvantages of the various traits that the local trout may adopt. Intuitively, richer, slower-flowing rivers should favour freshwater residence; low-nutrient, spatey rivers should favour sea trout.  However, it isn’t that simple. The overall production of sea trout smolts may be greater in richer rivers than in poor ones, although a smaller proportion of the juvenile trout may migrate to the sea from rich rivers.  Variation in marine conditions also must be taken into account.  If growth and survival levels for sea trout at sea are persistently low, the anadromous trait may be a poorer option than one of freshwater residence and will be selected against. On the other hand, if the post-smolts encounter rich marine feeding and grow big, the returning females carrying many eggs, the sea trout fraction may flourish.