Author Fran Southgate
As the rain falls outside, its not such good weather for us humans any more, but its good weather for ducks, and for some of our most mysterious fish! For reasons as yet unknown, Sussex is home to some of the largest sea trout in the UK. Sea trout and brown trout are the same species (Salmo trutta) but for reasons not really understood, some brown trout develop special marine adaptations. The sea trout breeds and spends its early life in freshwater rivers, before migrating to the sea and returning as an adult to breed.
The majority of adult sea trout spend one to three years at sea before returning to our rivers. Fish which spend one winter at sea are around ten times the weight of brown trout of the same age which remain in the river. It is not known how far they travel at sea. They may swim hundreds of kilometres over the continental shelf or to the Baltic Sea to feed, but they have a strong homing instinct which draws them back repeatedly to the river of their birth.
We know that populations of our sea trout on the Ouse, Arun & Rother rivers in Sussex are notably ďspecialĒ and appear to be of a much greater average size than other UK populations. The reasons for this are unknown but could be linked with rich local feeding grounds or unique genetic features. It is possible that the pure, clear water of our chalk and greensand streams help to create a special environment for our sea trout to thrive in.
There may even be two (or more) distinct genetic strains of sea trout in Sussex which have evolved since the last ice age. Some adult trout return to the Arun as early as May. Sussex Ouse sea trout spawn later in the year, typically from around the last week of November until mid February. We donít know why or if this is important.
At this time of year, both populations of sea trout are at their peak, and will be making their annual breeding journeys up to their spawning grounds Ė often over just a few days. So itís a very good time to keep an eye out for them. Look out for their silvery form darting up rivers, jumping up weirs and for their Ďreddsí (dents in river bed gravels which they make before they spawn). Perhaps in time we will discover what drives these mighty fish to make their special trips back to our Sussex rivers.