By Charlotte Owen
November signals the start of the wild trout spawning season. Just like their close cousins the salmon, famous for persevering against the current, trout will be making their way up-river to their spawning grounds – but some will be travelling much further than others.
The brown trout and the sea trout are both the same species, Salmo trutta, but they lead very different lives. Brown trout live in our rivers and streams all year round, but only in those with sufficiently clean, cold water and gravel areas for spawning. Due to a slightly mysterious combination of genetic and environmental factors, which we still don’t fully understand, some brown trout decide to swim out to sea, leaving their freshwater brethren behind to feed out in the open ocean. So while brown trout will migrate several miles to return to their spawning grounds, sea trout may travel hundreds.
We don’t know exactly how far they roam but most sea trout spend a year or two at sea before returning to our rivers to breed, and they will be substantially bigger than their freshwater counterparts when they do. Trout are voracious predators and there are richer pickings out at sea, where they can bulk up on sand eels and sprats rather than insect larvae and flies. Fish that spend one winter at sea can be ten times heavier than fish that remained in the river, and populations of sea trout on the Ouse, Arun and Rother here in Sussex are particularly big. It’s not clear why but maybe the pure, clear water of our chalk streams helps to create the perfect environment for them to thrive; or maybe it’s in their genes. Salmo trutta is one of the most genetically diverse species on the planet and there are at least two distinct genetic strains of sea trout in Sussex. Remarkably, the level of genetic variation across British populations of wild brown trout far exceeds the variation between any populations in the entire human race, so this is an amazingly adaptable fish. Look out for the silvery forms of migrating trout and listen for their distinctive ‘splosh’ between now and the New Year.