Friday 27 April 2018

Sea Trout can migrate up to 300km for feeding

Sea trout
Sea trout

Jim Hurley - Nature Trail

Following collaboration by fish experts in Ireland and Wales, Inland Fisheries Ireland has published a report called The Celtic Sea Trout Project. The report addresses significant knowledge gaps around Sea Trout in the Irish Sea. Importantly, the research carried out reveals new information about the species that will inform its conservation management.

Our native Brown Trout is a variable species of fish. The Sea Trout is one such variation. It has developed a taste for leaving freshwater rivers and going downstream to estuaries and inshore seawater because of the increased availability of food there. In the process of moving, it loses its brown colour and becomes silvery.

Because of its taste for being in seawater and because of its different colour the Brown Trout variety is popularly known as both the Sea Trout and the White Trout. As it moves between the sea and freshwater its colour changes from the pure marine silver with hints of blues and greens to being uniformly dark.

This migratory form of the Brown Trout goes to sea to feed for up to two years but returns to freshwater to breed in summer. Unfortunately, Sea Trout numbers are in decline. The aims of the recent EU-funded Celtic Sea Trout Project were to assess the size of the remaining stock, to try to find out why the population is falling and to identify its conservation needs.

The first of the key findings from the project is that the Sea Trout is not just one type of fish; nine genetically distinct groups of Sea Trout were discovered within the Irish Sea. Also, while most Sea Trout occurred in marine waters near the rivers in which they originated, some migrated up to 300 kilometres for feeding purposes.

The survey also discovered that shorter rivers of low alkalinity in areas with poor nutrients, but which had good spawning and nursey areas that were easily accessible from the sea, tended to be the better Sea Trout rivers. And, the warmer rivers of the south Irish Sea had bigger fish than the cooler more northerly ones.

The keystone prey species of Sea Trout at sea were found to be Sandeel and Sprat, both high-protein and high-fat species much sought after by many other marine fish and seabirds.

These findings will be particularly important in contributing to understanding both the fish's long-term conservation needs and the potential impact of climate change for Sea Trout and other species.

The Argus