Unlocking the sea trout enigma

Called The Celtic Sea Trout Project (CSTP), it was produced by Inland Fisheries Ireland (IFI) and addresses significant knowledge gaps around the enigmatic species. This migratory trout has a significant fisheries value, but some sea trout fisheries in parts of Ireland and the UK bordering the Irish Sea are suffering decline.

Key findings

There are nine major genetically distinct sea trout groups within the Irish Sea. Although the majority of sea trout occurred in marine waters in the vicinity of the rivers in which they originated, some migrated up to 300 kilometres for feeding purposes.

Sea trout abundance, based on angling catches, varied considerably over time and between rivers and regions around the Irish Sea but catches were shown to demonstrate a degree of synchronous variation pointing to some common factors. The river size and the fishing effort in the area was recognised as a dominant factor controlling the catch. It 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.

Sea trout growth, which is strongly tied to environmental factors of temperature and food, is an important measure of subsequent smolting and of marine survival. The spatial variation in marine growth in sea trout in this research was significantly positively correlated with mean annual sea temperature. Fish from more northerly rivers were smaller at age .0+ (finnock) and .1+ (maiden) than fish from southerly rivers.

Sea trout diet at sea is mainly fish-based and their preference for high protein/lipid prey can be met by sand eel or sprat, depending on which is available. Enhanced feeding opportunities are a major factor in determining the benefit of trout migration to the sea and sea trout are dependent on these keystone prey species (sand eel and sprat), as are many other marine fish and bird species. The study demonstrated that the marine habitat and ecosystems are strongly structured. The combinations of current, depth, seasonal fronts, temperature and salt content within this marine environment appear to influence productivity and growth opportunity for sea trout.

Various models were developed over the course of the CSTP project which will contribute to enhancing the decision making framework for managers of sea trout in both the freshwater and marine environments. These will be particularly important in contributing to understanding the potential impact of climate change for sea trout and other species.

Report reaction

Sean Kyne, Ireland's Minister with responsibility for Inland Fisheries, said: “I particularly welcome this report and the exemplary collaboration between Irish researchers at Inland Fisheries Ireland and other bodies and their international counterparts. The research has resulted in a better understanding of the sea trout stocks in the Irish and Celtic seas and this will underpin logical and well-informed decisions on the management requirements that are needed to safeguard these stocks into the future and to ensure the maximum social and economic contribution is secured.”

Dr Cathal Gallagher, Head of Research at IFI, added: “This is a ground-breaking multi-agency partnership investigation which aims to fill some of the information gaps around sea trout conservation. The first study of its kind, the Celtic Sea Trout Project is a wide-scale comprehensive, cross-disciplinary project which has provided valuable insight into many important research needs in this area, which were first identified at the International Symposium on Sea Trout in 2004. Its primary purpose of improving understanding of sea trout stocks in order to support better management in the freshwater and marine environments has been achieved.”.

To view the full report, click here.