Sea Trout Research Programme

The Trust’s sea trout research programme is based on the recommendations of the Sea Trout Workshop we organised in 2011. The Workshop identified some key questions which need answering. These include the basic issue of why some trout go to sea and others do not, where sea trout go and how they behave once they reach the sea and the role of small streams for sea trout (and salmon) production. The AST’s research programme on sea trout is designed to help answer these questions. Most of the work we have funded so far is in freshwater, but we are hoping to be able to do more at sea in future

With this context, projects funded under the programme  primarily address four  issues:

(1) Why do sea trout go to sea?

Sea trout and brown trout form a single species and interbreed to form single populations. So why do some trout go to the sea and others stay at home? On many rivers, it is the females which go to sea, so that with more abundant food they can grow larger and produce more eggs, while the males remain in freshwater. But there are many variations, with rivers where both males and females go to sea, and trout in apparently very similar streams may behave completely differently.

A better understanding of why trout behave as they do is crucial if we are to improve the management of sea trout fisheries, managing all trout within a catchment as a single stock complex and developing targeted conservation measures for sea trout and for the populations which produce them. Currently, managers tend to focus on the sea-going part of their stocks, but the resident element may be equally important. Moreover, it is clear that the environment is a crucial factor, and climate-driven changes to the environment (for example, drier summers) may alter trout behaviour in ways that are difficult to predict.

(2) Sea trout life cycles and population dynamics, and the development of Biological Reference Points

Biological Reference Points (BRPs) are stock levels based on the biology and stock dynamics of a species that help fishery managers keep their numbers at a healthy level. Establishing such reference points is critically dependent on our understanding of the fish’s biology and on the information, we have on breeding numbers.  Salmon Conservation Limits are an example of a BRP; it has been possible to develop these because we know a lot about the biology of salmon and about salmon stock dynamics. This is not the case with sea trout; it is, for example, not easy to set a BRP for sea trout without knowing the contribution made to the population by resident trout. Work under the previous heading will obviously make an important contribution to this work, as will an improved understanding of sex ratios within sea trout populations, which is the aim of the project we are supporting on the river Tamar.

(3) The Role Small Streams Play in Sustaining Sea Trout Populations

Small streams provide invaluable spawning and juvenile habitat for trout, and contribute to the macroinvertebrate richness of catchments. Their significance, though, is often not appreciated, and they are very vulnerable to damage. Small coastal streams provide important spawning habitat for sea trout, with large fish moving into small, and sometimes very small, streams. We know little about these streams, or of the contribution that they make to the overall number of sea trout at sea. A project we funded exploring the genetics of sea trout populations in six small Cornish seams provided an important indication of the potential significance of small streams for the genetic diversity of trout in the UK.

We have also developed, with partner organisations, a system for assessing the state of small streams – Small Streams Classification System (SSCS); this will give managers a means of identifying problems affecting small streams so that these can be dealt with.

(4) Sea trout distribution and behaviour at sea

We need to know more about sea trout once they have left freshwater. Questions that need answering include where they go, how behaviour different between different age groups, how important are estuaries, (particularly to post-smolts), where do they overwinter, and what do they feed on. We also need to know, as with salmon, what the main causes of mortality are and whether they can be mitigated. We already know that, as in freshwater, different populations adopt very different behaviour patterns, with some migrating long distances and others staying close to their rivers of origin.

As with salmon, one way of answering some of these questions is to use acoustic tags to track sea trout at sea, and we need to ensure that tracking projects cover both species.

The Projects

The individual research projects are listed below. Where it is helpful they are grouped in categories, and details of each project can be found under these headings.

Using Stable Isotopes to Identify the Maternal Origin of Trout Fry

• Freshwater  / Marine Origins of Trout Fry in the Tweed Catchment
• Exploratory study of Freshwater / Marine Origins of Trout Fry in the Deveron Catchment
• Validation of Stable Isotope Analysis: Tweed and Deveron

Genetic Analysis of Sea Trout Populations

• Cornish Small Streams Project
• Sea Trout Sex Ratios in the River
• Shetland Sea Trout
• Why does a ‘brown’ trout become a ‘sea’ trout?

North Norfolk Sea Trout Project

• This uses both Stable Isotope and Genetic Analyses

Celtic Sea Trout Project Follow-on Projects

• Population Dynamics Analysis of Sea Trout Populations around the Celtic and Irish Sea
• Lipid concentrations as a measure of the condition of Sea Trout in the Irish Sea

Outer Hebrides Sea Trout Project  

Position of salmonids in the carbon cycle of a peatland lake ecosystem