South East Rivers Trust

What have ‘The Sea Trout’ ever done for us?

Ok, they haven’t built any aqueducts, roads or made any wine… However, recent research has demonstrated how important sea trout are in maintaining freshwater resident brown trout populations. This gives greater incentives for freshwater conservationists to improve fish passage so migratory sea trout can return to their natal areas to spawn.

Same species, different life-style

Sea trout and brown trout are the same species (Salmo trutta L.), but have different life history. Sea trout are ‘anadromous’, meaning they migrate from freshwater to the marine environment to feed and grow. Whereas brown trout stay resident within the freshwater river their whole life. Both anadromous (sea) trout and freshwater-resident (brown) trout freely interbreed to produce fertile offspring. Like salmon, sea trout return to their natal river to spawn, this has made each rivers salmon and brown trout populations genetically distinct. The homing behaviour of the fish effectively isolate each population from each other, although some gene exchange can take place when fish ‘stray’ into another river and spawn there. Each river catchment presents slightly different conditions; this has led to local adaptations through natural selection to the local environment, meaning the genetics of each rivers population are slightly different. This genetic diversity and local adaptations makes each rivers population unique and best adapted to life in that river, this could be lost forever if populations suffer chronic events e.g. pollution incidents.

Sea Trout, Dave Brown

A proportion of any brown trout population migrates to sea, and thus become Sea Trout. This is known as ‘Partial Migration’, where a fraction of the population exhibits migratory behaviour while another does not. Different populations will exhibit different tendencies to migrate, some populations being dominated by freshwater-resident trout while others may be near wholly migratory; this may have a genetic link, but it is also known to be influenced by the environment. For example, low food availability is known to influence migratory behaviour. Trout populations from nutrient poor streams draining the acidic New Forest, for example, have a high proportion of individuals exhibiting migratory behaviour. While populations from more productive chalk streams, due to their stable temperature regime, tend to have a lower proportion of individuals migrate to sea. An increasing body of evidence is suggesting that stress plays a key role in the ‘decision’ to migrate to the sea or to remain in the river. Stress can be triggered by low growth, accounting for the differences discussed above, or be caused by specific events such as drought-associated low-flows. Therefore, the proportion of any population migrating to sea will differ from year to year, according to in-river conditions.

Benefits of seaward migration.

After 1-3 years in the freshwater environment, trout may undergo a process called ‘smoltification’, which is a period of great morphological, behavioural and physiological change when juvenile salmonids develop various adaptations that enable them to survive at sea. These changes include, a change to a silver coloration, to aid camouflage in the open water marine environment, and the ability to osmoregulate (regulation of an organism’s body fluids, in relation to internal and external salt concentrations) at sea.

Migration to sea is risky. The sea contains many predators that make for a risky environment for a young trout, and the journey down river is in itself associated with an increased mortality risk. However, the sea also contains numerous large prey items, like small fish and sand eel,  that trout grow quickly on. With much higher food resources available at sea, those trout that do enter the marine environment exhibit significantly increased growth. This can be observed when reading scales from trout that have spent time at sea. Fish scales, like tree trunks, have growth rings that can be read, to give the age of the individual, and the distance between growth rings gives an indication of the growth rate during that year / growing season.

Rapid growth at sea makes for large individuals returning from the marine environment. Freshwater wild brown trout will grow up to about 2 lbs in southern England, with fish up to about 4 lb being exceptional individuals. However, after a couple of years at sea, sea trout can grow to about 6 – 8 lbs, with some older individuals growing to well over 10lb. These large individuals are often females, and as a result of this increased growth rate, these individuals can produce many more eggs than a freshwater-resident trout. The eggs of a large female sea trout will also be much larger in size, and therefore contain more yolk which will fuel egg development and early growth. As a result, the offspring of sea trout emerge from spawning gravels earlier and at a larger size, giving them an advantage over the progeny of freshwater-resident trout.

Sea Trout on Redd New Forest Stream

Sea Trout research

A recent research paper from scientists from Exeter University, Queen Mary University and Game and Conservancy Wildlife Trust highlight the importance of a small number of large female sea trout in   maintaining freshwater-resident brown trout populations. The paper is: Goodwin et al. (2016) A small number of anadromous females drive reproduction in a brown trout (Salmo trutta) population in an English chalk stream. Freshwater Biology 61, 1075–1089.

Using a novel combination of stable isotope analysis and genetic analysis, the study aimed to;

  1. Quantify parentage by sea trout (ST) and freshwater-resident (FR) brown trout of newly emerged juvenile trout (fry) in a lowland English chalk stream (River Frome, Dorset)
  2. Assess relative parental fitness, in terms of number, size and time of emergence, of offspring.

The results demonstrated:

  1. An overwhelming contribution of anadromous (sea trout) parentage (both female and male) to fry production, with 76% of the juveniles sampled being parented by sea trout females and 63% parented by sea trout males. Despite the numerical dominance of freshwater-resident adults (6F & 12M [ST] vs 22F & 56M [FR]).
  2. Offspring of anadromous females emerged earlier and at a larger body size than offspring of resident females. Overall, this study suggests that sea trout offspring have an adaptive advantage and greater fitness in early development, and that a small number of female sea trout (six of 96 adults sampled) are the main drivers of trout production in this river.

Restoration downstream can bring benefits upstream

Given this evidence, restoration of fish passage at downstream barriers will allow sea trout to return to their spawning areas, often in the very upper reaches of river systems where preferred trout habitat exists. Allowing sea trout access to these upstream areas will see these fish to once more contribute to the spawning stock and allow their offspring, with their greater fitness thanks to their parent’s migration to the sea, to increase local populations of trout.

Great Stour (Kent) Your Fisheries Plan and Workshop

The South East Rivers Trust has created a pilot ‘Your Fisheries’ plan for the Great Stour in Kent, to drive rehabilitation of the sea/brown trout population in this catchment. The plan is available here:

A workshop will be held on from 7pm – 9:30 pm on Thursday 15th March 2018 in the Committee Room at the CDAA headquarters: 14 Mill Road, Sturry, Canterbury, CT2 0AF. The workshop will aim to feed back on the consultation carried out to create the plan and, specifically, to identify local opportunities to deliver projects to address the issues raised. Your local knowledge is crucial to this aspect of the workshop and we would like to draw on this, in an interactive session, so potential projects are aimed at addressing your priorities.

Please RSVP to confirm attendance:



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