What have The Sea Trout ever done for us?

To paraphrase Monty Python’s Life of Brian, what have sea trout ever done for us? They haven’t built any aqueducts, roads or made any wine…. but scientific research has demonstrated just how important they are in maintaining freshwater resident brown trout populations.

Enabling migratory sea trout to return to their spawning areas is just one reason freshwater conservationists work to improve fish passage and the ecological connectivity of rivers.

The South East Rivers Trust’s head of Science and Partnerships, Dr Chris Gardner, illustrates below an example of the trout to underline the importance of overcoming barriers to fish migration.

Weirs in the way of nature

Structures such as weirs – like the one pictured here on the River Darent – disconnect sections of rivers, preventing fish moving upstream to their preferred spawning habitats, or to the sea.

Weirs were put in place for industrial purposes, for example to power watermills. They slow the flow of the river and also limit the movement of its gravels, which should move freely along the waterway to help build diverse habitats.

Weir removal or mitigation also increases a river’s resilience to pollution incidents. If fish can move freely within a catchment, they can avoid poor water quality episodes by moving out of the way and repopulate sections of river after localised mortalities.

Since 2012, SERT has removed many weirs on our rivers and addressed fish passage at the catchment scale as part of its Hogsmill Connectivity Project.  Where it was not possible to remove weirs, we have mitigated their impact on fish passage by installing either rock ramps or fish passes.

A weir fragmenting habitats on the River Darent. Photo Chris Gardner

Same species, different lifestyle

Sea trout and brown trout are the same species (Salmo trutta L.), but have different life histories. Sea trout are ‘anadromous’, meaning they migrate from freshwater to the marine environment to feed and grow. 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 birth river to spawn. This has made each river’s 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 river’s population unique and best adapted to life in that river. This could be lost forever if populations are wiped out because trout cannot migrate.

Large sea trout. Photo Dave Brown

Reasons trout migrate to sea

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. Yet fewer trout tend to migrate to sea from productive chalk streams, because of that type of river’s stable temperature regime.

An increasing body of evidence is suggesting that stress plays a key role in a trout’s ‘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 or poor water quality episodes. Therefore, the proportion of any population migrating to sea will differ from year to year, according to in-river conditions.

Dead brown trout following a major agricultural pollution incident. Photo Chris Gardner

Benefits of seaward migration

After one to three years in the freshwater environment, trout may undergo a process called smoltification. This 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.

The sea contains many predators that make for a risky environment for a young trout and the journey down river is, itself, associated with an increased mortality risk.

However, the sea also contains numerous large prey that trout feed upon and grow quickly on, such as small fish and sand eels. Significantly increased growth 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 an 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 2lbs in southern England, with fish up to about 4lb being exceptional. However, after a couple of years at sea, sea trout can grow to about 6-8lbs, with some older trout growing to well over 10lb.

These large individuals are often females which, because of this increased growth rate, can produce many more eggs than a freshwater-resident trout. The eggs of a large female sea trout will also be much larger 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.

Brown Trout. Photo Chris Gardner

Sea Trout driving population growth

A research paper from scientists from Exeter University, Queen Mary University and Game and Conservancy Wildlife Trust highlights 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); and
  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.
Sea Trout on redd in a New Forest Stream (photo: Chris Gardner)

Restoration downstream can bring benefits upstream

Given this evidence, restoration of fish passage at barriers such as weirs will allow sea trout to return to their spawning areas, often in the very upper reaches of river systems where preferred trout habitat exists.

Giving sea trout access to these upstream areas will allow 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 native wild brown trout.

Historic water milling structures block fish migration pathways. Removal or migration by fish pass installation is required to bring benefits to fish communities catchment wide. Photo Stuart Merrylees