Sea Trout Watch

Our work with the migratory Sea Trout
A Sussex Ouse Sea Trout (Salmo trutta)

A Sussex Ouse Sea Trout Salmo trutta

Sea Trout Watch

Every year between November and February OART undertakes its catchment wide sea trout watch. This project counts the number of redds observed across the catchment. Having done this now for many years we have a large data set which shows annual changes in the first spawning activities as well as changes in redd locations which can subsequently be investigated and any problems can be addressed. These surveys further enable us to direct management and task force activities to the right locations in order to clean gravel areas which have become smothered in silt.

As you can imagine these surveys are a huge undertaking and we always need more help, if you would be interested in finding out more about our monitoring and becoming part of the team then please contact us at

The Sea Trout


An introduction

The sea trout is a migratory form of the brown trout, Salmo trutta. It is what is termed an anadromous fish; one which breeds and spends its early life in freshwater, before migrating to the sea, where the bulk of its growth occurs. It then returns as an adult to freshwater to breed and perpetuate the species. Its life cycle is very similar to that of the Atlantic Salmon, to which it is closely related and resembles in appearance, although adult Sussex Ouse sea trout are actually larger than many salmon. Lowland rivers such as the Sussex Ouse generally have very limited, or often no sea trout population and the fact that the river supports a good population of the fish is thus rather surprising. There is no accurate information on exactly how many adult sea trout migrate up the Sussex Ouse each year, but it is certainly at least several hundred fish. These form the basis of an important recreational fishery; the number of sea trout caught by anglers each year is generally the highest for any river entering the several hundred km of coast between Southampton in Hampshire and Whitby in Yorkshire.

Spawning – Redds

The life cycle of the Sussex Ouse sea trout starts in late autumn and winter when the adult fish, having returned from the sea to their home river, migrate up the Sussex Ouse and into its tributaries to find suitable spawning grounds. The adult sea trout are large and powerful fish, generally about 2.5 kg – 3kg in weight and 55-65cm long, but may reach 7kg or more and exceed 75cm in length. They need to find suitable areas of gravel in where they can excavate a nest, termed a redd, in which to deposit their eggs. The redd is excavated by the female (hen) fish, using her tail, which she fans very rapidly over the gravel to excavate a hollow depression in which she lays her eggs, before covering then with a mound of gravel. Suitable gravel for the excavation of a redd is typically between the size of a marble and golf ball; larger fish generally chose gravel beds where the indidual pieces of gravel are larger, but this is not invariably the case.

Sea Trout Redd in one of the Sussex Ouse tributaries

Sea Trout Redd in one of the Sussex Ouse tributaries

A completed redd is usually approximately circular or oval in shape (although it may be more elongated) and comprises a raised mound of gravel, in which the eggs have been deposited, often with a pit visible in front of the mound where gravel has been excavated by the fish. Redds vary in size; many are around 50cm across and 10cm in height, but they can be large and conspicuous structures 1m or more in diameter, which may occupy the entire width of a small tributary stream. Large fish tend to excavate larger redds (although this is not always the case; very large fish of 5kg or more may make a redd only 30cm across) and in some cases more than one fish will excavate a redd together; it is not unusual to see two or thee female fish working the same redd and in some cases, particularly in areas where suitable gravel is scarce, larger numbers of fish may be involved.

As well as proper redds, which actually contain eggs, more tentative and incomplete excavations (often called “scrapes”), which do not contain eggs can frequently be seen – these are the result of a fish having started to construct a redd but then abandoning it, possibly because the gravel was unsuitable. Fish need fairly loose gravel so that they can excavate it. Compacted gravels, which may be caused by silt building up within them (in part as a result of more silt entering rivers due to changing agricultural practices), may be too solid for the fish to break up to construct a redd; additionally a high silt content can clog the redd and reduce water flow through it, preventing sufficient oxygen reaching the eggs and suffocating them before they can hatch.

Once the redd is excavated to the satisfaction of the hen fish, she is joined by a male (cock) fish, which has remained close by during the excavation of the redd. The male rubs his flank against the side of the female, and as she releases her eggs, he releases a cloud of sperm into the water upstream of them. The female fish then excavates further gravel to cover over the by now fertilized eggs, which may finally be buried 10cm or more inside the redd. An individual female fish may repeat this process several times, excavating a separate redd on each occasion, before all her eggs are deposited. While spawning typically involves a pair of fish, male and female, two or sometimes more males may be present on the redd with the female, and an individual male fish may spawn with several females. Resident brown trout, which are the same species as sea trout but have not migrated to sea, may spawn with sea trout. Redds, when newly excavated, are easily to spot, especially in small streams, and counting them gives a useful indication of how large the population of spawning fish is.

The number of eggs laid by a particular female sea trout is determined by her size. Very small adult sea trout (although small fish are unusual in the Sussex Ouse) may only produce a few hundred eggs, however a typical adult Sussex Ouse sea trout will produce several thousand and very large fish may produce 10,000 or more eggs. Each egg is spherical and around 5mm in diameter; the size of a small pea. Fertilisation takes place in the water at the bottom of the redd almost immediately after the eggs and sperm are released and is achieved by the head of a sperm entering an egg through a tiny hole termed the microple. A high proportion of the eggs are usually successfully fertilised, usually 90% or more.


The length of time it takes the eggs to hatch is determined by water temperature, but is typically between 2 and 3 months. While there are few actual observations, most Sussex Ouse sea trout eggs probably hatch between March and May. Eggs develop and hatch more quickly in warmer water; spring water from the Chalk aquifer has a uniform year round temperature of about 17degrees C, so it may be expected that eggs deposited in the Chalk stream tributaries (e.g. the Northend and Plumpton Mill Streams) would hatch earlier than those in redds in rain fed tributary streams (such as the Batts Bridge Stream) . Before they actually hatch the black coloured eyes of the developing young fish are visible through the shells of the eggs; such eggs are know as “eyed eggs (or ova)”.

Alevin & Fry stages

The newly hatched fish (called alevins) are about 1.5- 2cm long and have a large yolk sac attached to them, which provides an initial food supply. They have very limited swimming ability at this stage, and at first remain within the shelter of the redd, but as the yolk sac is used up they work their way out of it into the main flow of the stream and start feeding on tiny invertebrates. With the yolk sac gone, the young fish, now called fry, disperse away from the redd and start to establish their own territories. They grow fairly slowly, feeding mostly on invertebrate food items such as freshwater shrimps and mayfly larvae; after a few months they are still only a few cm long, and at a year old are generally in the 8-12cm size range. By this stage they have developed an overall brownish coloration with red and black spots and about 10 characteristic bluish or purple oval spots in a line along each flank. These distinctive spots are called parr markings and the young fish at this stage is itself called a parr. After another year, at age 2, they typically range from 15 -25cm in size.

Parr & Smolt Stages

Sea trout parr may spend anything from 1-3 years (occasionally longer) in fresh water before migrating to the sea. Sussex Ouse sea trout typically spend two years in freshwater. These young fish are indistinguishable from non migratory brown trout, which remain in the river system for their entire life. Scale readings of Sussex Ouse sea trout generally show that the parr grow more quickly during their second year. This may be because they spend the first year in the tributary stream in which they are spawned, before dropping downstream into the main river during their second year. The greater food supply (including small coarse fish) and deeper water in the main river may lead to faster second year growth. Some young Sussex Ouse sea trout do show a considerable increase in growth during their second year and may be as large as 30cm as a two year old fish, which is exceptionally large for a sea trout parr.

A Sea trout parr from the Bevern Stream

A Sea trout parr from the Bevern Stream

As the time approaches for a parr to migrate to the sea, its appearance changes as it becomes what is known as a smolt. It becomes bright silver in appearance (this is achieved by the deposition of crystals of guanine in the surface covering of the scales), with black, and often still some red spots. Its pectoral and pelvic and anal fins often become bright yellow. It takes several weeks for the silver coloration of the smolt to replace the brownish hue of the parr; in the Sussex Ouse some parr start showing signs of this change by September, though others may not become smolts until the late winter or spring. Smolts tend to group into shoals as they move downstream and in tandem with changes in their physical appearance there are changes in their physiology which will allow them to survive in sea water – only a very few species of fish are able to modify their physiology in this way – most freshwater fish species, if accidentally washed out of a river into the sea will quickly die.

There is a peak in downstream smolt migration in the spring, from March to May; however in the Sussex Ouse, smolts do migrate downstream in smaller numbers from the late autumn and throughout the winter. While the majority of smolts probably drop down through the tidal river and directly into the sea, it appears that a (possibly significant) proportion of them, rather than migrating directly to the sea, having entered the tidal river in autumn, instead of continuing towards the sea, temporarily migrate up, and overwinter in certain streams, particularly the Winterbourne in Lewes. In spring they migrate downstream out of these winter refuges into the main river again to continue their migration to the sea. This overwintering behaviour is unusual for a sea trout population and is a subject which warrants detailed research. There have been incidents when large numbers of smolts have become trapped in the Lewes Winterbourne in late spring when it starts to dry up (a winterbourne is, as its name suggests a seasonal stream which only flows during the winter) and have had to be rescued by the Environment Agency and returned to the main river.


By late spring the smolts will all have entered the sea, where the greater food supply relative to the river allows them to grow much more rapidly than non-migratory brown trout. In many British rivers a large proportion of the smolts return to the river very quickly, after only a few months at sea. These fish are know in different regions of the UK by various local names such as finnock, herling, school peal or whitling (the term whitling is usually used for Sussex Ouse fish) and in the majority of British rivers with a sea trout population, large shoals of these small fish (usually 25 – 40cm in length) dominate the sea trout run, with a smaller number of larger fish which have spent a year or more at sea. The situation on the Sussex Ouse is different to this typical population structure; whitling comprise a small proportion of the run and the majority of sea trout entering the river are large fish which have spent a year or more at sea. A similar situation, i.e. a population dominated by large fish which have spent at least a year at sea, does occur in some other rivers entering the east coast of England, such as the Yorkshire Esk.

Small numbers of whitling do however enter the Sussex Ouse, the first arriving around July, and continuing to arrive in low numbers until around Christmas. As these fish have only been at sea for a relatively short period, probably only a few weeks in the case of the first of those to return, it is likely that they have not ventured in the sea far from the mouth of the Sussex Ouse, although there is no reliable information on how far they do travel. It is also uncertain what proportion of these small whitling are actually mature fish which spawn in the river – some certainly do, as there are observations of them spawning, however it seems likely that at least a proportion of them run up the river to overwinter in it, but then drop downstream again without spawning. Very bright silver, apparently unspawned, whitling are for example regularly seen in the Bevern stream around February, apparently dropping back downstream without having spawned. Whitling returning to the sea will typically spend another year there before they return again to the river, as mature adult fish.

Adult Fish

The majority of adult sea trout to enter the Sussex Ouse are fish which have spent a full year or more at sea before their first return to the river. Fish which spend one winter at sea before returning are generally from 2-3kg in weight. This is around 10 times the weight of non-migratory brown trout of the same age which have remained in the river. Adults spending two winters at sea before they return may be considerably larger and are usually in the 3-5 kg range, although they may reach 6kg or more. Exceptionally, adults may spend 3years, or even longer at sea before they return as very large fish.

It is not known how far Sussex Ouse sea trout spending a year or more at sea travel within the English Channel or North Sea before they return to the river. They may well swim hundreds of km over the continental shelf and it has been suggested that some may even travel to the Baltic Sea to feed there. Studies have shown that adult sea trout from other rivers may travel very long distances – fish migrating from the River Tweed on the Scottish borders to East Anglia are documented for example – and it likely that the Sussex Ouse fish travel comparable distances. Interestingly, sea trout captured at sea in nets in the Friesian Islands, off the coasts of the Netherlands and Germany, look very similar in appearance to Sussex Ouse fish and it is tempting to speculate that fish from the Sussex Ouse may regularly cross the North Sea. However, tracking studies of individual tagged fish would be required to establish how far, and to where, the Sussex Ouse sea trout do travel at sea.

Upstream Migration – “Run”

Returning adult sea trout usually first appear in the tidal reaches of the Sussex Ouse in May, although there are records of the first fish of the year arriving before this. There is usually an appreciable run of adult fish in June, although its timing is dependant on the prevailing flow conditions. Low water delays the run, as they remain at sea awaiting rain, while spate conditions, which provide better travelling conditions for the fish, encourage them to swim upstream. Fish often enter the river together in groups when conditions are right; an influx of fish is called a “run”. A sea trout which has newly entered the river from the sea is called a “fresh run” fish. These fish still often carry sea lice, a marine parasite, on their bodies; these parasites drop off after a few days in freshwater. Adult fish continue to return throughout the summer, autumn and early winter. Fish arriving at spawning time, and in spawning condition, travel very quickly to the spawning grounds – it is probably that they leave the sea, travel up river and reach the spawning grounds in perhaps a few days or a week. If they quickly return to the sea, they may only have been in the river for a few weeks in total. However, fish which arrive earlier in the year are not yet sexually mature and may have up to 8 months in the river before they are ready to spawn. Once they have entered the Sussex Ouse, many of these early fish will remain in the lower reaches of the river, and may lie in one particular favoured place for weeks or even months. When they are finally ready to spawn, around the New Year, they make a final upstream push to the spawning grounds.

Winter Spate at Redbridge Weir on the Bevern Stream

Winter Spate at Redbridge Weir on the Bevern Stream

Sussex Ouse sea trout spawn later in the year than is the case for many other rivers, when spawning is at a peak in October and November. The Sussex Ouse fish rarely spawn before late November, and fish ready to spawn are often still entering the river at the start of the New Year. Spawning takes place predominantly in accessible tributary streams, but also in the main river where there are suitable gravel beds. The Sussex Ouse spawning season extends typically from around the last week of November until mid February, peaking in early to mid January, but may be delayed in winters of low rainfall, as the fish may not enter the river until spates occur into the New Year. During the exceptionally dry winter of 2004/5 fish were still spawning in late March, and in the main river as opposed to the tributaries, as low water levels prevented the fish entering smaller streams and migrating upstream past weirs.

The appearance of the adult fish changes as spawning approaches. Sea trout entering the river in spring and early summer are inevitably bright silver, with black spots – the extent of spot marking is variable; some Sussex Ouse fish are almost entirely silver with only a few black spots, but more typically they are heavily spotted in appearance, more so than in many other rivers. After a few weeks in the river their appearance starts to change. The silver colour gradually becomes dull; in females the colour typically fades to grey – sometimes a pewter or lead shade, perhaps with a purple sheen – or dull brown; males frequently initially develop a pinkish sheen and eventually a deep reddish brown colour. The existing spots become more prominent and new spots may develop, often to the extent that the back and flanks of the fish are entirely covered with spots.

In male fish, the jaws grow larger and the lower jaw may develop a prominent hook, called a kype, at its tip. This is assumed to be used for fighting with other competing male fish to secure a place on the spawning grounds and access to female fish, although displays of such aggression are infrequently observed in the Sussex Ouse catchment. Fish (male and female) which have adopted their spawning colouration are often termed “coloured fish”.

While sea trout arriving in spring and early summer are always silver, those entering the river later in the year, from around September onwards, may have already lost their silvery sea going appearance by the time they enter the river. While some of them may still be almost as silver as the early season fish (although they may have developed some red as well as black spots), many will have started to adopt their spawning colours before they entered the river.

Prominent Hook jaw, or Kype of the Cock Fish

Prominent Hook jaw, or Kype of the Cock Fish

Fish may still be entering the river as late as Christmas, but these will nearly all be in full spawning colours. Such fish are virtually indistinguishable in colour to non migratory brown trout in spawning. Before the Sussex Ouse was modified by man’s activities, adult sea trout probably managed to enter virtually every minor stream within the catchment. At the present time, only around half the total length of potentially useable spawning streams within the catchment are accessible, due to man made obstructions. These include weirs, culverts and dams to create artificial lakes. Adult sea trout are, like their relatives the salmon, powerful jumpers, and can surmount fairly high obstacles such as weirs a metre or more in height. However a number of obstacles on the Sussex Ouse are so severe as to completely prevent fish getting over them, and others can only be passed under extreme spate conditions. Fish passes have been constructed on a number of weirs, to allow fish to surmount what would otherwise be difficult or impassable obstacles.

Seaward Migration

Once spawning is completed the fish, know now as kelts, return to the sea. The physical exertion of migrating to the spawning grounds, and the act of spawning itself, take their toll and not all fish survive. It is not certain what proportion of fish do successfully return to the sea, however only small numbers of dead kelts are actually seen each year, suggesting that mortality is low. Additionally, kelts themselves are only seen for a short period after spawning has finished; this suggests that they may actively, and quickly, return to the sea, rather than passively drifting dropping downstream with the current. Some of the spawning streams used are very small, and only really accessible to adult fish briefly in spate conditions, when water levels are high. It is likely that adults very quickly enter these streams after rain, spawn within a day or two, and then actively return downstream, first into the main river and subsequently to the sea.

Once they have reached the sea, they commence feeding actively and regain condition. Many of these will survive to return to the river to spawn on a number of occasions, unlike their close relative the Atlantic salmon, of which only a small percentage survive to undertake a second spawning run. The salmon is well known for its ability to return to the river of its birth and, while possibly not as well developed, the sea trout shows the same homing instinct. Sussex Ouse sea trout and those in several adjacent rivers in Sussex and Kent (such as the Rother) have a very distinctive appearance, typically being deep bodied, heavily spotted fish. They show exceptionally fast growth – Sussex Ouse sea trout have possibly the heaviest average weight of any English or Welsh river- and differ in behaviour in some ways from stocks in other rivers. They are notably different in appearance and size to sea trout entering south coast rivers further to the west. This suggests that they are a discrete local strain, which homes reliably to its river of birth, in a way comparable to salmon.

Scale readings of Sussex Ouse sea trout, which have been undertaken for many years by Dr. Clive Fetter, have provided much valuable information about the life history and growth of individual sea trout. Sea trout scales have rings, similar to the rings on a tree trunk, which can be read so as to reveal information such as the fish’s age, growth rate, and key stages in its life such as the age it became a smolt and migrated to sea. However, detailed tagging studies and study of the genetics of the population would be needed to discover more about the accuracy of the homing instinct and whether the fish are indeed a discrete and possibly unique strain of sea trout. In the 19th century it was though that there were over 15 different species of trout in the British Isles. Subsequently scientists concluded that they all belonged to one very variable species, Salmo trutta, which has both non-migratory and migratory populations. However, there are undoubtedly significant differences between many trout populations and it is possible that studies using modern techniques for genetic analysis may indeed demonstrate that, while not actually a different species, the Sussex Ouse sea trout is nevertheless significantly different from other sea trout populations.

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