Andy Fergusson

the author of this paper, examines why some brown trout go to the sea, while other individuals stay in the streams where they were born, migrating instead to the mainstream, estuary or lake (FF&FT October 2020).

The simple answer to the question „ What is a sea migratory brown trout?“ is thatit is a migratory brown trout that spends time feeding in the sea before returning to freshwater for further feeding, overwintering, or spawning. Generally, trout spawning takes place within small tributary streams. As illustrated in the life-cycle diagram, while some brown trout may spend all their lives within a few hundert meters of where they were born (stream-resident), most brown trout are migratory. In addition to optional migration to the sea, migrants can also occur to the downstream mainstream of a  river, to brackish estuaries and lagoons, or to a lake.


They are all smolts

It is now recognised that all migrations share many genetic and physiological features in common and thus all juvenile migrants can be referred to as smolts, not just those migrating to sea. Except for the south of England, there was no freshwater connection between Britain and Ireland and mainland Europe  at end of the last Ice Age, some 12,000 years ago, and colonisation had to be by sea. Thus, native brown trout populations here, irrespective of current life-history, are mainly descended from sea trout. In order to manage sea trout correctly, we need to understand why some of these original sea trout populations have become purely freshwater trout.

In regions such as Ireland and Scotland, where there are many thousands of lakes, river-lake migration is now numerically the most common life-history, and these lake-feeding trout are consequently the most important for anglers. Stream-resident and migratory trout can all arise from  the same population and individuals can change their migratory behaviour during their lifetime. Thus, in some populations sea trout regularly move between the sea and freshwater and former sea trout may adopt a freshwater life-history. Previous recognition  of sea trout as a diffrent species, or other management unit, from freshwater brown trout is no longer tenable. That, is both sea trout and freshwater trout must be managed together, contrary to current legislative arrangements.

Why are they silver?

Silvering of the body is often regarded as a charactersisic of see trout.However, silvering is camouflage for life in mid-water resoluting in the trout not standing out against the surface when viewed from below, and it is not in any way specific to life in the sea. Similar silvering is seen in mid-water feeding trout in lakes, and their smolts, leading in some cases to erroneous reports of sea trout being present. In some lakes both silvery plankton-feeding trout and more typical brown trout colorations of bottom-feeding trout are present, as well as often silvery fish-eating ferox.  Trout feeding in shallow brackish waters don‘ t require silvery camouflage and many have a sandy coloration to mach the background, such as the classic „yellow-bellies“ of Loch of Stenness, in Orkney.

What is a slob trout?

Alternatively, colaboration may not be recognisably different from trout remaining in the river. Trout feeding in deeper brackish water become silvery as for true sea trout and distinguishing the two is again difficult on coloration alone. The difficulty of identifying estuary feeding trout results in the imoprtance of estuary feeding beeing underestimated in many rivers. Although many trout use estuaries as staging post for onwardmovement to the open sea, some never move bayond the estuary. Brown trout that feed in shallow estuaries are frequently referred to as bull trout or slob trout, the latter name derived from the Irish term for mudflats that are typical of many such estuaries.

pstruzi FergussonSaltwater adjustment

Migration from freshwater (less than 0,05% salt) to full-strenght sea water  (more than 3 % salt) requires physiological adjustment. In freshwater, a trout‘s body fluids are more concentrated compared to the surrounding water and it is faced with the uptake of water by osmosis and the loss of salts by diffusion. To counteract this, copious urine  is produced, and salts are actively taken up by special cslls in the gills. The reverse is the case in full strenght sea water where the body fluids are less concentrated, with water bsing swallowed to counteract loss and excess salts gained are actively expelled. Many estuaries, lagoons and encosed seas are brackish water. At around 1 % salinity, a trout body fluids are the same concentration as the surrouding water,  and water and salt regulation are not required, with a cosequent saving in energy. Thus, the divide is not between freshwater and sea water, but between below and above 1 % salt concentration. Indeed, much of so-called „seas“ as the Baltic and Caspian are such than 1 % salinity ans are effectively freshwater, as least as far as the trout‘s physiology is concerned.

Why migrate?

The advantage of migration, irrespective of destination, is that more food becomes available allowing greather growth. For females, this means that many more eggs can be produced allowing a greater contribution to the next generation. For males, the advantage is less since enough sperm can be produced at small size, although larger males are more able to gain and defend mates. This explains why more sea trout and freshwater migrants are female and, in some rivers, almost all females are migratory and all males stream-resident. Stream-resident trout raraly exceed 0,5 kg in mass. However, by migration to better feeding a much larger size is achievable.

Brackish water has provided the largest brown trout specimens with, for example, trout in the Caspian Aea having pereviously been recorded up to 57 kg, although the size has reduced pn recent decades. Baltic „sea trout“ can reach 18 kg. A trout of 13,6 kg was taken in the Loch Stenness, the largest brackish lagoon in Britain, which still regularly produces specimen trout. In Britain, the record rod-caught sea trout is one of 12,85 kg caught in Solent  in 1922. The British  record river-lake migratory trout is a ferox of 14,4 kg taken from Loch Awe  in 2002. The relatively low abundance of prey fish, such as Arctic charr, in lakes in Britain  and Ireland means that only a small proportion of individuals can adopt to piscivory – feeding of fish – compared to the greater abundance of prey fish at sea. However. In some Scandinavian lakes, where fish prey is more abundant, almost all lake-feeding trout are piscivorous and the average size exceeds 4 kg.

Migration „trade off“

Whilw there are benefits to migration, there are also costs. Considerable energy is expended in making the journey tothe feeding destination and back again to the original tributary stream for spawning, especially if there are obstacles in the way. In general, more predators are present in the downstream parts of river systems, in lakes, and especially at sea. Parasites and diseases are also more prevalentin the sea.Thus, the risk of death increases with migration, especialy to the sea. Migration is therefore a „trade off“ between better feeding on the one hand, and increased energy expenditure and risk of mortality on the other.

The best feeding destination for a migratory trout varies widely  among river systems, depending on local conditions. In some river systems feeding may be better at sea then in freshwater. But in others the opposite is the case. Thus, where a lake with good feeding is present, sea trout are often absent even though there may by no barriers to movement to and from the sea. River-lake migration also reduces the likelihood of being predated or parasitised compared to at sea.

On the other hand, in river systems in low-nutrient regions such as peatland in western Scotland and western Ireland for example, sea trout are frequently present irrespective of the existence of lakes. Where there re no lskes in a nutrient-rich catchment, then feeding in the mainstream of the river may be the best cost-benefit strategy. Estuary feeding can also be better than going to the sea and many reduce the risk from predators and parasites.

The migration gene

Trout only migrate where the benefits from doing so outweigh the costs in terms of eventual survival and reproduction. Since an individual has no way of assessing these directly its decision in informed by the genes, which have been shaped by the experiences of its ancestors, together with current environmental conditions and its own nutritional status. If the ancestors that migrated were more successful and left more offspring than those that remained stream-resident, then the genes responsible for migration will have increased in that population. In other words, the migration genes have been subjected to positive natural selection.

The proprtion of individuals that migrates varies among populations as well as between sexes. Sea trout and other migrant parents can give rise to young that become stream-resident, and vice versa and both forms have been shown to interbreed. However, there is a strong tendency to follow the parental life-history, especially the maternal one. Owerall, around 50 %of the variability in migration versus stream-residency is due to genes,with the rest resulting from environmental factors.This is why, as already noted, the migration decision is flexible, and migratory trout can give rise to stream-resident ones purely as a result of changes in environmental conditions experienced by the juveniles, irrespective of parental life history.

Decision to migrate

On of the principal environmentalfactors involved in the decision to migrate or not is feeding quantity and quality. This decision occurs a year or more before external signs of becoming a sea trout are visible in the form of smolt transformation, the preparation stage for migration. If an individual‘s nutritional status, especially the amount of energy stored as fat, is below a genetically determined treshold lrvrl the individual „decises“ to migrate to find better feeding, If above the energy status treshold, it remains in the stream and becomes sexually mature.Thus, the hogher the treshold level thr more likely is in than an individual will migrate, as it will be more difficult to reach the condition required for residency. Female trout have higher tresholds than males resulting in females being more likely to the migratory. Different populations also can have distinct thresholds resulting in varying propensities for migration. Experiments on both sea trout and river-lake migrants have shown that low food availability increases the proportion of migrants, more so for females than males. Where the number  of young trout and salmon is greater, that is, where there is more competition for food, then more trout individuals migrate. Offspring of sea trout reared under standard hatchery condition often become residents, simply because they are too well fed! Since larger smolts are more successful at migration this can occur at different ages depending on when an individual reaches its optimal size.

Where to?

Once to decision to migrate is taken, the next consideration is where to migrate to, that is, the destination of migration. This decision is mainly under genetic control, ie, what did their parents do? However , some individuals can‘t readily categorised as to their life-history type and spend part of their life in a river, or in a lake, and part in the estuary or at sea, with regular movements between these habitats. As with whether to migrate or not., individuals can‘t directly determine the best place to migrate to in their river systém. Again, they rely on the experience of their ancestors as encoded in their genes trough natural selection. Thus, if ancestors that,say, migrated to a lake were left mostsuccessful, they will have left the most offspring, and the genes responsible for river-lake  migration will ptedominante in that population. In the case of trout populations above a waterfall or dam impassable to upstream migration, since migrants are lost from the migrants are lost from the population, natural selection results in the removal of genes responsible for migration, with the population becoming purely stream-resident.

Humans v migration

We now have a grater understanding of the genetic and environmental influences that determine what producess that determine what produces a sea trout and other brown trout migrants, although there are still many gaps on the genetic side. We also know many of the human impacts that result in the reduction of migration, especially to sea, and these will be covered in the next article. Only withfull kowledge of all factors involved can we hope to protect and restore sea and freswater brown trout migrant runs, especially in the face of growing threats to our rivers and seas by an ever-increasing human population.

Andy Ferguson was Professor of Biology at Queen‘s University Belfast. He has studied salmon and trout, especially genetics, for 50 years. Retired to Galloway, he tries to make scientific studies accessible to a wide audience and fishes for trout when possible.