INTRODUCTION
When the last ice age ended some 11,500 years ago and the ice retreated this major global event allowed the colonisation of our rivers by the sea trout.
Those early sea trout were not only the ancestors of our current sea trout but also of our native brown trout for it is now a well documented and accepted fact that although the resident Brown Trout you may encounter in your local river and the fresh run Sea Trout sheathed in its metallic silver coat may look entirely different creatures they are in fact both the same species – Salmo Trutta.
LIFESTYLE
Our sea trout is also a coastal migrant with a range extending from its natal river to several hundred miles away. Evidence collated from tagging and genetic testing has shown that although they frequently “stray” between river systems the pull to return and spawn in their home rivers remains strong.
That said they clearly have the ability to colonise new and distant waterway systems as has been shown historically when they “founded” almost the entire global trout populations in our freshwater lochs and rivers during the retreat of the last ice age.
The enigmatic sea trout is wide ranging, extending from as far afield as Iceland to the Atlas mountains in North Africa and from Ireland to the Ural Mountains and the Caspian Sea.
Furthermore sea trout have now succesfully colonised - after having being introduced to the region - the vast majority of all accesible streams as far afield as the Falkland Isles and areas of Patagonia.
Whilst some brown trout may move only short distances and spend their entire life cycle in the rivers, lochs and streams of their birth, others, due to a mix of genetic imprinting and differing environments, perhaps a lack of food in their natal river, take the perilous decision to migrate to sea for the much greater availability of food and thereafter return to spawn, a lifestyle - like that of the Atlantic Salmon - known as being “anadromous”.
MIGRATION
Along with the sea-wards migration, the young Brown Trout has several alternative migratory paths to choose from.
Whilst the migration to sea is known as being anadromous the much more common freshwater migration, which is generally undertaken for spawning purposes, say from an upstream tributary to a mainsteam river or alternatively 'tween connected lochs within a river system, is known as being potamodromous.
Given the vast numbers of freshwater rivers, lochs and interconnected waterways in Scotland and Ireland this potamodromou migration is by far the most common, however some simply alternate between both salt and fresh water migrations throughout their lifetime.
Salt water migration itself opens up the availability of much richer and varied food sources which in turn leads to much greater growth and increased weight gain.
It has been noted that the female sea trout that migrate are larger in size which allows for much greater egg production whereas the smaller male trout are still easily capable of producing sufficient milt to fertilise the eggs.
Whilst improved food supply and increased growth are benefits of an anadromous sea migration this does however require much greater levels of energy whilst journeying to sea and returning to the natal river to spawn.
There is also many dangers along the way with predation by seals, otters, pike, cormorants and mergansers, to name just a few, all these increase the dangers throughout the journey and decrease the chances of survival.
TO STAY OR GO
Whether or not to migrate to sea is a delicate balance, as can often be the case in nature, bettween several factors. If the trout is in good physical condition and has reserves of energy currently stored in body fat then it is logical to presume the trout may well be inclined to simply stay put within its home environs.
Conversely if food is in short supply then possibly the trout may well be more inclined to migrate in search of additional nutrition, whether this migration is within freshwater or by migration to sea is the difficult decision that arises.
This is were the genes of individual trout inherited from past generations come to the fore.
Natural selection along with the survival experiences of their ancestors have left a genetic imprint giving them insight into the best way forward.
In short whatever its ancestors did, and survived in any given situation, will most likely be copied.
Migration within freshwater has many advantages over the sea-wards migration with much less energy expenditure and the absence of the aforementioned sea predation aspect.
In both lochs and rivers where the resident brown trout have good feeding there may well be no requirement for trout to migrate at all or perhaps a short migration downstream or from a stream to adjacent loch may well be enough to tip the food scales balance in the trouts favour.
In springtime under normal circumstances a certain percentage of young Brown Trout, having reached an age of two to three years and attained a length in the region of between thirteen to eighteen centimetres, make the decision to leave their home rivers and migrate to sea.
In order to survive in this extremely harsh high salinity environment the brown trout must go through certain physiological, morphological and behavioural changes as it begins a quite remarkable transformation into the sleek and streamlined silver sea trout that we are all familiar with.
These changes are known as smoltification and during this time they transform from their natural freshwater brown trout colouration into metallic bars of silver chrome.
This transformation involves the formation of guanine crystals in/on their scales, their eyes grow larger, and critically their internal organs adapt to cope with the change from fresh to salt water and once complete they have the ability to survive happily within a saline environment.
These small sea trout are now called smolts and around March and April, having reached around ten inches in length, shoal together and when darkness cloaks the land begin their seaward migration. This nocturnal migration increases their survival chances as they avoid predation from at least some of their many adverseries.
Some smolts however make only a partial journey down river to the brackish water of estuaries and sea lochs where they live out their lives without travelling much further afield. These trout are often referred to as “slob” trout and their appearance can vary from looking very similar to ordinary brownies through to fully silvered sea trout.
AT SEA
Small sea trout in their first year at sea are voracious feeders and will consume great varieties of marine life from small crustaceans through to sand eels, sprats and young herring.
They are now known as finnock in Scotland and referred to as white fish in Ireland. These young sea trout move back and fore between river and sea with the tides, travelling many miles up and down the coast.
Having now reached the rich feeding grounds of the ocean they quickly outgrow those brown trout who have remained resident in the river.
Although leaving the relative safety of the natal river is a high risk strategy with much greater risk of predation the greater benefits of a much superior food supply clearly outweigh the considerable dangers of predation from seal, cormorant etc.
The inherent need/urge to grow large and pass on superior genes to future generations allied to the greater egg production of larger hen fish makes the risks worth taking.
RETURN MIGRATION
The upstream migration of the early returning sea trout in Scotland generally occurs in April though until June.
These returning sea trout may travel around several river estuaries before eventually returning to spawn in their natal river whilst some occasionally spawning elsewhere.
On returning to the river these sea trout are easy enough to catch however shortly they seek out and begin to take up residence in the quiet deeper pools in rivers where they seek the cover of sheltered areas under the cover of trees where they lie at rest during daylight.
Once dusk arrives they show more and can occasionally be seen - and more often heard - “splooshing” as night casts its shadows, it is now, under cover of darkness that they can travel unoticed in remarkably shallow water with just their dorsal fin showing and power their way upstream.
Similar to salmon on first entering the river as bars of silver they soon begin to lose their metallic sheen and slowly revert to their previous brown trout colouration
Once this silver sheen disappears it becomes much harder to distinguish between a brown trout and a sea trout with the only real remaining visible clue being the sea trouts larger size.
Indeed at times the only reliable means to determine whether it is a brown trout or sea trout is to have a scale reading taken.
These scale readings show the periods of growth similar to the growth rings on a tree and these scale readings will show a period of rapid growth coinciding with its time at sea.
SPAWNING
Unlike the Atlantic Salmon the majority of sea trout, around 75%, survive spawning and will return to sea and then thereafter once again return to spawn.
In many of the rivers the vast majority of sea trout are females and their larger size allows them to produce greater quantities of eggs which is a much greater resource than the males milt which is required in much smaller quantities.
The hen sea trout can produce upwards of 800 eggs for every pound she weighs and it is important to note that older hen sea trout can produce a much higher ratio than this and these eggs from the more mature hen fish are considerably larger than those of younger females.
This makes it even more critical that these larger hens are returned healthy for future generations.
Some of these Sea Trout smolts will begin returning to the river of their birth after only a few months at sea sometimes as early as April although most will arrive in the early summer months through until Autumn.
Most sea trout in Scottish and Irish rivers generally become sexually mature at three years although this can vary quite dramatically from one to six years of age.
Whilst some return from sea to spawn after just the one year, it is most common for them to do so after two years at sea. Once again there are some variation with others returning after three years and even four years - although the latter is quite rare.
In Scotland sea trout in excess of ten kilos have been caught and these larger specimen fish can be divided into two groups, the fast growing fish of about five years of age having returned after just a couple of years at sea and the repeat spawning fish that have attained their large size by simply living longer.
GENETICS
Whilst it is far from being understood as to why some juvenile brown trout take the decision to leave their home rivers and go to sea there is strong evidence that suggests genetic imprinting plays a major role in this decision.
When trout eggs are incubated at a constant temperature - as opposed to the natural daily fluctuations of reality in the wild – allied to an “easy existence” and the growth rate of reared trout fed a regular controlled diet compared to the harsh reality of a wild existence will most likely have a dramatic influence on their decision making process going forward.
For example if a trout is raised in an ideal environment for growth with ample food they are more likely to remain (in fresh water) whereas if food is in short supply then in all likelihood this will lead to a “desire” or more accurately a necessity to relocate.
One study has shown that trout bred in artificial rearing tanks, before being released into the wild, show a marked tendency to thereafter remain in freshwater and not migrate to sea.
Another possibility is that a harsh existence in the wild may influence such things as an earlier onset of sexual maturity possibly resulting in a decision by the trout to remain at “home” in their natal river.
Whilst in all likelihood sea trout will be found in any loch, stream or river that has access to the sea there then follows a myriad of complex and inter-related circumstances all of which influence the proportion of trout that actually take the momentous decision to go to sea.
Whilst it might be easy to take a rather simplistic view that slow rivers with good feeding might well be condusive for resident brown trout to simply remain at “home” and those living in the harsher environment of low nutrient fast flowing spate streams might choose - or be forced – to migrate to the richer feeding of the salt.
Other factors requiring consideration involve the number of those smolts that go to sea and actually encounter rich feeding in their new marine environment. If they are allowed to feed well and return strong and healthy to their natal rivers to spawn then the migratory gene aspect will be passed on for future generations to follow. If conversely marine feeding in any given year is poor, or predation is high, then it's likely much fewer migratory fish with the andonomous trait will return resulting in a higher proportion of next seasons brood perhaps having a tendency to remain in freshwater.
Also to take into consideration is well fed female sea trout returning from the sea will be carrying many more eggs and again the sea going migratory aspect has a greater chance of flourishing and being inherited by future generations.
Most sea trout in Scottish and Irish rivers generally become sexually mature at two years although this can vary quite dramatically from one to six years of age.
Whilst some return from sea to spawn after just the one year, it is most common for them to do so after two years at sea. Once again there are some variation with others returning after three years and even four years - although the latter is quite rare.
Sea trout spawn throughout our rivers and streams slightly earlier than salmon and generally in much smaller streams sometines as narrow as one metre wide.
A spawned sea trout is referred to as a kelt, just like a salmon, however unlike most Atlantic Salmon who rarely manage repeat spawning (less than 10%) a high proportion of sea trout kelts (75%) do return to sea around November after spawning whilst others may choose to remain in the rivers and estuaries before once again migrating back out to sea much later in the spring.
PREDATORS and PARASITES
During both the sea trout and salmons marine phase the sea louse - Lepeopheirus salmonis is a naturally occurring parasite. However the coastal rearing of salmon in pens creates far greater concentrations of these parasitic infestations.
With these parasites already well adapted to both locating and adhering to their wild and relatively rare hosts, it must surely come as no surprise to learn of the severe levels of infection and mortality rates in sea trout smolts as they swim in the same coastal waters of the already infected caged salmon.
Although in recent years a slightly improved level of control has been achieved with new chemicals such as “Slice” recent research however has not only indicated that it is already becoming less effective in controlling sea lice but is also a potential danger to humans
STOCKS and GLOBAL WARMING
Whilst undoubtedly some of the blame for falling sea trout numbers can be attributed to fish farming aquaculture, especially in the NW of Scotland, this does not necessarily show the complete story or indeed accurate picture.
Firstly let's agree it is in no way scientific or indeed reasonable to expect sea trout stocks to remain at previous levels given our changing climate.
Apart from the well documented catastophic effects of global warming there are many many other critical factors affecting sea trout survival rates.
Before we enthusiastically point the finger of shame at fish farms we might do well to consider the many other factors at play here.
Sadly the list is long - predation from seals, cormorants, otters mergansers and many others who lie in wait to ambush our sea trout on its journey to and from the sea.
Water abstraction whether for hydro power or land drainage, droughts, spates, pollution from field/farm run-off, intensive farming, forestry - I could go on but it is suffice to undersand fish farming is just one of many many problems facing the survival of our sea trout in Scotland and Ireland.
Simply from an angling viewpoint we should at the very least strive to play our own small part in the sea trouts conservation story by ensuring whenever possible we return any fish caught and thus allow them to swim another day pass on their incredible genes to future generations.
