Trout (Salmo trutta)

Trout Life History

Adults

Adult trout tend to occupy territories within the river, this gives them a good food supply and shelter from predators. They tend to prefer deeper pools with slower flows and are quite often found where there are undercuts or tree roots. In Autumn/winter they migrate upriver to spawn.

Spawning

In November/December female trout dig a nest (Redd) within the gravels, she then releases eggs into the depression. These eggs are fertilised by males and then covered with gravel. Smaller substrates are required for trout than that of salmon, although both species can spawn in the same areas.

Eggs and Alevins

Eggs or ova remain in the gravel for a couple of months before hatching, this is temperature dependant. Once hatched the alevins remain in the gravel and live off their yolk sac. They then emerge as fry and set up territories.

Fry and Parr

Both fry and parr are territorial, instream habitat is vital for their survival. They require lots of cover from rocks and stones. Imagine the river bed as a mosaic of territories, the more substrate there is, then the more territories are available, this is the same for salmon. Around 5% survive their first year.

SEA TROUT

Sea trout and brown trout are the same species. It is thought that genetics and environmental factors such as a reduced food abundance results in some trout migrating to sea. They have the same lifecycle as the resident brown trout except that after 1-3 years they undergo a number of physiological changes in preparation for the transition to the marine environment. They turn silver, just like a salmon are also called ‘smolts’. They usually migrate at night in the months of April/May.

By moving out to sea, they have access to a richer food source, this enables them to grow much bigger and faster than resident river brown trout. In many rivers, sea trout are generally female, because they are bigger, they produce more eggs. The bigger the fish = More and larger eggs, these fish are therefore valuable and should be returned.

Sea trout will return to their natal river to spawn, they can return after only a few months at sea. These small Sea trout are often known as ‘Finnock’ in Scotland. Most, however, will return in Summer and Autumn and lie up in deep pools until spawning time, they often tend to move at night.

FEROX TROUT

Ferox trout are brown trout, however, they have a different life strategy. As juveniles they feed in the same way as brown trout, occupying the same habitats. Once they reach a size of approximately 35cm they switch their dietary preference in favour of fish, in particular Arctic Charr.

They are found throughout the brown trout’s native range and where large glacial lakes/lochs exist and often where charr are present. In Scotland it is thought they are present in less than 10% of the sites where Brown trout occur. This suggests that these fish are vulnerable.

These trout tend to be long lived, the oldest recorded was captured at Loch Killin and was estimated be 23 years of age. Ferox trout, as a result of the highly nutritious piscivorous diet show marked increases in growth once they make the switch, one tagging study recorded one ferox that increased in weight by 10lb (4.5 kg) in 4 years. The current British rod-caught record is a 14.4 kg (31.7lb) fish from Loch Awe, Scotland.

In most lochs, Charr are the main prey species, however they have been shown to switch prey species. In Lough Corrib they have switch to Roach following the extinction of Charr and the introduction of roach.

So, what makes a Ferox a Ferox?

Genetics

Debate continues as to whether the Ferox is a separate species, Salmo trutta displays numerous genetic variations. During the last ice age, (10-15000 years ago) ice cover shifted, ebbing and flowing and this is reflected in the trout and charr populations. For example, Loch Melvin has three genetically distinct brown trout, Gillaroo, Soneghan and Ferox, this distinction has remained by spawning in different locations and adopting different feeding strategies.

The ferox in Lough Melvin in Ireland and Lochs Awe and Laggan in Scotland have been found to be genetically more similar to each other than to other (non-ferox) trout in the same lakes and suggests an ancestral lineage.

The International Union for the Conservation of Nature (IUCN) and the Freshwater Fish Specialist Group regard ferox as a separate species. Ferox is listed in the IUCN Red List of Threatened Species with its status being given as ​“Data Deficient”