I find the diversity of trout species fascinating. In this article, I just want to share just how diverse Brown Trout are within a geographical area as small as the British Isles.
At a glance, all brown trout seem to be just Brown Trout, and in some ways they are. But, Brown trout are a very diverse species of fish, and even in an area as small as Britain, they are extremely genetically diverse.
They even form populations, that are genetically distinct from each other, sometimes these populations have been inhabiting the same lochs or lakes for thousands of years. Yet, they do not seem to interbreed which has kept their behaviour and unique characteristics pure.
I will start by discussing the main species of brown trout we all know and love.
When most of us, think of Brown Trout we think of S. Trutta, the stereotypical brown trout native to North West Europe bordering the Atlantic.
Even within Salmo Trutta there are populations of trout, that keep themselves genetically pure from each other.
Northern Irelands Lough Melvin is inhabited by three genetically distinct forms of Brown trout. sonaghen, gillaroo and ferox.
These forms have different feeding, behaviour and even breeding patterns. Which keeps them genetically distinct from each other.
Genetic analyst has even found that isolated populations of ferox trout living in Scottish Loches are gentically closer to the ferox inhabiting Lough Melvin than they are to other forms of Brown Trout inhabiting the same water.
One of the main splits within the brown trout population is the separation between the river dwelling fish and the sea run trout. Sometimes, the sea run variant is given the name Salmo trutta trutta. These sea trout share a lot in common with the standard river dwelling S. Trutta and most people will agree they are the same species.
The main differentiating point is a strong desire to migrate out to sea. The difference between a Brown Trout and a Sea Trout is comparable to the difference between a Rainbow Trout and a Steelhead. It is widely accepted that they are the same species, but one just have a preference to head out to sea.
It is often debated whether Ferox trout are a species within their own right, what is and is not a species can be a hotly debated topic. What is clear, is that Ferox Trout are quite different from the usual Brown trout.
Remanent populations of Ferox trout survive in isolated populations within clear, cold oligotrophic lakes/lochs across the British Isles.
S. Ferox are behaviourally and genetically distinct from other brown trout variants they share the lakes with. Differentiates in spawning location and behaviour typically prevent hybridization between the different populations of trout.
The behaviour of this Brown Trout variant quite closely resembles that of the North American Lake Trout. They are typically found living deep in glacier formed lakes, where they predominantly predate upon other fish.
S. Ferox are known to grow both larger, and live longer than what is typical for British Brown Trout. Some even reach over 20lbs in weight and can be over two decades old. This is quite the contrast to the typically small Brown Trout that inhabit the lakes and streams across Britain.