Considered by some to be even feistier than the rainbow the silvery blue iridescent sheen of a blue trout is a sight to behold. This coloration is not as you might expect due to skin pigmentation but instead caused by the light scattering appearance of the fishes skin similar to the “Tyndall effect".

Alongside rarity - as can often be the case - comes a degree of mystery myth and legend with blue rainbow trout considered by some to possess more explosively acrobatic and hard fighting qualities than their traditionally coloured siblings - though this is open to debate.

First of all it's best to begin by clarifying that there is no such natural born creature as a “blue trout”, the blue trout we fish for is in fact a rare naturally occurring genetic mutation of our common rainbow trout which has then been selectively chosen to breed with other similar trout to supply and stock our fisheries.

This selective breeding results in the majority of their offspring taking on the distinguished blue colouration - which can vary quite dramatically from sky to a much darker cobalt blue.

It is worth noting that - although a much rarer occurrence - this genetic anomaly does also occur in the brown trout.

The blue colouration is thought to be determined by a recessive genetic anomaly in some hatchery bred rainbow trout which results in these trout lacking the middle lobe of the pituitary gland giving them their predominantly cobalt silvery blue appearance.

When trying to ascertain conclusively whether it is a blue trout - when it's not glaringly obvious - pay close attention to the area around the head and particularly the eyes as it is usually here that the most vivid colouration is displayed.

Though perhaps less rare than one in a million, this mutation results in a most beautiful fish displaying an iridescent body with metallic silver flanks and underbelly making them much sought after by many anglers.

To understand what makes a triploid we must dive into the murky depths of the genetics pool.

Wild and unadulterated trout have just two sets of chromosomes and are called “diploids” whereas “triploid” fish, including our stocked rainbow trout, whilst appearing almost identical to diploids, have three chromosomes.

To cause a trout to retain an extra chromosome - which it would under normal circumstances discard - requires human intervention.

Although there are various ways to achieve this goal, two of the most common methods are by exposing the egg to either heat shock or to external pressure.

Under normal circumstances once an egg and sperm combine and fertilisation commences one chromosome is discarded leaving behind either male chromosomes (XY) or female chromosomes (XX).

A critically important component of the triploidy conversion process involves the retention of an extra – superfluous - set of chromosomes already present in the egg prior to fertilisation known as the “second polar body”

Under normal circumstances - shortly after the sperm cell enters the egg – certain chemical reactions are initiated which lead to the expulsion of this second polar body before the chromosome fusion between sperm and egg nucleus occurs.

Having allowed the eggs a sufficient length of time to develop naturally to the point whereby the polar body is about to be discarded, the eggs at this critically important stage are now exposed to either shock or external heat which causes the second polar body to be retained.

The result of this human intervention and chromosome manipulation is that there are now three sets of chromosomes present instead of the normal pair.

When the eggs hatch these sterile triploids – whether standard rainbows or selectively bred blues – are nurtured and grown on in much the same way as diploids before subsequently being sold to fisheries as stock trout.

Although the male triploid rainbows may still develop gonads and show spawning behaviour the females do not and this additional set of chromosomes renders both male and female infertile.