Why are some trout very dark nearly black in color?

Occasionally a trout will be caught that is much darker, almost black. Such a trout often differs significantly from the normal fish in the river.

Why are some trout much darker than others, and what might be the cause? When I first started fly fishing, that was a common question on my mind.

Trout can darken for many and varied reasons. Often dark pigmentation is the result of a disease or infection. Such trout are unlikely to survive. Other times, the darkness is a natural adaption to better camoflauge with the local conditions.

1) Fatigue and sunburn.

The first brown trout I ever caught was a long, skinny snake of a fish. His back was very dark, bordering on black, being new to fishing I did not realize that there was something a bit different with that fish. Looking back, I can only speculate that he was a very old, battle fatigued jack who was on his last legs.

Over the years, I have noticed that ill or weak trout can spend a lot of time in the shallows, where they are exposed to direct sunlight, and just like us their pigmentation darkens as they get sunburnt. Why do sick trout spend so much time in the shallows, again I can only speculate but it is probably to stay out of the current and conserve their energy. It might also offer them a degree of protection from more dominant fish deeper in the water column.

2) Pathogen infection

Sickness and poor condition is a common causes of darker pigmentation in trout. Another significant cause of such pigmentation is viral, fungal, or bacterial infections. These infections often start as a result of improper handling by fishermen who inadvertently removed the trout’s protective mucus layer that helps keep such infections at bay.

I have seen too many trout dying in the shallows with a human handprint ‘burnt’ into their side.

In 2018, one of the major causes of black pigmentation in brown trout was finally identified. It was a virus that predominately affects Salmon in both the Pacific and Atlantic oceans. For some reason, brown trout are particularly vulnerable to it. The virus was first isolated by a team from the Technical University of Munich who was trying to find the cause of the brown trout die off in central Europe.

Another common pathogen infection that can cause skin darkening in Salmonaes is Bacterial Kidney Disease (BKD) which is caused by the bacterium Renibacterium salmoninarum. For more details, about BKD and other potential pathogens, I suggest checking this fact sheet.

The pathogens, are most commonly seen when the trout are under stress, typically caused by low oxygen conditions and high temperatures.

3) Genetics

Not all darkly pigmented trout are sick or dying, some are just naturally much darker than others. One such example is the Kamloops rainbow trout from British Columbia that are naturally much darker than other rainbow trout subspecies.

Dark ‘birthmarks’ are also quite a common occurrence on some hatchery raised Triploids trout.

With regards to brown trout, genetics seem to be less important than environmental variables when determining how dark a trout will appear. Trout that live in brighter environments will appear paler, while trout that live over dark substrate will darken to better blend in.

4) Habitat

The habitat or environment where a trout spend a lot of its time can affect its pigmentation. Trout that spend time in open waters such as lakes or oceans often become more silvery, while trout that spend time in rivers and streams become darker and more brown.

Just how quickly can a trout change color in response to its environment? Will, evidence suggests that they can change color within minutes, to a few hours. It is a relatively rapid transformation.

There is quite a few discussions online, about trout becoming darker when they live in shaded environments, such as overgrown streams, canyons, or from the bottom of deep dark holes.

Trout that live in heavily stained, or tannin water also darken up, assumingly an adaption to better blend in with their surroundings.

Although, there is an exception. Trout that live in the complete absence of sunlight, such as cave dwelling rainbow trout found in the lost sea, Tennessee or the Blanched brown trout found in various caves across the British Isles typically become extremely pale, almost white. Trout like as, do require some sunlight to darken up. Once blanched brown trout are exposed to sunlight their pigmentation quickly returns and they quickly color up like a normal fish.

6) Diet

A trout’s diet can also affect its coloration. A diet that is rich in carotene, typically from Crustaceans can enhance the coloration of trout.

The richer pigmentation colors gained from high levels of carotene in a trout’s diet is probably not usually enough to cause a trout to appear very dark bordering on black.

High carotene levels can magic a trout’s flesh appear much richer and brighter in color so is a popular supplementary for trout raised in farms.

7) Spawning

As the time to spawn approaches, trout become more aggressive and their pigmentation typically darkens and becomes more vivid. For this reason, trout that are caught close to spawning are much richer in colors.

With that said, this darker pigmentation is not usually to the same extreme as a sick or dying fish.

Is it safe to eat black trout?

As someone who once ate a ‘sickly’ black trout, I can quite honestly say it tasted horrible. The flesh was bitter and there was not much of it.

If cooked well, it is highly unlikely to make you ill, but I still will not recommend eating one. Simply because ill trout are likely to be in poor condition making them taste bad.

If the trout is black from living in dark water, then there is likely to be nothing wrong with it. So consider it as safe to eat as any trout.