How Big Do Rainbow Trout Grow?

According to the IGFA The heaviest Rainbow trout weighed 21.77 kg (48 lb 0 oz) and was caught in Lake Diefenbaker, Canada (2009), while the IGFA All-Tackle Length World Record belongs to a 94cm (37 inch) Rainbow caught in the Tekkapo Canals, New Zealand (2020).

Although, I have personally seen photographic evidence of a 98cm (38inch) trout caught from the same Canals, and apparently the world record 48lb Rainbow unofficially measured in at a massive 42 inches long. So there are certainly fish out there larger than the current world records.

In 2021 there was also a 39.25-inch steelhead caught in the South Fork Clearwater River.

What is the average size of rainbow trout?

According to the US Fish and Wildlife Service, the average rainbow trout is around 16 inches in length (source). A 16inch Rainbow typically weighs just under 2lb.

The chart below shows an approximation of the growth rate of rainbow trout from a creek in Alaska.

As can be seen, after reaching maturity Rainbow trout continue to grow at a fairly consistent rate. They also grow heavier faster than they increase in length.

This suggests, older more mature fish are typically in better condition.

I will note, that harsh environmental conditions can easily stunt or slow growth. A rainbow trout in an Alaskan creek, where the growing season is short will grow slower than a fish further south in a more temperate climate.

So it is not uncommon for 4 year old fish from a highly productive river to be larger than much older trout from rivers where the growing season is heavily restricted.

Few trout get older than 8 years, and those which remain are usually large fish. I suspect that large strong trout are more likely to reach an old age which could explain the acceleration in growth rate towards the end of their life.

How big do ‘wild’ rainbow trout grow?

The world record Rainbow trout was a genetically engineered fish to make it infertile. There is certainly quite a bit of controversy regarding the legitimacy of such fish.

Well, many large ‘wild rainbows’ have been caught from the Tekapo Canal in New Zealand. These trout were born in the wild, and usually, spawn. The longest I have seen was 98cm and some do weigh in at over 40lb.

But, some might say. These trout live in man-made canals and often feast on artificial fish pellets. Are they truly wild fish? How big do wild rainbow trout get when they grow in a wild river or lake?

Well, the longest Rainbow trout I know of caught from a ‘wild river’ was the 39.25” Steelhead from the South Fork of the Clearwater River in Idaho. Although, the Idaho Fish and Game believes it was probably a hatchery fish. So not quite a truly wild bow.

Factors that influence the growth rate of Rainbow trout

1. Food Supply: To grow to their potential rainbow trout need readily available food. The largest rainbow trout are ones that typically live in one of three places. Beneath fish farms, where they gorge themselves on pellets, In the tailwater of hydroelectric dams where they get a constant source of food washed through the turbines, and finally in the nutrient rich waters offshore.

2. Water temperature: Water that is too warm, or too cold can slow the growth of rainbow trout. Rainbow trout which live in very cold climates tend to grow slower than fish that live in more temperate waters. In climates that are too warm growth also slows. Rainbow trout grow best in waters that are cool, but consistent in temperature. Such conditions are usually enjoyed by trout that travel out to sea or who live in larger lakes or tailwaters.

3. Genetics: There is a lot of genetic variation within rainbow trout populations. Some strains of Rainbow trout grow faster. Hatchery and farm strains are usually selected for maximum growth rate. This has been further enhanced via genetic modification and the breeding of sterile fish.

Age at which Rainbow trout reach their maximum size?

Rainbow trout continue to grow for most of their life, but most trout reach their maximum weight potential around 10 years of age. Trout older than that, might grow in length, but due to the onset of age, they struggle to maintain their condition and can even start losing weight.

Few rainbow trout actually reach that age, most rainbows do not survive beyond their 8^th year in cold climates, and their 6^th year in warmer waters.

Rainbows survive longer in cold climates because they grow slower, and often do not spawn until their third year.

Do steelheads grow larger than Rainbow trout?

They are the same species of trout, so they actually grow to very similar sizes. Many line class records for steelheads range from 30-35lb. That is lighter than the monster river rainbows. Although, the average size of a Steelhead is larger than the average size of a river rainbow.

On average steelhead does grow larger and more powerful because of the more consistent supply of food out at sea.

With regards to length, the longest Steelhead I have heard of was 39.25inches, while the longest rainbow measured in at an unofficial 42 inches.

But Steelheads are still wild fish, they still have to hunt down their food, escape from predators and go on long, often exhausting spawning migrations. All of which reduces their growth potential.

While the largest rainbow trout, usually have a fairly lazy life. Many of which rest beneath fish farms, and just engulf the endless supply of fish pellets that wash beneath the farms. While other Rainbows, like the world record fish caught in Lake Diefenbaker have been genetically engineered to be sterile. That means, they do not ‘waste’ any energy or condition spawning. All their energy goes into growth, so these genetically engineered rainbows, when given the correct conditions can grow to massive sizes.

Do Rainbow trout grow larger than Brown trout?

The IGFA world record for Brown Trout is 20.10 kg (44 lb 5 oz), which is smaller than the 21.77 kg (48 lb 0 oz) world record Rainbow Trout. Although, the World Record Rainbow was a sterile genetically modified fish that allowed it to reach such a large size.

When comparing unmodified fish, then the largest Brown trout and Rainbows are very comparable in weight and size. So both species of trout, in the wild, under ideal conditions both species can reach 40+ pounds.

What is the largest Rainbow trout caught in the United States?

The largest Rainbow trout caught in the United States measured 41 inches and was estimated to weigh 38.2lb. It was caught in the Feather River just downstream from the Oroville Dam, California.

At 41” it betters the current IGFA world length record for Rainbow Trout but is 10lb short of the current world record for weight.