Life History of a Fish: Rainbow Trout

By Scotty Kyle

I was born in a smallish, cold stream high up in the mountains of what humans call British Columbia and my kind have been here in abundance since before there were any humans in the region. I supplied indigenous and new people alike with much of their daily food requirements and helped them survive inhospitable and long winters. Human scientists call me Oncorhynchus mykiss, but hundreds of millions of people around the world know and value me as rainbow trout

My parents were what people call “steelheads”, due to the sides of their heads being grayish against the more silver sides of their bodies. Steelheads, like other rainbow trout, are born in the upper reaches of rivers but, after the first couple of years, we migrate to the ocean and spend time there before returning to spawn. In the ocean life is good, there is an abundance of food of many kinds and, if we can avoid predation and capture, we will return to where we were born to continue the cycle of life. The wonderful amount of food also enables us to grow much larger than most other freshwater-bound rainbows. 

My parents swam upstream, possibly into the same stream where they were born, until they found a suitable “riffle” – which is a shallowish, fairly fast-flowing section of river with a gravelly bottom.  There, my mother scooped out a trench by flicking her large tail and, just upstream of the trench, began to lay her eggs. My father was waiting for this and swam in close to my mother where he fertilized the eggs as they were produced and many of these eggs came to rest in the trench. Then the mother tried to cover them with more gravel. 

Many rainbow trout

Apparently, a female rainbow trout can produce two to three thousand eggs per kilogram of her weight. At a preferred temperature of 6-7 degrees centigrade (42 – 44 degrees Fahrenheit) the eggs take 4-7 weeks to hatch into what humans call alevin or fry. These digest their yolk sacs, become “parr”, which have dark vertical stripes down their bodies and then slowly feed and grow as they drift downstream. 

Some, like myself, feel the call of the ocean and head out into it where we become “smolts” and then steelheads, both of which are more silvery than those that remain in the freshwater. In the Great Lakes of North America there are rainbow trout that enter them in the same way as I entered the ocean and these are also called “steelheads”. 

We are not very fussy about what we eat, as long as it fits into our mouths when we are hungry. We prefer insects and their larvae when we are young, and then eat larger items, like small fish and frogs, as we grow. We also eat some algae and I heard that some humans are trying to raise “vegan” rainbow trout in captivity but, personally, I prefer a mixed bag of what I like to eat. There are records of us eating small snakes, frogs and even birds but, generally, we are seen as less aggressive feeders than our brown trout cousins. 

Our rate of growth and size attained, depend on where we have grown up and also food availability. In rivers, especially cold, low nutrient ones, we grow relatively slowly while in warmer lakes and dams with higher nutrient levels, we can grow very rapidly indeed. The “bottom line” is that we can apparently only live for a maximum of 11 years, compared with sometimes over 20 years for brown trout. During this time, however, while cold river trout may only attain about 1 kilogram (2.2 lbs), a similar rainbow trout in a rich, warm dam, may attain 9 kilograms (20 lbs). Apparently, a human in Canada caught a lake-living member of my species that had reached 22 kilograms (48 lbs.).

rainbow trout in the hand

Historically, my species was limited to the rivers and waters of the northern Pacific Ocean rim from the Kamchatka peninsula, around past the Aleutian Islands and Alaska, right down to northern Mexico. Humans, however, soon realized my value and started not only commercially harvesting rainbow trout but, as early as 1870, he began breeding us in captivity and rearing us in ponds and cages. Added to this they introduced my species to new areas of North America before taking us to other countries around the world. We can now be found in about 50 countries where previously we did not occur. In some of the colder areas of South America, Africa, Europe, Australia, New Zealand and Asia, we can now be found benefiting mankind as food, commerce and also what they call, “sport”. 

In some places our impact on the new environment was negligible, but sometimes we can be a problem. We breed well, and are efficient predators, and so can decimate naïve indigenous species not used to our predation. Often, we can simply out-compete other indigenous species causing localized extinctions and disturbed balances between certain other species. We have also been known to bring in diseases and parasites and so the “gay abandon” with which early introductions were carried out, has been replaced by careful assessments of all potential impacts before new introductions are made. 

In many areas rainbow trout bred in hatcheries are released into the wild to streams, lakes and dams where they do not breed. In these, if the ongoing introduction is stopped, we will disappear within a few years. Otherwise, to remove breeding rainbow trout from any water requires an enormous effort and severe damage to the rest of the ecosystem and its components. Poisoning the entire system is usually the only option and this means killing all fish and many other organisms. Despite this, however, we have become established in many new areas, cannot be removed from others and are now “managed” to advantage.

In many places in all continents, except Antarctica, people fish for us and we are considered to be a top freshwater “sport” angling species. On the other hand, we are now listed in the “top 100 invasive species” worldwide. The fact that I can eat a wide variety of foods (much more efficiently than brown trout), convert it into fish mass, makes me a top fish farming species. In the USA I am considered one of the most important fish farming species for both food and stocking fishing waters. 

rainbow trout on the table

Humans have long perfected the methods of capturing wild rainbow trout on spawning runs, stripping the females of eggs and adding the milt from males to fertilize them. Hatchery or fish farmed rainbow trout are sold widely as food, introduced repeatedly to “put and take” fisheries or stocked into waters where they do not naturally breed to improve sport fishing. Huge amounts of money are generated and local economies boosted through the use of my species. So, love me or hate me, I am an important and valuable member of many freshwater systems.

Many of the people who love me do so because they enjoy catching my species on rod and line. I can be caught in many ways – spinning, bait or fly, but I am renowned for being unpredictable and also rated as one of the best “fighters” among freshwater sport fish species. Many, many people spend vast amounts of time and money planning to outwit my species and then, when they think they have worked it all out, they still find it very difficult to catch me. 

To me, fly fishers epitomize this in that each one has a few very special fly patterns, sometimes carefully guarded, that he/she will make and use. They can range from the huge, amazing and brightly coloured “Dolly Partons”, to the much more conservative, and usually much smaller and duller, nymphs. One angler will swear by a large colorful fly, and catch well on it one day, only to hook nothing on the same fly the next day. Some fly tiers will diligently try and imitate exactly a dragonfly nymph and swear as to its effectiveness, while I have no idea what a “Dolly Parton” imitates, although it sometimes looks very tasty. 

Some people do not realize that before we take a fly, lure or bait there are many factors that combine to determine if we will bite or not. Some, such as temperature, barometric pressure, wind and cloud, are easily recognized and known, but others such as our internal thinking, state of hunger plus many more, are well beyond the knowledge of any man or woman. The net result is that experienced anglers who plan trips, use good equipment, learn from experience and try hard, are more successful than those who do not. On the other hand, there are many times when the novice will stumble onto a fly, lure, or bait that simply looks appealing to us and he or she will score.

a rainbow trout on a river bank

The best way to fish for me usually ends up being the one that a fisherman likes most.  In many parts of the world only fly fishing is allowed and, sometimes, it is only catch and release (my personal favorite). This can, however, be fun, exciting and productive to the sport angler. Whether it is bank or boat fishing the angler tries to cast the fly to a suitable looking spot and then retrieves it in a manner that he or she thinks will be appealing. We are generally an open water species and do not especially favor structure or cover and so may be caught throughout a waterbody. We also spend much time patrolling for food or whatever and so, although one spot did not produce a fish, it does not mean it will not later in the day.

We are mostly daylight feeders, but tend to shun extremely bright light and very warm, calm days. Mornings and evenings are generally best for us but, while one day we may be in the shallows or near the surface, the next day for no obvious reason, we may be in the depths. Similarly, we may favor slow moving food down deep one minute, but the next take a fast-moving surface fly. The bottom line is that there are few absolute rules for fishing for me but lots of generalizations that can increase the angler’s chances. 

Where anglers are allowed to fish by any method, many will spin with a wide variety of lures, depending on the water and size of fish they hope to catch. While we take “natural” looking spinners we also like bright flashy things. Once again, we may be in the shallows or depths and go for fast or slow recoveries. Bait fishing is more predictable and can be productive and, again, a wide variety of natural baits can be used. Live baiting can be successful, usually using a small fish, while a worm or a similar organism can work well, too. 

Much of our appeal, beyond apparently tasting good, is our fighting ability. We are renowned for repeatedly leaping high out of the water and powering away just under the surface leaving a substantial bow wave. When hooked on a fly, our first few runs can be almost unstoppable and when we leap out of the water we usually shake our heads in an effort to loosen any hooks. Our fighting ability, combined with our rapid growth to a substantial size, make rainbow trout one of the most sought-after freshwater fish. Many freshwater anglers have treasured memories of a first run by a large rainbow – powerfully and inexorably stripping the line off a reel then leaping high out of the water several times followed by the satisfaction of landing and releasing a magnificent rainbow trout. 

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