Fish Behavior During Spawning Periods

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by: Jacob Noteboom

It is that wonderful time of year when the rain has returned to our beautiful, wet corner of the United States. The leaves have begun to change color and you decide to take a hike along your favorite tributary stream. As you walk, you notice a bright flash in the water. Your curiosity brings you to a stop and you realize it is a Chinook salmon, digging a redd. You take a moment and observe the wonder that is spawning. Moments later, you catch sight of a smaller flash less than 15 feet behind the large, dark kings. It is a rainbow trout, and a smart one at that, because it has chosen to key in and feed on the eggs of these salmon, as many trout do this time of year.

You decide to walk further upstream, but are startled by a splash you see ahead. You eagerly investigate, and see a strange white line swimming in place. Then that white line turns into three. You have just spotted the white leading edges of a bull trout’s fins, a rare and beautiful sight. You take a few moments more to observe what the boss of the stream is up to. Suddenly he bolts upstream, chasing a small whitefish, rushing to gain those vital calories needed to survive his upcoming spawning.

But what does it take as a trout to be able to spawn? The short answer is frequent changes in behavior. But to get the long answer, we should start at the beginning.


Size Matters

It is late spring. Rainbows have long wrapped up spawning and their alevin (egg sac fry) have begun to emerge from the gravel. These fish still have a couple weeks before they absorb their sac and “button up,” becoming parr (juveniles). It will be 2 years before the fry reach sexual maturity, but there are other factors that play a role in successful spawning, size arguably being the most important.

Size contributes to spawning success in many ways. Trout compete for the most fertile gravel, so the bigger fish have an advantage when it comes to shoving each other around to get the best seat in the house. Larger fish have larger tails, making it easier for them to dig larger redds. This allows more oxygen flow to the eggs, which in turn means a better survival rate for them as well. The most important advantage of size is fecundity. Fecundity is the ability of the female fish to produce an abundance of eggs.

Good Diet = Healthy Fish

While it is obvious that the larger the fish, the more eggs it can hold, it is not the only factor in egg production. The amount of eggs a fish will produce in a spawning period is heavily influenced by water quality and dietary behavior as well. If a watershed is low in oxygen, high in temperature, or low in a food source, a trout simply will not produce as many eggs. It and will oftentimes be in rough shape by spawn, which affects the survival rate of the trout itself.

A good diet translates into a healthy fish, and the dietary shift of fish pre-spawn is very drastic. Fish will go from being picky and feeding on smaller meals, to needing all the food they get pre-spawn. This means fish get more aggressive, less shy, and sloppy when it comes to what they eat. Fish could go from feeding on Baetis and midges during the winter, to feeding solely on Sculpin and stoneflies.

Protecting their Turf

Fish also become extremely territorial before spawn, often pushing each other out of prime habitat. Fish on spawning gravel have been observed picking up sculpins, shaking them around, and spitting them out away from the area. Biologists believe this is to protect already existing redds from predation or to evict them from the area a fish may want to dig. The interesting thing about this observation is that the trout do not eat the sculpins when this takes place, they do it simply to kill them or get them out of their territory.


Streamer Action

What does this mean for we fly fisherman? Streamer action, obviously. It is no debate that you can catch fish on streamers twelve months out of the year, but a month before and a month after spawn are arguably the most effective time to chuck big, meaty flies. As I type this, it is September here in Oregon, and the char and true trout are getting feisty. Brook trout have begun stocking up in the tributary creeks that feed some of our mountain lakes, and these fish eagerly feed on small fish and insects that come from these streams.

A couple of us here at the shop like to play a game where we see what the biggest thing these fish will eat, throwing mouse patterns, large articulated streamers, bass poppers, etc. It amazes me how aggressive these fish will get. In some of the larger cascade lake reservoirs that have connecting channels, large brown and brook trout will migrate up to either spawn, or follow smaller species of fish such as kokanee or chub to predate upon before winter.

Some great patterns to throw at large, hungry trout are Big Gulp Sculpins, Sculpzillas, Egg Sucking Sculpins, Sticky Tube Sculpins, Dolly Llamas, Mr. Hankey, Morrish Mouse 2.0, and any woolly bugger. If you want to know more info on fishing large flies for large fish before and after spawning periods, give us a call or stop by the shop on your way to your favorite creek mouth.

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