Trout as they grow older become more territorial and the most dominate trout in a pool will chase weaker fish out of their lies or beats. When there is a large enough difference in size, a large trout will even eat a smaller one. A fingerling is no different from a baitfish for the master of the pool. When two mature trout of comparable size, it is not uncommon for them to fight to determine who gets the best spot in the pool.
When out fishing, I spend a lot of time just watching trout. Sometimes, I am waiting for the opportune time to present my fly, but often I am just curious and enjoy watching them. There is nearly always a constant jostling for position between the fish in a pool. The smaller ones trying to sneak into the best lines, the dominant ones keeping them.
In still water, it can be even more obvious. Trout, and in particular brown trout often cruise along a feeding beat. They generally follow the exact same route, which can take them several minutes to complete. It is interesting, to watch the dominant and largest trout usually take the lead, with the smaller fish sheepishly following a few yards behind.
In such situations, the trout’s natural aggression can be used to our advantage. Casting to such fish can be difficult, and it is generally best practice to wait until they are swimming away then cast from behind. The problem is, that if you cast to catch the big fish, the fly line will likely land on top of and spook the follower. This would then cause a chain reaction with both trout darting to safety.
Luckily, for us, Large trout are usually quite cranky, and every so often they stop and chase the smaller trout away. They always return in a few minutes but it does give us an opportunity to cast at the bigger fish with the smaller ones out of the picture.
This video below shows two rainbow trout having a real brawl. I personally have never seen them fighting so severely. Usually, a lot more chasing each other around.
When are trout the most aggressive.
Trout are at their most aggressive while feeding and while spawning. They are aggressive when feeding to protect their source of food. They are aggressive when spawning to protect their eggs and to keep away competing fish. If you ever watch spawning trout, it is not uncommon to see them chase each other.
Trout are less aggressive when they are young, they typically swim in large schools of similar age fish. This is certainly a case of protection in numbers.
Trout are also much less aggressive when they are just resting. I have spent quite a bit of time diving to the bottom of pools in trout rivers, and it is not an uncommon sight to see large groups of trout in all sizes just resting on the bottom.