Is it a big deal if I accidentally kill one small trout?

Let's Do Some Fishy Math

How long would the fish have lived anyway?

A 10" trout is approximately 2 years old.  Trout live on average for 7 years.

So, if I accidentally killed 1, that is just 1 less fish in the river for someone else to catch, right?

Well not exactly.  We have to consider what that trout would have done in the next five years. SPAWN!

How many more fish could that one little 10" trout have spawned in its lifetime?

Female trout lay on average 2000 to 3000 eggs per kg of weight each year.  So a 2 year old, 10" trout will lay 340 to 510 eggs that year.  The next, year it will lay between 940 and 1410 eggs.  Each year as it gets bigger and will lay more.  By the time it reaches the end of its life, it could have laid between 13,000 and 21,000 eggs.

That is a lot of eggs, but how many would have survived?

The mortality rate of wild trout in the first year is 95%. So only 5% would have lived to be 1 year old.  However, 5% of 13,000 is 650, and 5 % of 21000 is 1050.

Accidentally killing one 10" trout theoretically is a loss of 650 to 1050 more trout over the next 5 years!