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Fall Spawners

August 02, 2010 | by Bruce Auchly

The spring spawning run of rainbow trout is over. In the Missouri River of north central Montana, the spawners are back in the river's mainstem, the blue ribbon section downstream of Holter Dam.

Meanwhile the young trout hatched this spring in nests, called redds, in tributaries and in the mainstem of the Missouri now are several inches long. Because the Missouri is warmer than its tributaries, the young fish in the Missouri are slightly larger, but most the young of the year rainbows now are about 4 inches long.

That is those that have survived this far. A young trout's life is perilous.

A basic rule of fisheries science is about one percent of rainbow eggs survive to become a one-year old fish. At that point the 8- to 10-inch fish is "recruited" into the population.

That means if an adult female rainbow deposits 1,000 eggs (and that's about average) into a redd, approximately 10 will survive to celebrate their first birthday. The odds obviously are not good. But if you think of it another way, it only takes one egg to survive to replace the female adult trout. And ten female spawners can increase their population tenfold, from 10 to 100. Sounds better that way.

And what perils await a young trout? Let us count the ways: floods, drought and predators – mammals, fish, birds and man.

This year, melting snow and rain made for tremendous runoff in the tributaries of the Missouri, especially in June. Torrents of rushing water can disrupt and destroy redds or dash very young fish against rocks and kill them.

Fisheries biologists can predict when trout eggs will hatch based on water temperature and when the female trout spawned. Given the right conditions, eggs spawned March 30 will hatch May 9; eggs spawned April 15 will hatch May 18.

With a later runoff, young fish hatched earlier in the spring have a better chance of surviving a heavy runoff than eggs not yet hatched. On the Missouri, peak spawning occurs mid-April. So this year, those fish stand a better than average chance of surviving heavy runoff.

Drought is simple. No water, no fish. No fish, no eggs.

Yet nothing is that simple. Tributaries might dry up in drought conditions, but many rainbows spawn in the Missouri's mainstem. With drought, however, even the Missouri's flows are depleted forcing young fish into the same waters as adult fish. And as we all know: It's a fish eat fish world out there.

In fact when fish are concentrated, it's easier for all predators, whether kingfishers from above or otters from below, to find a meal.

That's not even taking into account human pressure. From 1995 to 2001, angling pressure increased from 88,000 angler days to about 136,000 angler days. That's the same as adding the fishing pressure on Flathead Lake to the Missouri in just seven years.

In the years since the pressure has dropped back to about 100,000 angler days, down from the peak but still considerable.

While many of those anglers practiced catch and release there is always hooking mortality. Like when a fish is caught and released improperly or played too long on a hot day when the water is warm.

No one said it was easy being a fish.