I spent yesterday afternoon visiting my old friend Tom and his buddy
Pete. Tom is 68 and Pete 87. These two Ennis Montana old timers are
among the oldest (former) Montana fishing guides still kicking
around.
Tom and Pete told me stories about fishing the Missouri below Tosten
dam in the late 1960s, when mayfly hatches were so thick the surface
of the river looked like it had the whirlies.

Today, some 40 years later, the hatches there are nearly
non-existent. What's made the difference? Tom said he thinks it's
agricultural pesticides. The Missouri down near Wolf Creek still has
some
good hatches (although nothing like they used to be) but that water
comes out of a three dam, deep-water filtering system that dilutes
the
chemical concentrations. The Missouri in the Tosten area is still
shallow,
containing undiluted runoff from millions of acres of fields,
all sprayed regularly with an ever increasing diversity of noxious
chemicals.

Tom said he'd asked various Fish and Game biologists about his theory.
They all said "Sure, we think that too. But we have no baseline data
from 50 years ago, and therefore no way to prove the allegation."
Beurocrats (scientific or otherwise) learned long ago to keep thier
mouths shut, if they don't have strong evidence to back them up.

Still, as the old timers can tell you, the hatches are nothing like
they used to be.