Diurnal Periodicity

If you look at the daily temperature graph of the Smith River in late June in Montana:

http://waterdata.usgs.gov/mt/nwis/uv/?site_no=06077200&PARAmeter_cd=00060,00065,00010

you see a remarkably well defined 24 hour cycle, from coldest water temperature to warmest following a repeating pattern that swings down below optimum temperature (14 degrees celsius) to above (18 degrees celsius). As the season progresses the shape of the daily curve widens around the daylight hours and moves higher, sometimes plunging down into optimum activity temperatures only at night.

The interesting part is the predictability of the cycle--if you have access to the data, that is. Trout are opportunists, so they'll feed when the have to. But it's also widely accepted they're most active beween 14.5 - 18 degrees. So it makes sense to pay attention and plan your time on the water to coincide with those (highly predictable) temperatures.

I remember reading an academic paper about seasonal brown trout activity patterns. That researcher observed a single period of maximum activity centered around midday in winter, a single period of late night peak activity in late summer and two split periods of peak activity morning and evening during the changing seasons. That researcher tried to explain this behavior in terms of circadian rhythms (the 24 hour light dark cycle) as well as diurnal temperature periods.

But he too was--when you get right down to it--talking about highly predicable activity patterns. I'm going to have to start paying closer attention to the USGS water temperature tables. And carry a thermometer too, I guess; to see if they're getting it right.