It happens each spring at the onset of the first mayfly emergences, right around mid-April. Anglers go forth with high expectations, hoping for large hatches and rising trout. And each year, there are the same tales of woe from friends who have been on Catskill rivers and observed large hatches of flies, but few or no rising trout. I have heard this story so many times that I decided to research the issue before writing about it.

All salmonids are cold-water fish, in that their behavior and metabolism are controlled by water temperature. Eastern fly hatches begin when water temperatures rise to around 50°F. So that would appear to be a temperature favorable for trout to be feeding. And from what I learned, it is—but with exceptions.

I called two fish culturists and asked if trout in their hatchery fed actively when water temperature was at 50°F. One man explained that his trout grew well at that temperature, provided that it was constant. The other advised that his trout grew best with water at 58°F. Several published studies show that trout grow best when water is between 54 and 66°.

With this knowledge in mind, I wondered why our stream trout are not chowing down on early season hatches when the water reaches 50°F. I had a thought, and called one of the culturists back, and asked whether his trout would feed steadily if there was a significant drop in water temperature during the night, say from 50° to 40°F . He responded with a firm “no,” explaining that changes of that magnitude would definitely put trout off their feed.

He also advised that trout become lethargic and don’t move much to feed when nighttime temperatures drop into the 40s. He speculated that what little feeding there was would probably be near the bottom, not at the surface. Further review of the literature found that trout stop feeding when temperatures drop to 45°F.

To follow-up, I consulted the USGS website, which provides real-time flow and temperature data for several Catskill rivers, and found that in the spring, it is not uncommon to see diurnal changes of 10 degrees or more. While daytime levels may reach 50°F, nighttime temperatures can drop to 40°F. These changes are large enough to require several hours for water to warm back to 50°F, causing trout to be lethargic, and not to feed at the surface or very much at all.

Another factor is light sensitivity. In my own fishing, I have found that some species, especially the larger fish, tend to feed on cloudy days and at dusk, rather than during periods of bright sunshine. On sunny days, even with favorable temperatures, large browns are seldom found rising even during heavy hatches. It turns out my own observations are borne out by some hard research: studies conducted by students at the University of Arizona found that brown trout were much more light sensitive, and less likely to feed, on bright, sunny days, than other species.

What does this all mean to fly fishers? Low water temperatures early in the season combined with bright sunshine act as deterrents to trout feeding. These conditions are perfect for anglers to observe large hatches completely ignored by trout. And even as water temperatures warm to favorable feeding levels, sunlight will still put trout off their feed.

Anglers may wish to keep these factors in mind, and have a thermometer handy when going off to a favorite river.