Predicting Hatches

A flotilla of duns making their way placidly down river on a sparkling spring afternoon.

My friend Jack texted me the other day with a prediction: Hendrickson hatches will start on the Battenkill on April 19th. All things considered, a pretty good guesstimate. The winter we just left behind has been colder than those in recent memory. The snowpack, while far from record breaking, has persisted in the higher elevations and there is still a bit of snow to fall. As I write this the water temperature on the Battenkill in Arlington is 38 degrees and dropping with the cold that is settling into the valley. A chilly forecast for the next 10 days will ensure that the water will not warm up dramatically. An unusually early hatch seems unlikely.

For many years, when the first hatches of Hendricksons would begin was somewhat predictable. There was a steady progression from early spring chills to those bucolic days of late April and early May. The sort of days where one can feel a hatch coming on. Numerous books provided hatch charts that gave reasonably accurate tables for when and where the Hennies would show. Fly shops that serviced streams both famous and not, could offer refined and up to the date information. Anglers scheduled vacations and time away from home with reasonable assurance that they would find trout rising to the first big, succulent mayflies of spring. Then things changed.

In 2012 there was a snowless winter in much of the East and a very mild March, with temperatures hitting the 70 + degree range for a number of days at month's end. Anyone that adhered to the generally accepted hatch charts found out the hard way that the normal late April/early May hatches were wildly inaccurate. Bugs began to hatch on rivers like the Beaverkill and the Battenkill as early as the first week of April. My friend Rich Norman recalls seeing them on the Battenkill on April 1st. The hatch was well and truly done before the calendar flipped to May. Since that time the emergence of Hendricksons has often taken place well earlier than the "norm". Hatch charts have less value today than they did in the 1970's and 1980's when so many bug books were published.

My personal fishing journals dating back to 1990 through 2011 show Hendricksons hatching before the 25th of April on only four occasions. Dozens of other entries demonstrate that I encountered the hatch from the 25th of April on out to the 22nd of May. From 2012 onwards, I have encountered Hendricksons as frequently before the 25th as I have after that date. From my experience, there are as many years that see the hatch start earlier than the traditional norm as there are years that see the flies hatch on the late side.

While hatch charts have been the most common reference point for anglers, other methods for predicting when hatches will take place have been around for decades. The late John Merwin offered an interesting look at the relationship between when trees and shrubs begin to flower and the beginning of a variety of hatches in his excellent book The New North American Trout Fishing. This relationship is known as phenology, defined as the study of cyclic and seasonal natural phenomena in relation to climate and plant and animal life. Merwin notes that Hendricksons begin to hatch when maple leaves are the size of mouse ears. From a personal point of view, I have noticed that fiddlehead fern is noticeable in the woods as the Hendricksons begin to hatch and by the time they have unfurled, the hatch is on its way out.

As fiddlehead ferns lose their namesake shape, the Hendrickson hatch is on its way out.

Fly fishing literature is replete with similar references between blossoming trees and bug hatches without attaching the scientific name to the occurrence. Ernest Schwiebert offers an interesting piece that first appeared in the William Mills catalogue in 1968 titled "Time of the Hendricksons". He notes:

Bloodroot is often blooming when the Hendricksons are hatching on the Raritan and Nissequogue, and there are dogwoods in blossom when the hatch reaches the Neversink and Battenkill further north.

One can find this essay in a collection of stories written by Schwiebert in the book Remembrances of Rivers Past. It's a worthwhile read.

Not all prominent anglers subscribe to the notion that phenology is a valid tool for predicting hatches. I view it as simply another way to try and make connections between what is happening on the river and the larger world, or at least that world adjacent to the river. In fact, I often associate the sounds one hears along the river with certain hatches. The drumming of grouse on mild spring evenings has often coincided with Hendrickson spinner falls in my experience. When the spring peepers begin to fade, I know that the Hendricksons are done and with the trilling of brown toads comes the first hatches of March Browns. These are my experiences, and I am certain others have similar "tells" for when a particular hatch will begin.

Of course, another tried and true method for trying to divine when the Hendricksons will begin is to keep track of water temperature. The entomology books will tell the reader that our favorite spring mayfly will begin to show when water temperatures hit 50 - 52 degrees. Unfortunately, the nymphs apparently do not read the same books because it is not uncommon to see blanket hatches of these little sailboats when the water is 47 or 48 degrees. The trout are rarely impressed when water temperatures are that low. Why might bugs hatch earlier than when the textbooks tell us?

Once again, lets refer to John Merwin. While discussing the biology of mayflies in The New North American Trout Fishing, Merwin notes that the nymphs will begin hatching after accumulating a certain number of thermal units over the course of their development. By this reasoning, if a river is warmer than normal for a period of time in February, for instance, then the nymphs will accumulate more thermal units than normal. This will result in earlier hatching because the nymphs have developed more quickly than normal. It is a valid concept worthy of consideration.

Immature Hendrickson nymph sampled in March.

All of this brings us back to the here and now. Just when will the Hendricksons hatch? I am tempted to say, "who knows" and leave it at that. But that is a bit of a cop out and I'd be lying if I wasn't trying to decipher roughly when we will begin to see our first fishable hatches, even at this early stage. Which brings us back to my friend Jack's prediction of April 19.

Battenkill old timers will tell you that April 25 was the "normal" beginning of the hatch along the lower river, roughly defined as Shushan on downstream. May 1st is the traditional start of the hatch on the "upper" river above Shushan. My personal average peak over the years (prior to 2012) has been the 7th of May, with spinner falls carrying us to about the 20th of May at the latest. Last year the bugs started before the 10th of April and the last good hatch of duns that I experienced was on the 28th of April. Spinner falls persisted for about a week after that.

Another Hendrickson nymph. One of many found while turning over stones along the NY stretch of river.

I'll go out on a limb (a pretty thin one at that since it is still March!) and grudgingly say that I agree with Jack's assessment. Give or take a day or two. One thing I can say with certainty: I'll be out looking on the Battenkill and a couple other streams as well come April. I hope that you too will find yourself on the water when the Hendricksons show, and the dry fly season is upon us.

Soon enough, we will be casting a line to rising trout that are feasting on the first big hatch of spring, the Hendrickson.