1. Troutnut Forum
›
2. Fly Fishing
›
3. Mayfly Hatch Decline?

The following question was posed to me by brntrout on another forum:



And, I answered it thusly:



However, the question has really sensitized me to the issue, and I’m wondering whether (or not) others may have noticed a significant decline in mayfly hatches (similar in circumstance to the SE Minnesota observation) in their own home waters.

another bit of anecdotal evidence: on Dickey's Run in south central PA, the sulphur hatches used to "boil the water", according to our elderly informant. in the three years we've fished there, we've seen obvious hatches, but nothing that would begin to suggest that vivid image, and E. I. has expressed his regret at the passing of something else in the natural world.

This is always a troubling and puzzling question, Roger. As a fervent follower of the hatches in the rather broad range of streams that I'm proud to consider my homewaters, I've noted over about forty years both decreases and increases in mayfly populations that seemed to be cyclic, some that were probably attributable to catastrophic events like sustained severe flooding and subsequent slow recovery, and others that were more insidious and disturbing.

Here in the Cumberland Valley, for example, we have a number of small limestone spring creeks that once held outstanding populations of "sulphurs" (especially E. invaria, but also others). I mention this example because invaria seems to be a particularly resilient and adaptable member of the Ephemerella genus and because these spring creeks are generally quite stable and fertile environs. While the wild trout populations in these creeks are still pretty good in many cases, they are sustained now primarily by scuds, sowbugs, and midges. The land surrounding these streams has long been prime agricultural real estate, but land use trends are rapidly changing, and suburban sprawl and its associated baggage are consuming more and more of this fertile ground. The pressure for farmers to produce more on less land along with the pressure to develop the land makes me worry about the future of many of these streams. In a few, I seem to see an increasing ratio of sowbugs to scuds, which is usually not a good sign.

In contrast, I can point to a few waters where improvements have occurred and mayflies are on the increase. The invaria populations are often the first mayflies to respond to these positive changes. There are also baffling incidents that I cannot explain. Last season on the Yellow Breeches (one of the two main streams that cut across the valley) we saw the first good hatch of Hendricksons (E. subvaria) that most of us can remember seeing in many, many years. (They used to be quite strong and a reliable harbinger of the renewal of the fly-fishing season.) Of course, we all took this as a happy and encouraging event.

Different mayfly species have different sensitivities and face different challenges. In my other home region, the Poconos, multiple years of severe flooding have altered stream courses, overwhelmed sewage treatment plants, and flushed trout and insects from any stream section where the flood waters were unable to spread out. I expect that the streams can recover from this over time, but the resulting "cleanup" that follows in the wake of such events is often done in thoughtless and damaging ways, and further exacerbates the problems for trout and mayflies. (Most of the Easterners here know that the Catskill streams and upper Delaware Basin were also hit hard by these events.) Only time will tell what the result will be for trout, mayflies, and fly fishing there.

Before the devastating floods of Hurricane Diane hit the Northeast in the '50s, the Brodheads (and a number of other Pocono streams) had nice hatches of green drakes (Ephemera guttulata). Some attribute their loss directly to the floods, while others blame the widespread spraying of DDT that followed for their demise. Whichever is the case, except for a very small relict population in a short section of the Lehigh, they seem to be gone forever.

This is certainly one of the sadder sides of fly-fishing today, and I also wonder about the larger compounding effects of climate change and global warming. The future of fly fishing may seem like a petty concern in the light of issues as large as these, but it is certainly precious to me, and without a hatch to match it loses much even now.

I haven't been fly fishing long enough to observe the long-term trends myself, but I've heard about the same trends Taxon and Gonzo describe many times, and in many places.

The major directly human-caused factors seem to be

• Watershed development often introduces undesirable chemicals to the water, and the removal of shading streamside vegetation warms the water. It can result in excessive silt.


• Acid rain, depending on location, may seriously dampen the productivity of a stream or even make it unlivable.


• Global warming - The slow rise in mean annual temperature probably hasn't directly affected trout streams yet, although it probably will within a few hundred years. But global warming is also predicted to make brief extremes more frequent, something we seem to be seeing lately with uncanny hot spells. Trout and cold-water insects don't have to worry about water temperature during most of the year, only during the worst extremes, and those extremes may worsen much faster than the annual average.


• Floods and droughts are seemingly natural events which have a big impact, but their presence or severity can often be traced to human activity, too. Wetland filling, channelization, and paving over the watershed all serve to worsen floods, and the severity of both floods and droughts can probably be attributed to global warming's predicted consequence that we get our rain less often and in larger doses.

None of these things bode well for trout or cold-water insects. Luckily many streams are protected by park services from the worst impacts of watershed development, but global climate factors are cause for concern over almost any river.

I wouldn't anticipate a large net loss in insect biomass from these changes (although I'm not really qualified to speculate on that). I would guess that the result, instead, will be a shift in the dominant species. Gonzo described some cases of that above. So the problem isn't really that the invertebrates are disappearing, but that the ones we like are going to disappear first. Many of the most treasured mayfly hatches (like the Hendricksons) are canary-in-the-coal-mine species.

That is my impression as well, though I have also seen the Hendrickson described as "hardy." I suppose that begs the "compared to what?" question. The habitat requirements of subvaria and its sensitivity to changes seem pretty acute, even when compared to the closely related invaria species. The invaria seem to occupy a wider range of habitats (even within a stream section) and seem able to hold on after subvaria has dwindled or disappeared in a stream. Epeorus pleuralis also seems to be a particularly sensitive Eastern species (perhaps even more so than the Hendricksons), and is noticeably on the wane in many streams of my experience.

Jason, I know that when benthic invertebrates are surveyed as a tool to assess the overall health of a stream or section, they are given a rating based on their relative sensitivity. Do you know if these ratings are sufficiently species-specific to provide a point of comparison with my (admittedly anecdotal) observations?

Most invertebrate biomonitoring is done at the order or family level, I think. I'm sure some experts use genus as well, but comparison of individual species for that purpose is probably rare if it's used at all. I don't know all that much about it.

Thanks, Jason. As just another point of interest, does anyone know anything about the effects of large-scale spraying against black flies or mosquitoes upon other aquatic insect populations?

Spring Creek here in Central Pa used to have Green Drakes, they're gone now due to a Chemical spill from PSU. We also have experienced major floods in the past ten years, I can tell you, the sulphurs(dorthea?) are slim to none now. But to me, that seems a little weird seing that we have an over abundance of Tricos(naming?). The baetis hatchs last year seemed to suck, but then again, we had a flood two years ago, at beatis time.

My experiences with the little olives on Spring Creek in State College PA match Jeremy's. In 2004 there were heavy hatches. In 2005 and 2006 I found some fish rising to olives in March, but not nearly as many as in '04 when there were bugs everywhere each grey, drizzly day I was able to get on the water. Folks I have talked to suggest that these differences may be due to natural cycles.

In 1996, 1997 and 2001 I fished great Hendrickson hatches on a small tailwater that is not so well known, and will remain nameless. Local Trout Unlimited members I talked with told me that it is believed a spill in 2002 virtually wiped the Hendricksons out. I came looking after that but the bugs were gone. Last year one angler told me they were making a comeback in the upstream waters, and all the local flyfishers I've talked to are hopeful that the hatch will reestablish itself. In the past this had been a magnificent hatch, bringing up lots of big fish that now are rarely, if ever, seen feeding on top.

I'd second Gonzo's observation that it appears that some variations are cyclic, that some are caused by human intervention, and that it's difficult without hard evidence to know which at times, or if some combination of natural factors and the absence or presence of pollution best accounts for immediate changes. I also believe that climate change may have some long term effects on reducing habitat for both mayflies and trout.

Thanks, David, for clarification on both points. Can you point me in the direction of the literature you mention? And Louis, I'll pry the name of that little tailwater out of you one way or the other! :)

Thank you very much, David. I can see that I've got my work cut out for me, but I greatly appreciate your thorough response.

Thanks TAXON, for posting my question on this forum.

From reading the replies to this question it appears that what is happening in S.E. Minnesota is also taking place in other locations of the country.

I wonder if the loss, or severe reduction in the numbers of certain invertebrate species will cause any long term health issues(poorer body condition/survival) for trout?

The reason I ask this question is the loss of some of the major invert. species has to have reduced the overall abundance of forage availablity for trout at DIFFERENT times of the year.

Since most mayflies can be readily captured by trout, their availbility as food source has to be superior to other forage species that aren't as readily available or as easilly captured. Will the loss of many differennt invertebrate species effect the health/survival/size of trout in many of our stream? What to you guys think?

WL Hilsenhoff (Univ. Wisconsin) put out several papers with aquatic insect species used to come up with a Biotic Index of water quality. Each has been assigned a tolerance value. I didn't notice his work mentioned in the discussion up to this point.

Example:
Hilsenhoff, WL. 1987. An improved biotic index of organic stream pollution. The Great Lakes Entomologist 20: 31-39.

I know the diversity and numbers of macros on Spruce Creek in Pa have been on the decline, the stream flows almost entirely through farmland. There is a COA, 2000 cow dairy operation on the stream which produces an ungodly amount of "stuff" plus uses 50 to 100 gallons of water PER DAY per animal (I would think the higher amount in this operation?) At 100 gallons per animal that would be 73 million gallons per year. Check the flow of the Little Juniata River and note the decrease in flow. (USGS site)