How aquatic insect populations are monitored and evaluated

Posted 11/3/23

As fly fishers, we spend a great deal of time discussing aquatic insect activity. Most of our dialogue has to do with the various mayfly and caddis fly hatches—or the lack thereof—that we …

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How aquatic insect populations are monitored and evaluated


As fly fishers, we spend a great deal of time discussing aquatic insect activity. Most of our dialogue has to do with the various mayfly and caddis fly hatches—or the lack thereof—that we have witnessed over the last several years.

Therefore, it is not uncommon for us to try and determine the reasons that aquatic invertebrate communities are on the decline.

And of course, considerable speculation is offered about causes, such as climate change and associated stream warming, pesticides, and flooding, to name just a few.

Yet there are no definitive answers, just a lot of grumbling and dissatisfaction with the fishing.

Speculation aside, most anglers are not aware of how aquatic insect populations are monitored and evaluated. Sadly, there’s a good reason why that’s the case. Most state natural resource agencies have not studied or do not study aquatic insect communities as a matter of routine. Fortunately, the New York State DEC finally established a biomonitoring unit that is dedicated to the study and analysis of aquatic insect communities on a statewide basis.

As background, anglers need to be aware that aquatic insect populations are monitored using the Surber Square Foot Stream Bottom Sampler. The device is made of a one-foot-square metal frame that is attached to a fine-mesh nylon net, such as the device shown in the accompanying photo. Biologists use the Surber Sampler to determine the qualitative (species diversity) and quantitative (productivity) of rivers. Samples are taken in late winter and early spring, when immature aquatic insects are abundant, almost fully developed, and prior to hatching.

That means biologists must deal with water temperatures in the low 30s Fahrenheit while collecting samples.

Before sampling begins, biologists conduct a field investigation of the chosen river, where several sampling stations are selected. Samples are taken along each side and from the middle of each station on a weekly basis, over a prescribed period of time. The Surber Sampler is placed along the bottom at each station, all the stones within the frame are removed and the invertebrates are allowed to collect in the net.

The samples are then removed to the lab, where the insects are identified and weighed. From the weight of the samples, the wet weight biomass of the aquatic insect community can be established. That weight can be extrapolated for the entire river, provided enough samples are taken, then compared to a table, and the productivity of the river determined.

In February of 1964, John Zuzel and I began an aquatic insect study along Lolo Creek in Western Montana. We began at its confluence with the Bitterroot River near Lolo (elevation 3,202 feet), extended to the summit near Lolo Pass (elevation 5,233 feet)—a distance of almost 30 miles and an elevation change of 2,000 feet.

Our goal was to determine if species diversity and productivity changed with elevation.

As students of the aquatic biology program at the University of Montana, John and I had the opportunity to elect independent studies with our professor, Royal B. Brunson. Over a period of two months, we took Surber samples at several locations along Lolo Creek. It was very cold work with lots of snow.

After each field trip, the samples were returned to the lab, where the collections were identified, counted and weighed. All specimens were preserved with isopropyl alcohol for future study. We usually stopped in at the Hiedelhaus House Restaurant, which was just outside of Missoula, MT on Route 93 on our way back from Lolo, to warm up and have a few beers.

All these years later, I don’t recall exactly what our findings were, in that I lost my copy of the report we submitted to Dr.Brunson. I do recall that it was very cold work, taking three samples at each station, with deep snow along the bank and air temperatures well below freezing. I do remember that we did find a difference in species composition as the elevation changed. I believe that had to do with the difference in water temperature, in relation to elevation.

So you see, there is a lot of fieldwork required to determine the status of aquatic insect communities in our trout rivers. Unfortunately, little historic baseline data is available against which to compare recent aquatic insect collections. Therefore, it is almost impossible to know if current populations are different than they once were without an accurate baseline for comparison.

What we can say is that aquatic insects are biological indicators, and are some of the first organisms to signal there are problems with the environment in which they live. We are hearing that both terrestrial and aquatic insect populations are in decline worldwide, and right now, we do not have a lot of answers as to why.

Hopefully as awareness increases, both terrestrial and aquatic biologists will increase their studies and determine both the causes and solutions to this worldwide issue. Because ultimately, the decline in both land- and water-born insect populations will affect us directly.

Whether it be the lack of honeybees to pollinate our crops, or of mayflies for trout to rise to and feed upon, humankind will pay the price if insect populations continue to decline.

As a side note, anglers who fish in New York will need a scientific collectors’ license from the DEC, should they decide to collect aquatic insects from trout waters.


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