There are fewer fish locally in the Animas River than there used to be, but the populations are not likely wanting for food.
A study of bugs, specifically macroinvertebrates, recently completed found a wide variety of species, including many that are sensitive to pollution, said Scott Roberts, an aquatic biologist with Ecosphere Environmental Services.
There’s evidence there are more species than there ever have been recorded before, he said.
He also found a wider variety of mayflies, stoneflies and caddisflies than what previously was recorded in the fall, which is a separate measure of river health. The case caddisflies, a species sensitive to pollution, in particular, were found to be more abundant than they have been in previous years.
But data also suggests declines in the dominance of other species, such as aquatic beetles.
Trout Unlimited funded the study to look into anecdotes that the bug population was changing, and it could be impacting the fish.
Roberts looked at communities of bugs at different sites, including at 32nd Street above Lightner Creek and at Purple Cliffs, behind Home Depot and Walmart.
He netted bugs in the same places and during the same times as in previous studies to make sure his data was comparable to previously recorded numbers. He sampled the populations last summer and fall.
But the macroinvertebrates in the Animas River near town have been studied only four times since 1996 in the fall, and the most recent study was completed in 2005.
So, the study could not take into account the variability in bug populations that occurs from year to year.
The short-term nature study makes it difficult to rule out a decline in food as a factor in the fish decline completely, he said.
“The scope of the study was pretty limited,” Roberts said.
But his findings align with what Jim White, an aquatic biologist with Colorado Parks and Wildlife, has found when he measures and weighs fish.
“They are not skinny; they are not fat, either. ... There’s enough groceries out there to keep them in decent shape,” he said.
The main problem is the young fish are not surviving to become mature adults.
“The bug study would suggest there’s enough food out there for young juvenile fish,” White said.
In addition to annual sampling, Roberts recommends studying the abundance of aquatic plants, such as moss, to help explain changes in bug populations.
Anecdotally, local anglers are noticing more vegetation in the river, which may be caused by changes in the nutrients and temperature. Warmer water can’t hold as much dissolved oxygen. While it’s not lethal for fish, it could be one of many contributing factors, White said.
Although he has found declining fish populations, the Animas River has maintained its gold-medal status, an indicator of the quality of fishing in a river.
mshinn@durangoherald.com
