IOWA CITY — Researchers found half as many mussels as they had hoped for last week during an intense survey of the Iowa River.

“We found 1,500 mussels, representing 20 species. We found some decent populations, but I had hoped for 3,000 or so,” said Scott Gritters, the Department of Natural Resources fisheries biologist who coordinates Iowa’s annual survey of mussels.

The catch rate during this year’s mussel blitz was 15.7 per hour, which Gritters described as low compared with rates achieved in six previous Iowa River searches since 2005. In their best year, 2011, the researchers collected 49 mussels per hour.

Gritters said he was disappointed that this year’s effort failed to find representatives of six species of mussels — the butterfly, fluted shell, lilliput, mucket, paper pondshell and round pigtoe — that have been collected periodically, although in low numbers, from Iowa River blitzes dating to 2005.

Recent extreme weather — floods, droughts and cold winters — coupled with excess nutrients and sediment covering mussel habitat likely contributed to this year’s low numbers, he said.

Though the 2014 mussel survey was disappointing in some respects, it was encouraging in others, according to Gritters.

The team collected six Higgins eye pearly mussels, which were close to extinction 40 years ago.

In an 11-year effort to establish Higgins eyes in Iowa’s interior rivers, biologists stocked stretches of the Cedar, Wapsipinicon and Iowa rivers with walleyes and bass whose gills had been inoculated with the mussels’ larvae.

They have since found Higgins eyes in both the Wapsipinicon and the Iowa, and this year they were hoping to find juveniles in the Iowa — proof that the endangered mussels are reproducing there.

Though they did not find any juveniles, two of the six specimens they collected were gravid females.

“Despite their extremely low density in the river, we know that males and females are finding each other,” Gritters said.

Another bright spot, he said, was this year’s “crazy good participation,” characterized by a record single-day turnout of 51 people, which includes volunteers and employees of conservation agencies.

“It’s gratifying to see so many river advocates freely stand up for our natural resources,” he said.

Most of the mussels were found using a technique known as pollywogging, which consists of crawling along a stream bed, probing the bottom with gloved hands.

The researchers also sifted the bottom contents of 620 quarter-meter grids selected in a stratified random manner that excludes areas that contain no suitable mussel habitat.

The grid survey enables researchers to find juveniles that would escape the notice of the pollywoggers. It also provides a scientific basis for estimating mussel populations, Gritters said.

At least 12 of the 54 mussel species that once lived in Iowa are gone, and about half the remaining species are listed as endangered or threatened.

Gritters said fish and mussels have “co-evolved” and depend on each other in certain respects.

Dense and diverse mussel populations go hand in hand with strong fish populations and good water quality, he said.