CRYSTAL LAKE | Thousands of Crystal Lake fish were killed earlier this week due to decaying algae and reduced oxygen levels, leaving anglers and fisheries biologists uncertain regarding the immediate future for one of North Iowa's premier fisheries.
Much of the shoreline was littered Friday with bleached carcasses, while the light breeze carried the pungent odor of decay.
"She's pretty sad," said Forest City angler John Gullickson. "The lake took a big hit."
The kill was a natural event caused by an alignment of environmental conditions, according to DNR Regional Fisheries Biologist Scott Grummer.
Iowa's shallow lakes are subject to periodic algae blooms fueled by their innate fertility along with nutrients from agricultural and urban runoff.
Several days of warm temperatures and light winds prior to July 4 contributed to an unusually heavy blue-green algae bloom.
"It just shows how nutrient-rich Iowa waters are," Grummer said.
The algae died rapidly earlier this week, probably due to abnormally cool temperatures and overcast days.
Decomposition of plant material removed oxygen from the water, while the weather also decreased the metabolic activity of oxygen-producing plants. Oxygen levels dropped quickly as a result.
"If that algae would have died off over a week or two period, we likely wouldn't have had an oxygen issue," Grummer said.
Oxygen levels became inadequate to meet the biologic demands of the system, causing the die-off of fish and other aquatic creatures.
Decomposition of the plant material and dead fish should progress rapidly, reducing the oxygen demand. Higher winds to aerate the water and warm, sunny days to stimulate oxygen-producing plants would help.
"I would expect things to gradually improve," Grummer said.
There is nothing DNR can do to accelerate the process, Grummer explained.
Collection and disposal of the fish and plant material would be prohibitively expensive and would potentially create ecological issues elsewhere.
"Environmentally, it's probably best to let the fish decompose in the basin that created them," Grummer said.
Crystal Lake is equipped with a winter aerator designed to maintain an ice-free zone, allowing ambient air to oxygenate the surrounding water. This creates a "refuge" to which fish can migrate as winter oxygen levels gradually drop.
Activating this system would have minimal impact during open-water conditions, Grummer noted, and fish would not likely be able to migrate to areas of higher oxygen concentration in response to a rapid decrease such as this one.
That's frustrating news for the many anglers who have been enjoying outstanding success on Crystal Lake in recent years.
"I've had 40-fish days, and that's keepers. Some were up to 4 pounds,” said Gullickson, who normally targets largemouth bass. "(Friday) I caught three fish, and they were the color of toilet paper."
"I come up here quite a bit," said Paul Friesth of Badger. "I've had no trouble getting a limit of nice crappies."
Although it's too early to determine the extent of the kill, DNR officials noted dead fish of a variety of sizes and species.
No changes to fishing regulations are planned. Grummer said fish remain safe to eat, although he noted algae blooms sometimes give fish an undesirable taste.
The kill is expected to have no lasting environmental impact, according to Grummer.
"The setback is the loss of the current fish," Grummer said. "A new population will develop and grow well."
DNR plans to conduct netting and electrofishing surveys this fall to assess the fishery. Some restocking, if indicated, could begin soon thereafter.
"Management decisions are going to be based on what we have left and how we move forward from that point," Grummer said.