Fish populations appear healthy after July flooding

Despite the heavy rains and flooding that caused widespread damage, it appears the fish populations of the rivers in Ashland and Iron counties came away from the storms of July 11-12 relatively unscathed.

Zach Lawson, a fisheries biologist with the Wisconsin Department of Natural Resources' Mercer office, said an overview of the population counts show while there may be some fluctuations at specific locations, the overall fish populations remain strong.

"We observed similar catch rates, similar size structure ... and similar species compositions," Lawson said in a summary of the data. "In many of the mid-sized streams (such as Devil's Creek, Vaughn Creek and Trout Brook), we observed similar trout densities. Perhaps a few of these mid-sized stream reaches contained slightly fewer fish following the large flood event, but we're close to pre-flood numbers and for the most part, these systems as a whole looked pretty good."

The population estimates are based on a survey DNR staff conducted in various locations beginning July 23 and running through mid-August.

Lawson said the population counts were conducted via electro-fishing, using backpack and barge shockers to stun the fish so they could be counted.

Given the rarity of major storms, Lawson said there wasn't much data to compare the numbers to.

"These flooding events are extremely rare to begin with, so documentation of fish population response to those events are actually pretty rare themselves," Lawson said.

The few case studies out there showed fish populations usually were significantly harmed by flooding, Lawson said.

"Many studies have shown large flooding events will reduce densities of adult fish and completely wipe out younger year classes," Lawson wrote in his summary. "In (previous studies) the 'seed' or remaining adult populations will take three to six years to fully recover from catastrophic events.

"However, in the case of the Ashland/Iron county systems, not only is there plenty of adult 'seed' to carry the population; we shouldn't have many missing year classes since it appears that plenty of juveniles survived flood conditions."

Further encouraging researchers was the fact there was poor visibility in the water, meaning there were likely more trout than counted.

In his summary, Lawson partially credited evolution for the high survival rates.

"It s hard to believe that a 2.5-inch fish can survive in a raging river that has the power to move a dwelling, or tear towering trees from nearby stream banks," he wrote. "While it seems impossible, we must remember these species have evolved in these high-gradient and flashy environments for millions of years, and as such, have the capacity to cope with both extreme and rapidly changing environments."

The storm also may have had a mixed impact on the fish habitats in the county, according to Lawson.

He said the storm had both positive and negative impacts on the waterways - exposing bedrock beneficial as a spawning area and adding debris for in-stream habitat, while removing vegetation from the shoreline often used for cover.

"While it kind of cleaned out the stream channel, creating quality spawning habitat in a lot of areas, at the same time you've got erosion of stream bank and the near-shore areas," Lawson told the Daily Globe Wednesday. "Along with that, a lot of the vegetation that was near shore is now placed in the stream. You've kind of got a lot of new in-stream habitat at the expense of all that near-shore (vegetation)."

The headwater areas of the streams and rivers were generally least impacted by the storm, Lawson said, with the damage increasing downstream.

"Habitat didn't seem to change a whole lot (in the headwaters), neither did the fish populations," he said. "As you go downstream, into some of the larger systems that have had a few tributaries up stream, those streams - like Vaughn Creek for example, or Devils Creek - those streams we saw (an increase in damage)."

While the loss of shoreline vegetation could have future impacts, such as erosion and less bug production resulting in a declining food source; Lawson said studies showed the biggest impact on fish populations are during and immediately after the storm - such as the actual loss of fish.

"There might be some changes but most studies have shown the largest impacts are felt immediately during or very shortly after these flooding events," Lawson said.

Above all, Lawson cautioned against reading too much into the results of a specific location, saying the researchers tried to get to a lot of locations - leading to smaller sample sizes to extrapolate into larger systems. Some areas produced higher numbers after the storm, some lower - but Lawson said the key was to look at the data as a whole.

"It's representative of what's there ... the take-home message is (the populations) are similar (to what existed before the storm)," he said.