Jason Rieke caught and released a trout in 2018 on Garvin Brook. Last fall, there was a fish kill on the upper reach of the stream.

Fish kill still mystery; trout return




Minnesota’s investigation into the September 2019 fish kill on upper Garvin Brook ended last week with no definite conclusion. Fortunately, experts report that the trout population there is on its way to bouncing back.

According to the report, 1,500 fish — including 1,300 brown trout — died in the event. Minnesota Department of Natural Resources (DNR) experts believed it wiped out every fish between the headwaters of Garvin Brook and its confluence with Peterson Creek at the entrance to Farmers Community Park, though a recent discovery by Winona State University (WSU) researchers suggest some slimy sculpin fish may have survived somehow.

“The agencies were not able to determine the exact cause of the kill because of the time lapse between the kill occurring and being reported. However, the agencies agreed that the fish kill likely occurred because of heavy rains leading to contaminated runoff that caused short-term toxic conditions for fish,” staff from the DNR, Minnesota Pollution Control Agency (MPCA), and Minnesota Department of Agriculture (MDA) wrote in a summary of their investigation released last week.

The closest investigators got to pinpointing a cause of the fish kill was their conclusion that “runoff from surrounding cropland during the rainfall events that occurred just prior to the fish kill may have been contaminated enough to contribute to the kill.”

The Garvin Brook fish kill was discovered a couple days after a heavy rain event that could have washed contaminants into the stream. Investigators believe that rain events on September 19-20 and September 24-25 may have played a role in this fish kill. A citizen reported the fish kill on September 26, and MPCA staff arrived to conduct water quality testing on September 27. By the time state agencies arrived, that contaminated water was already long gone, and they were not able to find signs of the fish kill’s source in stream water they tested.

“Whatever it was that killed those fish was gone from the water,” MPCA Watershed Pollutant Load Monitoring Network Supervisor Lee Ganske stated. “When we got out there just a couple days after the fish kill, the water looked great,” he added.

Similarly, DNR Area Fisheries Assistant Director Vaughn Snook said that necropsies — fish autopsies — did not produce any evidence because the fish were already too decomposed to make out a cause of death. “Necropsies are really only of value if you get the fish as it’s dying or right after it has died,” Snook explained. “These fish were already in rigor, and they were already fungus-ed up, and they were already decaying.” Snook added of the smelly job, “I’m glad there was a good breeze that day.”

Without clear-cut physical evidence, the only way to really pinpoint the cause of a fish kill is to have monitors already in place on a stream that automatically take water quality samples every time the stream levels rise from a rain event, Ganske said. “Those sort of monitoring setups are rare mainly because they’re expensive and difficult to maintain or operate. So, it’s not something that we would anytime soon — unless there are major changes in technology — see those systems used on a major basis on rivers and streams throughout Minnesota.”

However, MPCA staff did conduct real-time monitoring of Garvin Brook during another storm event shortly after the fish kill. Those results showed “elevated ammonia nitrogen levels, elevated dissolved phosphorus levels,” Ganske reported, “which are the types of things we see associated with runoff, whether it’s human waste or manure or something like runoff from, say, corn silage or food-processing wastewater — something like that.” He added, “Theres really no way to know for sure what different component it was. It could be manure; it could be one of these other things.”

The high levels of those organic chemicals observed in runoff after the fish kill led investigators to conclude that similar, but more contaminated runoff may have caused the fish kill, Ganske explained.

“We don’t know for sure because we weren’t able to collect those samples [at the time of the fish kill] but it could have been — to use a technical term — dirty enough to kill fish that first time around,” he stated.

“This provides an indication that runoff from surrounding cropland during the rainfall events that occurred just prior to the fish kill may have been contaminated enough to contribute to the kill,” the report states.

A stockpile of manure waiting to be applied to nearby fields was identified as a potential culprit, but investigators ruled it out because the stockpile was too far down in the watershed to have been the primary cause of the fish kill, according to the report. “People said they noticed that stockpile, and they said, ‘Wow, this could have been a contributing factor,’ just because it seems like kind of an obvious potential source,” Ganske said. The manure stockpile was near the top of a ravine that drains into Garvin Brook, but the ravine flows in the brook shortly before its confluence with Peterson Creek — nearly at the end of the area affected by the fish kill. Even if there was runoff from that stockpile, that runoff could not have been the primary cause of the fish kill because of the stockpile’s location, Ganske explained. “We can’t say 100-percent that there wasn’t some contribution … but that in and of itself couldn’t have resulted in all of the fish that were found dead upstream of that ravine,” he stated.

Like the even larger fish kill on the South Branch of the Whitewater River in 2015, agricultural runoff was identified as a possible factor in the Garvin Brook fish kill, but no definitive cause was determined. It is a result that left some citizens frustrated in 2015.

The good news is that trout are already spawning in upper Garvin Brook, and the population will likely fully recover within three to five years. Despite the fish kill, the number of trout spawning nests — called redds — keeps on increasing in Garvin Brook, WSU biology professor Neal Mundahl reported. Mundahl and his students have been counting the number of redds both up and downstream of the fish kill event since 2016 — when a trout habitat improvement project was completed. Late last fall, Mundahl and a graduate student found 63 redds upstream of Peterson Creek — the most they’ve found in four years. That means adult trout just downstream of the fish kill event migrated upstream to spawn, and while most of them will head back downstream after spawning, there will be lots of little, young trout growing in upper Garvin Brook, he explained. Some adult trout might even decide to stay there, too.

“Probably three, four, five years, we’ll see fish populations there — upstream of Peterson Creek — like they were before,” Snook stated. “That’s about how long it took for the South Branch of the Whitewater. The South Branch of the Whitewater is almost back to what it was,” he noted.

“The big assumption — the gorilla in the room is, we’re assuming it won’t happen again, and we’re assuming that whatever caused the fish kill isn’t lingering, isn’t something that’s persistent,” Snook said. “The chances of it happening again are probably pretty dang low, but if we don’t know what caused it, how do we know what actions [to take] to fix it?” he asked.

The investigation report may be found online at www.pca.state.mn.us/garvin-fish-kill.

State authorities encourage citizens to report fish kills to the Minnesota Duty Officer at 651-649-5451 or 1-800-422-0798.



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