Fish Kill Event - Lake Red Rock

Upon investigation,dead white bass and hybrid striped bass were observed, primarily along the north shore. Fewer than 10 fish were observed floating in the lake, while several dozen were observed blown up on the shore. All fish appeared to have been dead for days. The highest concentration of dead fish was found near the “sand island” area.

The cause of the kill was not obvious. The observed fish exhibited substantial decay and had likely been dead for between 1 and 4 days. No stressed or moribund fish were observed. All water quality data were normal, no extremes were observed in ammonia, dissolved oxygen, or temperature. The species make-up of the kill was exclusively temperate bass, specifically white bass and hybrid striped bass. A single freshwater drum was observed, but may not have been related to the other dead fish. No other species were observed. The size range of affected fish varied from approximately 14 inches to approximately 23 inches. However, most fish were white bass and ranged between 14 and 17 inches. It is unlikely that the kill is due to any specific contaminant. Similar die offs of temperate bass have been observed at other Iowa lakes in recent years. A similar kill occurred in 2002 at Rathbun Lake. No explanation for these die offs has yet been developed.

Given the species-specific nature of the kill and the wide-spread nature of the dead fish, complete counts were not possible. However, relatively few fish were present along other shorelines and fish appeared to have been concentrated by prevailing winds in the last few days. An estimate of dead fish was not completed; however, we suggest the number is in the 100’s lake-wide.