Nebraskaland

50 Nebraskaland • April 2023 MIXED BAG Winter Storm Diaz, the massive blizzard that moved across the state in December 2022, has claimed less visible victims — countless fish in at least 35 lakes of the Sandhills and other parts of northern Nebraska. Low oxygen levels were recorded in many of the region's shallow, highly vegetated lakes this winter, and ice anglers reported dead plants and fish on their underwater cameras, said Al Hanson, fisheries supervisor for the Nebraska Game and Parks Commission's northwestern district. Winterkills can occur when ice is thick and deep snow covers a lake for a long time. These variables combined prevent light from reaching the plants below. "When the photosynthesis stops, imagine that as a big hay meadow down there and all that hay has died," he said. "As it decays, it creates hydrogen sulfide, reduces oxygen levels, and creates toxic conditions for fish and other aquatic species. Unfortunately, we can't do anything about it." Low water levels from drought worsened the problem. Oxygen tests administered by Hanson and his coworkers found numerous lakes below the level at which fish begin to die, about 1½ parts per million. Fisheries staff are conducting surveys to assess winterkill severity. Hanson is hopeful some fish have survived to serve as breeding stock for coming years. Based on survey results, Game and Parks will prioritize lakes that need initial restocking with largemouth bass, bluegills and yellow perch, and hatcheries will do their best to meet demand. He is optimistic the lakes will recover and offer outstanding fisheries as they have in the past. "In three growing seasons, we can have perch up to 10½ inches in these lakes. Bluegill get about 7 inches and bass about 12 to 14. We'll rebound, but it's not going to be a one- year thing." Hanson said this winter is similar to those of 1996-'97, 1984-'85 and 1978-'79, when prior large winterkills occurred. Anglers wanting updates about specific lakes may contact fisheries staff at Game and Parks offices in Alliance or Norfolk. Low Oxygen — Not Just A Winter Thing Low concentrations of dissolved oxygen can be problematic to fish in summer, too. Oxygen is quick to escape warm water because molecules move faster than they do in cold. Algal blooms can prompt summerkill. Although algae produces oxygen through photosynthesis during the day, it consumes it during night. Furthermore, bacteria, which thrive in heat, consume oxygen as they feed on organic matter. Waters with poor circulation polluted by excess nitrogen and phosphorous are especially susceptible to fish die-offs. WINTERKILL By Justin Haag Deep snow and thick ice covers Walgren Lake in this bird's eye view captured about two months after Winter Storm Diaz. JUSTIN HAAG, NEBRASKALAND

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