Two days after the Arizona Game and Fish Department stocked Lower Lake Mary with nearly 8,000 trout, a sizable portion of those fish were found dead. On Thursday, the department collected 1,500 to 2,000 dead fish from the lake, department spokeswoman Shelly Shepherd said. The reason for the kill, water testing later revealed, was low dissolved oxygen levels in certain areas of the water, Shepherd said. The department hadn't tested the lake's dissolved oxygen levels before the fish were stocked, she said. "It was one of those rare occurrences and we were caught by surprise," she said. Shepherd explained that ice and snow that covered Lower Lake Mary for nearly two months over the winter blocked sunlight from filtering to the lake bottom, halting photosynthesis among the aquatic plants. In the absence of light plants do however, continue to respire. During that process, plants consume oxygen and produce carbon dioxide, which is what lowered oxygen levels in the water. As a result of the fish kill, fishing at Lake Mary will slow temporarily, Shepherd said. She said anglers should be on the lookout for signs of fish becoming active on the surface of the lake, which will signal that the fishing is good again, Shepherd said. The department will continue to monitor water quality and will restock more fish in Lower Lake Mary when conditions improve. Shepherd said she wasn't given an estimate on when that might happen. She did note that Game and Fish officials saw live fish as well when they were out at the lake on Thursday. In the meantime, the department advised anglers to check out Dogtown Lake south of Williams or Frances Short Pond in Flagstaff as alternative fishing locations.