Hypolimnetic oxygenation 1: win–win solution for massive salmonid mortalities in a reservoir tailwater hatchery on the Mokelumne River, California

Horne AJ. 2019. Hypolimnetic oxygenation 1: win–win solution for massive salmonid mortalities in a reservoir tailwater hatchery on the Mokelumne River, California. Lake Reserv Manage. 35:308–322. More than 300,000 hatchery fish supplied with deep hypolimnetic water from Camanche Reservoir, California died during two short, late summer events during the 1987–1992 drought. A criminal complaint cited the water agency for negligence. Fisheries agencies blamed the fish kills on low water levels but retaining more water would cut supplies to 1.4 million city residents, downstream farmers, and wild river fish. The reservoir was assumed to be unproductive due to its undeveloped mountain watershed but no measurements had been made. Hydrogen sulfide (H₂S) was suspected as the toxic agent since it acts rapidly. Later examination of sediment redox indicated the potential for H₂S formation which was confirmed by odor. A cure for H₂S in the hatchery and river below the dam was a Hypolimnetic Oxygenation System (HOS). In addition, a management system was devised to guarantee a minimum volume of cool (<16.4 C), oxygenated, hypolimnetic water. The two management strategies (HOS and cool pool) greatly improved conditions. H₂S was eliminated, fish moved into deep water in the reservoir, and no fish kills have occurred in 24 yr. The HOS was superior to permanganate treatments or maintaining high reservoir water levels during droughts. The criminal lawsuit was settled, no one went to jail, and inter-agency cooperation improved. Following HOS operation, Chinook salmon returns rose significantly (p < 0.05) from 3550/yr to 7660/yr (2009) and 19,867/yr (2018), with 10,000/yr spawning naturally in the river. Returns during a recent drought increased tenfold. Endangered steelhead trout returns rose from 8/yr to 1,168/yr (2017).