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

Figures & data

Figure 1. Views of the subject system. (a) Camanche Reservoir aerial view. Triangle is site of Speece cone close to the dam. Stars are main index sampling stations. Rectangle is site of fish hatchery below dam. (b) View from lower end of reservoir in autumn at normal elevation; white buoys show location of submerged Speece cone. Oxygen and electricity supply pipes are small black lines on far shore and the evaporator is the white structure at far side of dam. (c) Mokelumne River and Fish Hatchery directly below Camanche Reservoir dam.

Figure 2. Diagram of the Speece cone installed at the bottom of Camanche Reservoir near the dam. Poor-quality, low-DO water is pumped from 5 m above the sediments into the top of the cone, where it mixes with rising pure oxygen bubbles, dissolves them, and then is released as a highly oxygenated blanket over the sediments (from Brown and Caldwell 1995).

Figure 3. Relationship of redox potential in bottom hypolimnion water (dashes) and sulfide odor (solid line) in water samples collected at the same depth and station. Sulfide odor is shown in arbitrary units as absent (0), mild (100), and strong (200).

Table 1. Review of possible causes for observed fish kills in the hatchery downstream of Camanche Reservoir.

Possible cause	Comment	Final ranking
Hydrogen sulfide (1987 and 1989)	No data but likely present in deeper hypolimnion^a	High: H2S is a rapidly acting fish toxin. Deep water outlet sends H2S to hatchery in <5 min. Overturn occurred after the fish kills in 1989.
Early holomixis (1987 only)	Hatchery supply rose ∼2 C prior to fish kills	High: Depletion of hypolimnion induced holomixis, stirring H2S from sediments to water.
Turbidity	Sediment accumulation on fish egg trays	Moderate: Mean 9.7 NTU, range 8–17, rose prior to fish kills but then fell (Fig. 5).
Ammonia + pH	Hypolimnion ammonia high, x̄ = 702 max: 1700 µg/L.	Low: Hatchery ammonia low (mean 110 µg/L, range 110–140) and pH moderate (mean 7.1, range 6–5–8).
Heavy metals	Abandoned copper mine had caused kills in upstream river, low water hardness, high metal concentration in sediments	Low: Soluble metal levels in reservoir very low (dissolved Cu <2 µg/L at time of kills) so toxicity small even at low hardness (Horne and Jung, 2019). Dissolved Fe (mean 244 µg/L, range 260–590) not unusual.
Brown slimes in hatchery	Diatom chains and organic detritus	Low: Unlikely to produce mass sudden death.
Blue-green algae	Reservoir was eutrophic	Low: Algae probably like other years.
Low water level	Level fell from 32 to 21 m in droughts	Low: Reservoir well stratified at 21 m (as long as hypolimnion intact).
Fluctuations in dam releases	Water-hammer effect induced sediments stirring in reservoir	Low: No effect seen on fish kills in other years with similar flow regimes.
Low upstream reservoir releases	Cool, oxygenated water from upstream reservoir was thought to reach the hatchery	Low: Inflowing cold water does not reach the hatchery even at maximum upstream releases.
Temperature in hatchery	Mean 15.5 C, range 14.9–16.3 C.	Low: Temperature normal for October and increase not enough to cause mass deaths of fish of this size.
Low dissolved oxygen	Reservoir hypolimnion likely to have zero DO	Low: Hypolimnion anoxic for previous 30 yr, efficient prehatchery aeration system.^b
Methane	Is released from anoxic sediments at about the same redox as H2S	Low: Rates are low at ∼13 C reservoir sediment temperature. Methane is not very soluble and bubbles would rapidly be vented to the surface and/or oxidized to CO2 in epilimnion (Horne 2009b).

^a No sulfide data available at the time, but later it was found that H₂S odors were detected around the river and hatchery for years prior to the 1987–1989 fish kills (Messersmith 1989). In the 1980s hypolimnion pressure head of 30 m was used to drive an aeration fountain 20 m high, which then splashed back into a very large redwood tank built on a concrete base. DO in the water exiting the tank to the hatchery was at saturation. Estimated residence time from hypolimnion to hatchery was <5 min. Further mechanical aeration occurred at the head of each fish channel using propellers.

^b DO in hatchery was raised from <0.5 mg/L to ∼8 mg/L by aeration (Fig. 5).

Figure 4. Time course of the autumn 1989 steelhead trout fish kills. Most deaths occurred in 18 d starting 1 October. Fearing a repeat of the 1987 deaths, all the young Chinook salmon had been planted by 25 Sep 1989. Permanganate addition to oxidize H₂S was begun on 13 Oct. Based on Miyamoto (1989).

Figure 5. Time course of selected variables during the autumn 1989 steelhead trout deaths. Daily die-offs (solid bar) peaked on 10 Oct, while most other variables did not change much. DO supplied from reservoir hypolimnion (dotted line) was always low in DO but always high in DO at the head of hatchery after aeration (diagonally cross-hatched line). Temperature (pattern) and pH (circles) remained similar and suitable for salmonids. Turbidity (horizontal hatching) did increase before the start of the deaths. H₂S was not measured but odors were detected during the fish kills. Based on Miyamoto (1989).

Figure 6. Redox depth profile for a typical summer before (squares on dashed line) and after (dots on solid line) use of a hypolimnetic oxygenation system (HOS).

Figure 7. Distribution of fish (arbitrary units [triangles on dotted line]) in Camanche Reservoir with depth (a, 1992) before and (b, 1993) after HOS operation. Fish formerly squeezed around the thermocline moved throughout the entire water column after HOS operation. Also shown are DO (mg/L [squares on solid line]) and temperature (C [dots on dashed line]). Fish numbers are counts from the echo-sound reflection.

Figure 8. Seventy-three years of fall-run Chinook salmon returns (1944–2017) showing immediate increases due to HOS operation in 1993 and later improvements made possible by oxygenation. Also shown are droughts (triangles of varying width) and averages before and after HOS (dash-dot lines). Source: EBMUD and California Department of Fish & Wildlife continuous monitoring at the hatchery and fish counting facility at the downstream Woodbridge Irrigation District’s Dam.

Table 2. Chinook salmon returns in the Mokelumne River for various periods of time.

Time period	Total annual number returning Chinook	Standard deviation
1940–1987	3548	3,492 (98%)
1940–1992	3272	3,421 (105%)
1940–2011	4516	4,275 (95%)
1963–1992	2585	4,702 (182%)
1993–2011	7660	4,679 (61%)
2012–2015	12,145	63 (0.5%)

Table 3. Chinook salmon returns in the Mokelumne River during drought years before and after operation of a hypolimnetic oxygenation system (HOS).

Year	Chinook salmon in droughts before HOS (1940–1992)	Year	Chinook salmon in droughts after HOS (1993–2015)
1948	600	2000	7500
1949	1000	2001	8100
1959	2000	2002	11,000
1960	350	2007	1750
1961	400	2008	500
1962	400	2009	2200
1976	500	2012	12,091
1977	360	2013	12,252
1987	1800	2014	12,117
1988	500	2015	12,118
1989	400		Average = 7963
1990	500
1991	400
1992	1800
	Average = 786

Table 4. Comparison of hypolimnion volume decline in Camanche Reservoir between 2 yr representing drought (1987) and nondrought (1986) conditions.

Month/year	Outflow or net loss (million m^3)	Hypolimnion volume (million m^3)
Jun 1986	165
Jul 1986	47	118
Aug 1986	10	108
Sep 1986	27	81
Oct 1986	39	42
Total	288
Jun 1987	27	81
Jul 1987	26	49
Aug 1987	27	15
Sep 1987	26	0
Oct 1987	20	0
Total	126

Brown & Caldwell Inc. 1995. Camanche Reservoir oxygenation demonstration system: report on operation 1993/94. Oakland (CA): Brown & Caldwell, Alex Horne Assoc. & Biosystems Analysis Inc. for EBMUD.

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