The Harvest Moon is the full moon nearest the autumn equinox (in 2023, the equinox was on 9/22).  The Harvest Moon greets fall-run salmon returning from the ocean to the Central Valley.  Unlike the previous two full moons, the 2023 Harvest Moon is coincident with improved conditions for fish.

The last two full moons, the Sturgeon Moon and the Blue Moon, wreaked havoc on the Bay by contributing to warm water and algae blooms (Figure 1) and to the associated low dissolved oxygen levels that led to sturgeon mortality in the Bay (Figures 2-4).  Low Delta water inflows and outflows contributed to the problems.¹  The fish kills that occurred in the Bay were generally a consequence of poor water quality brought on by high summer water temperatures, associated algae blooms, and low dissolved oxygen levels (hypoxia).

Water temperatures above 20^oC/68^oF bring about stressful conditions, while those above 22^oC/72^oF lead to lethal conditions such as those that occurred in summer 2022 and summer 2023.  Other effects of warm water include plankton blooms and low dissolved oxygen levels that result from the algae die-offs after such blooms.  Though there is no direct evidence of the magnitude of mortality events, there is evidence that such events may have occurred in the Bay in summer 2022 and summer 2023.  Summer conditions in 2022 and 2023, and perhaps prior years, were likely major trauma incidents that had significant short-term and perhaps long-term effects on the sturgeon populations of the San Francisco Bay Estuary (Bay-Delta Estuary).

The warm water and algae blooms in the Bay abated early in September 2023.  Water quality improved with the advent of cooler air temperatures and with the higher Delta outflows (Figure 5) associated with the Fall X2 requirement from the Delta Smelt Biological Opinion (US Dept. of Interior).  The cooling of the water ensured that salmon could safely make their fall runs into the Central Valley rivers during the Harvest Moon, the last super moon of 2023.

Figure 1. River water stage (elevation) and water temperature at Rio Vista Bridge in Sacramento River channel of the Delta near exit to the Bay. Stage drops (draining of the Delta toward Bay) occurred prior to and after the four Super Moons of summer 2023. Note the warm water (74-75oF) draining from the Delta during the two mid-summer drain periods.

Figure 2. Water temperature (C), dissolved oxygen (DO, mg/l), and chlorophyll (mg/l) in western Suisun Bay in summer 2023. Note algae bloom at end of July that began during the warm water period. Note very low DO (<6 mg/l) after bloom die-off in August.

Figure 3. Chlorophyll concentration (mg/l) in 2022 and 2023 in Suisun Bay. Note two summer algae blooms in red circles.

Figure 4. Dissolved oxygen concentration (mg/l) in 2022 and 2023 in Suisun Bay. Note low levels after two summer algae blooms in red circles.

Figure 5. Daily average flow in summer 2023 and 25-year average at Rio Vista in the lower Sacramento River channel of the Delta leading into Suisun Bay. Late August and September increases are related to the Fall X2 requirement of Delta water projects in wet years.