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70 STATE OF THE ESTUARY FOOD CONTEXT No estuary can support a functional ecosystem, let alone larger crea- tures like sturgeon and herons and seals, without good food. Good food, for the San Francisco Estuary, starts with the growth of plankton and zooplankton at the base of the aquatic food web, which are in turn eaten by small fish and crabs, which themselves provide food for larger predators like salmon and cormorants. Each trophic level is import- ant and can be interrupted by changing con- ditions (such as upwelling offshore or shifts in the location of the low salinity zone in the upper Estuary), invasive species, or human disturbance, among many factors. For the purposes of assessing the state of the food web, this report examines three different tro- phic levels: zooplankton and small fish in the upper Estuary (see below) and big birds that eat fish in the lower Estuary (following pages). Zooplankton are small aquatic invertebrates and a critical trophic link between primary producers and fish. These tiny drifting ani- mals include some early life stages of shrimp, jellyfish and crabs, and like their plant coun- terparts, phytoplankton, grow and drift in many areas of the Estuary. When these food supplies are abundant in the low salinity zone of the upper Estuary, they tend to support a flourishing food web. Most larval and juvenile fish eat zooplankton, and some species eat them throughout their lives. Small fish also feed bigger fish. INDICATORS The first indicator of food Mysids (top) and copepods (bottom) 2015 web health examines annual zooplankton abundance in recent years, and compares it to estimates recorded since 1972 by the Inter- agency Ecological Program. Scientists chose two crustaceans called calanoid copepods and mysids for the zooplankton indicator because they are important food items for Delta smelt and longfin smelt. A state vessel collected samples in the upper Estuary in both the Suisun and Delta regions. Average annual biomass, in milligrams of carbon per Juvenile chinook salmon. Photo: USFWS cubic meter of water, was then calculated for 1974-2014. The benchmark derives from biomass measured between 1974-1986, before the disturbance to the food web caused by the arrival of the invasive clam Potamocorbu- la amurensis. The second indicator of food web health at a higher trophic level measured abundance of fish, both native and introduced, living in three major habitats — marsh, pelagic open water, and beach — found throughout the upper Estuary. The fish abundance indicator measured total fish caught per-unit-effort — reflecting the number of fish a predator would find in each year in each of the major habitats. Benchmarks for this indicator derive from average reference conditions in 1980- 1989 (for marsh and pelagic zones) and 1995- 2004 (for the Delta beach zone). STATUS & TRENDS For trends in the first indicator, long-term sampling indicates a decrease in zooplankton biomass in most areas of the upper Estuary since the 1980s,