The Green Sturgeon's Dangerous Diet

The enigmatic Green Sturgeon (Acipenser medirostris) patrols the benthos of the San Francisco Bay and near shore oceanic waters as a living relic of ancient seas. This large, long-lived fish species, which has persisted for millennia in evolving oceans, may have finally met its match in the environmental impacts of Bay Area development. The Southern Distinct Population Segment (DPS) of the Green Sturgeon, which is listed as threatened under the federal Endangered Species Act, swims through the San Francisco Bay to reach its only remaining spawning ground in the Sacramento River. As these animals swim through the Bay and Delta, they face deteriorating water quality, reduction of freshwater flows, potential poaching for caviar or bycatch in other fisheries, entrainment in water intake structures, and impassable upstream barriers. The principle threat to the Southern DPS is the disappearance of its spawning ground. Now restricted to a very narrow stretch of the Sacramento River, the elimination of the remaining spawning ground would mean the extinction of this genetically distinct Green Sturgeon population. The National Marine Fisheries Services (NMFS) is strengthening protective measures of the Southern DPS by proposing the same take prohibitions that are applied to species listed as endangered. Take prohibitions will make it illegal to hunt, harass, or otherwise harm the fish, including any action that degrades its critical habitat.

Unfortunately this step may not be enough to ensure the long-term survival of the population. A combination of human-influenced factors, now woven into the ecological fabric of the San Francisco Bay and Delta, has turned this estuary into a toxic environment where animals are actually poisoned by their food web. The Green Sturgeon feeds on invertebrates that it finds bottom of the Bay, including the invasive Overbite Clam (Potamocorbula amurensis). This prolific bivalve, which can be found in densities as great as 50,000 per square meter in some areas of San Francisco Bay, is likely to compose the better portion of the Green Sturgeon’s diet. The Overbite Clam itself has a voracious appetite, and it rapidly filters food particles out of the water column with amazing efficiency. Although it may be seeking tiny organisms and detritus, the clam inadvertently consumes the contaminants that pollute the water column including selenium, a bioaccumulative element that comes from oil refineries and Central Valley agriculture. The efficiency with which it incorporates selenium into its tissues makes the Overbite Clam a toxic meal for any predator it succumbs to.

The impact of selenium may not be immediately apparent in the individual adult fish, but it can cause massive reproductive failure. Selenium from a diet of contaminated Overbite Clams will bioaccumulate in fish tissues over the course of its lifetime. Although it may not harm the adult fish, selenium is transferred from a female fish to her eggs, which can cause embryonic death or fatal deformities upon hatching. These reproductive impacts can be severe enough to devastate a population. All fish species in the San Francisco Bay that feed on the Overbite Clam are at risk; however, the unique life history of the Green Sturgeon makes it more vulnerable to this poisonous prey.

In many ways the life of the Green Sturgeon is similar to that of a human. It has a remarkably long lifespan of up to 70 years, and it does not reach sexual maturity until it is at least 15 years old. As an adult it has an iteroparous reproductive strategy, meaning it allocates energy to multiple spawning efforts over the course of its lifetime as opposed to a “big bang” spawning effort once before death. The Southern DPS of the Green Sturgeon, which spends the majority of its adult life in the ocean, returns to the Sacramento River every 2 to 5 years to spawn. Even though it spends more time in the ocean than other sturgeon species, Southern DPS adults may live in the estuary for seven months of the year and juveniles can live here year-round, all the while exposed to dietary selenium.

Given its late age of sexual maturity, the Green Sturgeon will accumulate selenium in its body for at least fifteen years before it first spawns. The level of bioaccumulated selenium imparted to eggs will then increase for all subsequent reproductive efforts as the sturgeon ages. As a result the reproductive impacts of selenium are likely to be more severe for the Green Sturgeon than for a fish with a shorter lifespan and quicker maturation. This reproductive challenge can greatly reduce recruitment within the Southern DPS, meaning that fewer individuals are surviving to reproduce and maintain the size of the population. With this obstacle to the population’s survival so deeply rooted in the ecology of the San Francisco Bay and Delta, the actions that result in the take of an individual animal seem relatively easy to avoid. The proposed take prohibitions are a necessary step in slowing the decline of the Southern DPS, but they are by no means the solution. The elimination of selenium discharges to the San Francisco Bay and Delta may be the only way to ensure the population’s long-term survival.

(Photo credit: David Gotschall)