The Mysterious Death Of Thousands Of Fish In the San Francisco Bay

By Joanne Connery
Published: July 2006

A swimming sturgeon and leopard shark. Photo credit: Larry Lynch.

A number of possible scenarios are being examined.  The death of such a large number of fish could be the result, among other things, of chemical spills, oil spills or sewage leaks.  Natural causes are also being considered as possible culprits.  Bacteria require oxygen to breakdown organic matter and can significantly reduce the amount of dissolved oxygen available in the water.  Large-scale blooms of algae compete with fish for available oxygen, killing off large numbers of fish.  A large influx of freshwater entering the estuarine system from the melting Sierra snow pack from this winter’s heavy storms may have altered water temperature conditions enough to cause a significant number of deaths, especially among the leopard sharks.  Learning more about each of these animals, and how they depend on the estuarine ecosystem of the San Francisco Bay might provide clues to the cause of death and possibly help avoid losses like this in the future.

   The striped bass, Morone saxatilis, is a ray-finned fish belonging to the family Moronidae.  Even though it is a familiar fish to most Bay Area fishermen, the striped bass is not actually a native to the Bay estuarine ecosystem.  According to the California Department of Fish and Game, this fish was first introduced to the San Francisco Bay in the late 1800’s.  The striped bass is an anadromous fish, meaning it spends most of its adult life in saltier water but migrates to freshwater to breed.  Younger bass feed on crustacean plankton and invertebrates and older bass feed on appropriately sized fish, such as anchovies and herring. 

Sturgeon first appeared in the Jurassic Period of the Mesozoic Era.  They have changed little in form since they first appeared over 175 million years ago and so, retain the appearance of very primitive fishes.  Two members of the sturgeon family, Acipenseridae, can be found in the Bay ecosystem, the green sturgeon, Acipenser medirostris, and the white sturgeon, Acipenser transmontanus.  Both species are mainly bottom dwellers, spending much of their time in bays, estuaries and near shore brackish waters.  Sturgeons employ their flattened snouts and excellent sense of smell and taste to find and feed upon benthic organisms such as crustaceans, worms and mollusks.

The leopard shark, Triakis semifasciata, has a slender graceful body with two large spineless dorsal fins and one anal fin.  This shark is a bottom dweller, feeding mainly upon other benthic organisms such as clams, crabs, shrimp and small fish.  The leopard shark has good camouflage and small pointed teeth which it uses to capture its prey. 

   These fish and shark are prominent species in our bay’s ecosystem.  They feed on or depend on the benthic organisms in our bay and their deaths in large numbers raise concern about our bay’s health.  While much more than that, many people believe fish are indicator species of how an ecosystem is doing.  If you would like to see how the San Francisco Bay scored on The Bay Institute’s ecological scorecard in 2005, go to:  http://www.bay.org/ecological_scorecard.htm.  The more we know about the bay in our backyard, the better stewards we will be.  To read the July 7 Seattle Post-Intelligencer article by Associated Press writer, Marcus Wolsen, go to:  http://seattlepi.nwsource.com/local/6600AP_WST_Dead_Fish.html.

The San Francisco Chronicle published an article on July 15 connecting fish die-offs in the east bay with chloramine.  FMSA is trying to determine whether this chemical is connected to the die-offs:

http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2006/07/15/BAGAVJVLHG1.DTL&hw=chloramine&sn=001&sc=1000