By The Calanoid copepod, though paper-thin and usually about one millimeter in length, is arguably the most important species in the San Francisco Bay. These miniscule aquatic crustaceans play a pivotal role in shallow water food chains and bay ecosystems. Copepods are the largest source of protein for marine species among the oceans worldwide. They’re also the main food source for the world’s most economically important fish species. In the San Francisco Bay, the largest order of copepods is the Calanoid. These tiny zooplankton are abundant in the San Francisco estuary and are integral to larger fish communities and ultimately the health and diversity of the SF Bay ecosystem.
The San Francisco Bay is the most invaded estuary in the world. Since the beginning of the Gold Rush era, over 250 invasive species are now well established in the brackish water of the bay. It is currently estimated that a new invasive species is introduced to the San Francisco Bay every 14 weeks. Most arrive from the ballast water released from international shipping vessels, the enormous ships seen frequently arriving and departing as we cross the Dumbarton, San Mateo, or Bay Bridge. The most abundant copepods found in the Bay today were introduced to this region less than 75 years ago. This is true for the Calanoid, as well as several other orders of the copepod species. Though technically an invasive species, the Calanoid copepod has thrived due to its broad salinity distribution. Unlike most other copepods that can only survive in very specific salinities, the Calanoid can be found in freshwater, brackish waters, and saltwater. They can also occasionally move from one salinity to the next, transferring from a saltwater habitat to a freshwater one, a trait very rare in the copepod community.
A recent phenomenon has occurred that has sparked the interest of the local and national scientific communities. In the Delta, a freshwater portion of the San Francisco Bay Estuary, a steady decline of delta smelt, longfin smelt, striped bass, and threadfin shad has become commonly known as the Pelagic Organism Decline (or POD). One of several hypotheses for this kind of decline presumes that there is a significant drop in the Calanoid copepod population due to toxic algal blooms, which in turn reduces the protein abundance for the various fish species. Other speculations blame water quality, invasive species, and contaminants from local industry and agriculture for pelagic organism decline. Whether it is one or a combination of these causes that is leading to the decline in fish, one can’t help but notice that human actions are most likely to blame. If fish populations continue to decline, it could be devastating both economically and ecologically for the San Francisco Bay Area. Though tiny in stature, we must respect and give careful attention to the mighty Calanoid copepod, because our Bay rests on the backs of these tiny creatures.
