Exotic Species on the Pacific Coast of North America

Exotic Species on the Pacific Coast of North America
The earliest record of an exotic marine species on the Pacific Coast is an
Atlantic Ocean barnacle that was collected in San Francisco Bay in 1853. Recent
studies have documented hundreds of exotic species established within the reach of
the tides, including 64 exotic species in Puget Sound, 57 in Willapa Bay, 61 in Coos
Bay, 66 in Humboldt Bay, 65 in Elkhorn Slough and 106 in southern California bays.
However, San Francisco Bay remains the most invaded site on the coast, with more
than 175 exotic species established in its salt and brackish tidal waters, and over 75
additional species in the tidal freshwaters of the Sacramento-San Joaquin Delta.
Perhaps another 100-200 species are "cryptogenic" in the bay, meaning that we do
not know whether they are native or exotic.
More impressive than their numbers is how these species dominate many
important habitats. In the San Francisco Bay estuary, exotic organisms dominate
the muddy bottom of the bay, the fouling community, the salt marshes in the
southern part of the bay, the brackish and freshwater zooplankton, and the
freshwater fish in the southern part of the Delta. In these habitats, exotic species
account for over half to nearly all of the species, individuals, and biomass.
They have come from many parts of the globe: gobies from Asia, freshwater
fish primarily from the eastern United States, cordgrasses from the eastern United
States and South America, clams and mussels from Asian, Atlantic and
Mediterranean waters, snails from the North Atlantic, crabs from Europe, the
eastern United States and China, isopods from Australia and New Zealand, and
hydrozoan jellyfish from the Black Sea. These introductions have dramatically
reduced native populations, altered habitat structure and energy flows, and caused
direct economic damage amounting to billions of dollars.
Furthermore, new species are arriving in the San Francisco Bay estuary at an
exponentially increasing rate, from an average of one new species established
every 55 weeks between 1851 and 1960, to one every 14 weeks from 1961 to
1995. In the decade from 1986 to 1995, 43 new exotic species were discovered in
the estuary, with at least 33 of these becoming established.
Several mechanisms have been responsible for transporting exotic species to
Pacific Coast waters. Seaweeds, sponges, barnacles, clams, worms and other
organisms have traveled as "hull fouling," attached or clinging to the hulls of ships.
boats or barges. A few species—including shipworms (a type of highly-modified
clam) and gribbles (a group of isopods)—can bore their way into wood, and traveled
widely in the days of wooden ships. Other organisms have been carried with ballast,
which a ship takes on to adjust its trim or to sink the hull to a proper level in the
water when there is little or no cargo aboard, and then discharges over the side
when it is no longer needed. Until the early twentieth century, ships typically used
solid ballast—rocks, sand or mud—and various shoreline and intertidal plants and
animals traveled along with it. Moving rocks in and out of ships is time-consuming
and expensive, so modern cargo ships instead use large volumes of water as
ballast, which carries huge numbers of phytoplankton and zooplankton (small
drifting organisms) around the world.
Much aquaculture, in both marine and fresh water, is based on exotic species.
Some of these species are planted and grown out in the environment, while others
may escape from the facilities in which they are held, sometimes carrying exotic
parasites or diseases. While ballast water moves a much greater number of species,
aquaculture is probably a far more effective mechanism for introducing exotic
parasites, diseases and other pests of fish and shellfish. For example, Pacific Coast
oyster growers began importing and culturing Virginia oysters (Crassostrea
virginica) from the Atlantic Coast in 1869, and Pacific oysters(Crassostrea
gigas) from Japan in 1902, which resulted in many Atlantic and Japanese organisms
(including several oyster pests) becoming established on the coast. More recent
types of marine aquaculture (such as salmon and abalone farming) have also
released exotic species into Pacific waters.
Other vectors include the international and cross-continental transport and
sale of aquarium plants and pets, live marine bait, live seafood, and live organisms
intended for use in research and teaching. It is only in recent decades that there
has been any general recognition of the impact of exotic species on marine waters,
and the regulations and institutional arrangements needed to manage this problem
have not yet been developed. Until the "biological pollution" of exotic species is
regulated as rigorously as chemical pollution is, we can expect that large numbers
of novel exotic species will continue to arrive on our shores.