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CHANGE - 53
<br />ets, for aquaculture, and
<br />difornia were meant to
<br />this type was carp, now
<br />basses, and sunfishes,
<br />ies. This is not surprising
<br />sh native to California is
<br />introduced as reservoirs
<br />ve generally been small
<br />~panese subspecies), and
<br />y hitch, and threespine
<br />ahead minnow have also
<br />rs been mainly the result
<br />yen: introduced to help
<br />Quito control is likewise
<br />Oder, although it has also
<br />n fish, and aquaculture
<br />of and, as a result, other
<br />oharyngodon idella) are
<br />Js in ponds and canals.
<br />Imperial Valley.
<br />~ed by their proponents
<br />~g vegetable matter into
<br />hese exotics has resulted
<br />lure, when some species,
<br />ie main fishes raised for
<br />,es and trouts.
<br />:rs have released tropical
<br />the detriment of the
<br />t Valley, sailfin mollies,
<br />ter escaping from ponds
<br />eir charges and released
<br />for most wild goldfish
<br />,t plants.
<br />lions. Logperch came in
<br />~bably arrived as eggs on
<br />into the Delta from the
<br />mture of California's fish
<br />of the state's waterways.
<br />Bally responsible for the
<br />Kcies are most abundant
<br />st mostly in undisturbed
<br />ressive, predaceous green
<br />tbod regions they occur
<br />.~
<br />only as scattered large adults, while native minnows remain abundant. If a stream
<br />section is dammed, bulldozed, or otherwise changed the sunfish quickly take over and
<br />the native fishes become uncommon (Moyle and Nichols, 1974). It is interesting to
<br />note, however, that in trout streams native and introduced trout species may coexist,
<br />apparently subdividing the resources available through behavioral interactions and
<br />specializations. Nevertheless, it is possible in some situations for introduced species to
<br />eliminate native ones through duect interactions: competition, predation, habitat
<br />interference, and hybridization.
<br />Direct competition between two species for a resource (usually food or space) in
<br />limited supply, that results in one species being eliminated, is frequently invoked as a
<br />cause for faunal changes, but it is in fact very difficult to demonstrate. If a new species
<br />can survive in an undisturbed environment into which it has been introduced, it is
<br />likely that it will reach some sort of population equilibrium with the species already
<br />present, reducing the populations of the native fishes but not eliminating them
<br />(Johannes and Larkin, 196I). Thus, the introduction of golden shiners into a
<br />California trout lake usually results in decreased growth and reproduction of the trout
<br />population but the trout seldom disappear altogether. However, one native California
<br />fish species that seems to have been eliminated from its natural range due to
<br />competition from introduced species is the Sacramento perch. Its disappearance from
<br />the Sacramento~an Joaquin system was gradual (not obviously correlated with
<br />environmental changes), yet it is very successful in a wide variety of ponds, reservoirs,
<br />and lakes into which it has been introduced. The common denominator of these
<br />waters is the absence of ecologically similar but more aggressive species, particularly
<br />bluegill (Moyle, Mathews, and Bonderson, 1974).
<br />Predation by an introduced species on a native one is another mechanism commonly
<br />invoked to explain the disappearance of species. The classic example of this is the
<br />projected elimination, by sea lamprey (Pteromyzon mvTinus) predation, of lake trout
<br />and other large fishes from the upper Great Lakes_ Such situations, although
<br />spectacular, are also exceptional. Most predators will cease hunting a particular prey
<br />species long before it becomes extinct. It should be pointed out, however, that
<br />predation by green sunfish does seem to have been responsible for local extinctions of
<br />California roach, although habitat change may also have played a role. The sunfish
<br />invade the intermittent roach streams, which are ecologically similar to their native
<br />midwestern streams, and become trapped with the roach in the summer pools. Under
<br />these circumstances they can easily eliminate the roach from the streams (Moyle and
<br />Nichols, 1974).
<br />Habitat interference occurs when an introduced species changes the habitat by its
<br />activities and the change forces the native forms to leave or die out. Carp are the main
<br />villains in this category since they root up the bottom, digging up aquatic plants and
<br />greatly increasing the amount of suspended matter in the water. Fishes (including
<br />many gamefishes) -that require clear water for feeding or breeding may have their
<br />populations reduced or eliminated. In California the effect of carp is difficult to assess
<br />because they live mostly in disturbed habitats. It is possrble, however, that they may
<br />be partially responsible for the supposed reduction in the clarity of Clear Lake, Lake
<br />County and for the gradual disappearance of its native frsh fauna. Habitat alteration is
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