|
..:.
<br />~~ - -
<br />ECOLOGY 25
<br />le. Both populations and
<br />e range of environmental
<br />'rsh. Each species has its
<br />its habitat (or better, its
<br />ality members of a species
<br />limits, in habitats that
<br />duction (realized niche).
<br />s from one stage of its life
<br />knge occur in anadromous
<br />young in streams but as
<br />r, if more subtle, habitat
<br />understand the ecological
<br />age of its life history have
<br />sh species are many, but
<br />iient and flow, dissolved
<br />re, barriers to movement,
<br />populations, and aquatic
<br />ctors .as pollution, dams,
<br />gated systems, and the
<br />e also become important
<br />tors, such as temperature
<br />t~ution than others, they
<br />ct. Since different species
<br />different ways to a given
<br />it segregation on a broad
<br />and niche segregation on
<br />ties with similar environ-
<br />and interact to subdivide
<br />acies that occur together
<br />distinct zones, each zone
<br />'. The idea of zonation of
<br />attempted to classify the
<br />Hess (1) the Trout Zone,
<br />streams; (2) the Grayling
<br />Zone, dominated by grayling and trout, but with a few fast-water minnows present as
<br />well, in cool, fairly fast, midelevation streams; (3} the Barbel Zone, dominated by
<br />stream cyprinids with a few fishes from the -zones above and below, in slow,
<br />wazmwater stretches of stream close to the valley floor; and (4) the Bream Zone,
<br />dominated by deep~odied cyprinids and predatory fishes, such as pike and perch, in
<br />the warm, sluggish waters of the valley floor. The concept of zonation is useful mostly
<br />as a broad descriptive tool, since the exact composition of the fish fauna tends to vary
<br />from stream to stream as a result of minor environmental differences and accidents of
<br />distribution. Also, sharp boundaries between the zones are rare. The fish fauna changes
<br />gradually as the nature of each stream shifts from swift headwaters to slow river. The
<br />faunal change is generally less a replacement of one species by another than it is an
<br />addition of species as the habitat becomes more complex. Far example, in Europe
<br />trout are found not only in the Trout Zone but also in the Grayling and Barbel zones.
<br />Despite these limitations, zonation remains a handy way to describe the fish fauna of
<br />a region and its relationships to the different types of stream and lake habitats. In fact,
<br />fish zones can be described quantiatively, as has been shown for the fishes of the
<br />foothills above the San Joaquin Valley (Moyle and Nichols, 1973). There, 80 percent
<br />of the stream localities sampled had a fish fauna that could be assigned by computer to
<br />one of four fish associations (= zones). The following sections on the fish ecology of
<br />each -major California drainage system will, therefore, attempt to describe the fish
<br />zones both as they were before civilization and as they are today. They will also,
<br />where possible, describe how the species that occur together in these zones subdivide
<br />the habitat and its resources.
<br />SACRAMENTO-SAN JOAQUIN SYSTEM
<br />Despite the enormous size of this system, fish zones are surprisingly similar in its
<br />various subsystems. The differences in fish fauna between subsystems indicate the
<br />presence of zoogeographical barriers. Hardhead, for example, apparently were never
<br />able to invade the Pajaro-Salinas subsystem nor were Sacramento blackfish able to
<br />invade the Russian River subsystem. Both systems seem to have suitable habitats for
<br />the missing species. Similarly, the slow-water fishes of the Central Valley (splittail,
<br />blackfish, Sacramento perch, tule perch) have been unable to get over the falls on the
<br />lower Pit River, although suitable habitats abound far them in its upper reaches.
<br />Nevertheless, throughout the Sacramento-San Joaquin system five fish zones can be
<br />recognized: (1) Rainbow Trout Zone, (2) California Roach Zone, (3) Squawfish-
<br />sucker-hardhead Zone, (4) Deep-bodied Fishes Zone, and (5) Estuarine Fishes Zone.
<br />Rainbow Trout Zone. This zone is found in -clear headwater streams where the
<br />stream gradient is high (usually a total drop of 3 m or more for every km of stream).
<br />The water is swift and permanent with more riffles than pools. The water is also cold,
<br />seldom exceeding 21 °C, and is saturated with oxygen. The bottom material is
<br />predominately cobbles, boulders, and bedrock. The banks are well shaded and
<br />frequently undercut. Aquatic plants, submerged or emergent, are few except where the
<br />streams flow through boggy alpine meadows. The dominant native fish is rainbow
<br />trout but sculpin (usually riffle or Pit sculpin) and speckled dace are likely to be found
<br />
|