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The Southern California Steelhead Distinct Population Segment (DPS) occurs from the Santa Maria River to the Tijuana River at the United States and Mexican Border in seasonally accessible rivers and streams.〔 *NOAA. “2007 Federal Recovery Outline for the Distinct Population Segment of Southern California Coast Steelhead”. p. 5,9(56)〕 Steelhead (''Oncorhynchus mykiss'') in the Southern California Steelhead DPS, much like its relatives further north, requires sufficient flows in their natal streams to be able to return from oceans and lakes to spawn. Due to the extended periods of drought throughout their range, Southern California steelhead are most commonly seen during periods of increased rainfall, such as El Nino events.〔"San Mateo Creek Survey." Trout Unlimited California. Trout Unlimited, 2003. Web. 4 Feb 2011. ==Threats== There are many threats to the survival of the Southern California Steelhead DPS. Principal threats include, but are not limited to, alteration of stream flow patterns and habitat degradation, barriers to fish passages, channel alterations, water quality problems, non-native exotic fish and plants and climate change.〔 *NOAA. “2007 Federal Recovery Outline for the Distinct Population Segment of Southern California Coast Steelhead”. p. 20–28(56)〕 These threats pose a serious challenge to the persistence of Southern California Steelhead, and most threats are increasing in magnitude as human population grows in Southern California. Adequate stream flow is important to Steelhead, for both returning fish as well as young fish migrating to the sea. Lowered stream flows can impede passage for fish in both directions, as fish are unable to breach sand bars that form at the mouths of rivers and streams during the summer months.〔 *NOAA. “2007 Federal Recovery Outline for the Distinct Population Segment of Southern California Coast Steelhead”. p. 20(56)〕 Natural flows flush fine sediment and silt and provide for better spawning surfaces for the Southern California Steelhead.〔 *NOAA. “2007 Federal Recovery Outline for the Distinct Population Segment of Southern California Coast Steelhead”. p. 21(56)〕 Higher flows also bring water to riparian areas, which rejuvenate vegetation that provides shade and habitat for spawning fish. This riparian habitat will also better regulate water temperatures.〔 *NOAA. “2007 Federal Recovery Outline for the Distinct Population Segment of Southern California Coast Steelhead”. p. 20(56)〕 Anthropogenic structures such as dams also impede passage upstream.〔 *NOAA. “2007 Federal Recovery Outline for the Distinct Population Segment of Southern California Coast Steelhead”. p. 20(56)〕 Water quality is another problem that Southern California Steelhead face. Those fish that still have natal streams to return to often face degraded water quality. Steelhead eggs require clean flowing water to hatch. Sedimentation of streams from agricultural and urban runoff has the potential to smother eggs in a stream and greatly decrease the number of Steelhead surviving to return to the ocean. Urban runoff can funnel contaminants directly into streams. Runoff containing industrial or urban contaminants can directly cause fish mortality and decreased reproduction.〔 *NOAA. “2007 Federal Recovery Outline for the Distinct Population Segment of Southern California Coast Steelhead”. p. 22(56)〕 Point and non-point pollution sources also have similar effects. Discharges can also cause thermal stress on the fish, or can increase favorable habitat for non-native exotic species, such as Largemouth bass, carp and catfish, all of which will prey on juvenile Steelhead and eggs.〔 *NOAA. “2007 Federal Recovery Outline for the Distinct Population Segment of Southern California Coast Steelhead”. p. 22(56)〕 O. mykiss is used as an indicator species for water quality problems;〔“Whole Effluent Toxicity." United States Environmental Protection Agency. Web. 11 Feb 2011. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Southern California Steelhead DPS」の詳細全文を読む スポンサード リンク
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