Environmental Conditions

Culverts such as this one, partially visible behind the branches, can stop or impede fish passage.

Image courtesy of the San Mateo County Department of Public Works.

The reproductive success of anadromous salmonids depends upon freshwater habitat conditions. Each freshwater life stage – returning adults, embryos and alevins within redds, juveniles, and smolts – encounters a series of environmental factors that provide suitable, less-than-desirable, or unusable habitat. Essential habitat requirements are known as "limiting factors." Limiting factors are a group of related environmental factors that, if at sub-optimal levels, will prevent an organism from reaching its full biotic potential (Lincoln et al. 1998) (see the Factors Limiting Salmonid Production section for more information).

In San Mateo and Santa Cruz Counties, salmonid decline can be attributed to numerous factors. Stocking of both coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss) in the early 1900s potentially introduced disease as well as genetics poorly adapted for Central Coast conditions. Intensive logging in the 1900s caused sedimentation and removed shade from streams, and the construction of Highway 1 with its associated culverts changed passage conditions (Alley 2003). Water diversions and the increase of urban and suburban development caused changes in flow and watershed hydrology. Together with natural occurrences of floods and droughts, these activities severely impacted coho and steelhead populations in San Mateo and Santa Cruz Counties.

Degradation of environmental conditions such as those described above may not result in the immediate eradication of the population, but may substantially reduce the number of salmonids that a particular stream or stream reach can sustain. Although salmonids of all age classes might still be observed in the stream, the population size may be reduced to an unsustainable number. This reduced production can result from a single limiting factor or the gradual degradation of multiple habitat elements that combine to create an uninhabitable or minimally productive stream. Because conditions can decline slowly and may be the result of multiple factors, managers are challenged to identify the habitat improvements necessary to increase salmonid production.

References

Alley, D. 2003. Memorandum to NOAA. December 2003.

Lincoln, R., G. Boxshall, and P. Clark. 1998. A Dictionary of Ecology, Evolution, and Systematics. Second Edition. Cambridge, UK: Cambridge University Press.