II. Anadromous Salmonids of the MBNMS
On the central coast of California, coho salmon larvae emerge from their gravel nests in rivers and streams from January to May, depending on location and the local climatic conditions. Within the MBNMS, coho spawning currently occurs in Marin and Santa Cruz counties. Coho tend to spend six months to one year in their natal streams and estuaries along the coast, feeding on aquatic and terrestrial invertebrates in the water column (Nielsen 1992). During the late winter and early spring of the following year, juvenile coho salmon are transformed into ocean-going fish through a complete physiological transformation called smoltification. This life history shift involves extensive physiological, behavioral, morphological, and biochemical changes in preparation for marine life (McCormick and Saunders 1987). After entering the marine environment, coho remain in near-shore habitats close to their natal stream for some time, before traveling hundreds of miles along the coast in search of food. During their ocean sojourn, coho are primarily piscivorous, foraging on small fish and marine invertebrates. Coho from central California streams typically spend two years offshore before they reach sexual maturation and migrate back to their natal rivers to produce the next generation of salmon (Shapovalov and Taft 1954).
In the open ocean, Pacific salmon use several navigation systems to return to the exact site of their origins, including sensitivity to polarized light, temperature and salinity gradients. The mineral and organic characteristics of a particular stream are monitored by salmon using a highly developed olfactory sense (smell discrimination has been measured in parts per trillion; Smith 1985). Specific homing to a natal spawning site is believed to be under genetic as well as environmental influences (Hara 1993). The classic study of Waddell Creek by Shapovalov and Taft in 1954 documented an average spawning run of 247 coho per year for the decade from 1930-1940. Current runs in this creek have declined to less than 10 fish per year (Hope 1993). Wild coho in the San Lorenzo River once numbered over 2,000 fish. San Lorenzo's run of wild salmon is probably gone, having been replaced by a small population of hatchery-raised fish (Schmidt 1994).
The recent petition to list coho salmon throughout its range in Washington, Oregon, Idaho, and California as endangered under the federal Endangered Species Act (National Marine Fisheries Service 1994), considered evolutionarily significant units (ESUs) of coho as listable "species" under the act (see Waples 1991 for definition of an ESU). In their review of population viability and genetic integrity, the NMFS considered wild coho populations from Punta Gorda south to the San Lorenzo River a separate ESU that was in danger of extinction. Genetic data indicated that most samples from this region differ substantially from coho north of Punta Gorda (Bartley et al. 1992; Nielsen 1994). The timing of spawning runs were very late (peaking in January), timed to coincide with single, brief flow events. The strong and consistent coastal upwelling in this region results in a relatively productive near shore marine environment, affecting the ocean migration distance within this ESU. Federal status review of coastal coho will be determined by the National Marine Fisheries Service some time during 1996-1997 (see Special Status Table).
Historic references of chinook salmon spawning runs indicate they may have extended south along the California coast to the Ventura River (Moyle 1976). Today, wild runs of chinook remain only in the Sacramento River, with hatchery supplemented runs and net rearing programs extending into southern California. However, the ocean habitats along the entire MBNMS are used extensively during the chinook's 2-4 years at sea along the coast of California (see Special Status Table). Chinook life history is highly variable, with historic spawning runs swimming upriver from October to June. Most juvenile chinook along the California coast migrate out to sea within the first year of their life (i.e. "ocean-type" chinook) and spend three years in the open ocean. Some chinook, however, spend more than one year in freshwater bays or estuaries before moving into the ocean environment (Healey 1991). This "stream-type" chinook is uncommon south of 56°N, but recent genetic and life history associations found in small coastal bays and tributaries in central California suggest it may still play a minor role in the MBNMS stocks (Nielsen et al. 1995).
Dams which block most rivers in California have contributed significantly to the decline in abundance and distribution of the various life histories of California chinook. Genetic data, however, indicates that reproductive isolation among the temporal spawning runs in the Sacramento River chinook remains intact, despite a significant drop the number of returning spawners (Nielsen et al. 1994c; Hedrick et al. 1994). The MBNMS plays a significant role in the life history of chinook along the California coast, providing near shore habitat and forage opportunities contributing to the growth and successful survival of this anadromous species.
Chinook remain the most economically important species of Pacific salmon in California because of the valuable sport and commercial fisheries they support. The recent declines in salmon abundance has been critical in California's chinook stocks. With historic runs at an all time low, one subspecies, the Sacramento River winter-run chinook, is currently listed at endangered under the U.S. Endangered Species Act. A status review by the NMFS is underway for all chinook temporal spawning runs (winter-run, spring-run, fall-run and late fall-run) that utilize the MBNMS waters.
The range of anadromous steelhead in California extends from our northern border with Oregon south to Malibu Creek, and includes many coastal streams and rivers occurring within all five counties that border the MBNMS (Titus et al. in press; and see Special Status Table). The recent dramatic decline in population numbers for coho and chinook has also impacted our steelhead populations. The anadromous runs of steelhead that spawn in streams along the southern coast of California (Pajaro River south) have declined to less than 1% of their estimated pre-1940 abundance. Recent genetic studies have shown that southern steelhead utilizing near shore marine habitats represent a unique population subunit and evolutionarily significant unit (ESU) within this species (Nielsen et al. 1994 a & b). According to the NMFS status review for steelhead, the southern steelhead's genetic diversity, including those using the MBNMS, is unprecedented throughout the rest of the species range (National Marine Fisheries Service 1995).
The ability of the southern steelhead to exist and to sustain such unique genetic diversity, is probably contingent on special adaptations to the extreme environmental conditions found in this area (Nielsen et al. 1994a). Juvenile steelhead throughout California must deal with inhospitable warm water temperatures and dry stream reaches due to frequent drought conditions (Nielsen et al. 1994d). The periodic closure of the river mouth by sand bars in California coastal streams often prevents migration to and from the ocean environment (Goodwin et al. 1993), forcing steelhead to adopt extremely flexible life history schedules for smoltification and spawning.
A recent study of steelhead by the California Department of Fish and Game suggests that "the restoration of declining anadromous fish populations ....is intimately tied to the establishment of a new ethic for management of California's rivers and streams, an ethic that places a much higher priority on the continuance of essential physical, biological, and ecological processes" (McEwan and Jackson 1994). I would like to suggest that the near shore ocean conditions play an equally important role in the restoration and survival of these important anadromous species, and that the MBNMS is critical to the preservation of the unique genetic and phenotypic diversity they represent.
Section I. Overview of Anadromous Fishes of the
Monterey Bay National Marine Sanctuary