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Site Profile:
The Montebello

Credits

Each winter, hundreds of ocean-going steelhead, Oncorhynchus mykiss, return to the Monterey Bay National Marine Sanctuary to ascend local streams and spawn. A century ago, this number was almost certainly in the tens of thousands, but it has undergone a long decline. This is presumably due to the myriad effects of the growing human population on stream habitat; regional climate change; and especially the construction of impassable dams, culverts and other obstructions that block their freshwater migration routes.

The steelhead were listed as threatened (under the Endangered Species Act) in the 1990s, after the California drought put the situation in stark relief: from 1988 to 1992, only 16 adults were observed ascending the fish ladder at San Clemente Dam on the Carmel River. At that time, we knew very little about steelhead abundance in the other two inland systems -- the Salinas and Pajaro -- but we knew that the human impacts in those systems were at least on par with the Carmel River. We also knew that some modest runs occurred in various coastal basins between San Francisco and Cambria.

Since then, we have learned more about the steelhead -- findings both alarming and comforting. Much of the current situation derives from steelhead's dependence on accessible stream reaches with cool, reliable base flows during the summer in which their offspring can successfully 'oversummer' before migrating out to the ocean. Rainbow trout also play a key role, as do estuaries (see text below).

The alarming thing is the climate. Tree-ring data indicate that the climate has become warmer and wetter since the 19th century, when the Little Ice Age ended. Oxygen isotopes in shells at archaeological sites reveal a corresponding rise in sea-surface temperature, by about 2-3û C since 1700. The future looks to be warmer still, and possibly drier, according to forecasts by Mark Snyder and others at the University of California Santa Cruz (UCSC).

To a first approximation, stream temperature tracks air temperature; and summer base flow is a function of annual precipitation and watershed size. Matthew Goslin of UCSC, Fred Watson of California State University Monterey Bay and I used these relations to prepare maps of potential steelhead habitat, based on the climate of the past 40 years and the geomorphology of coastal stream networks. We used known occurrences of juvenile fish to estimate the species' tolerance limits along each environmental variable (known as a Bioclim or envelope model). The results advance the idea that oversummering habitat is largely confined to four areas: the immediate coast, the Santa Cruz Mountains, the Carmel River and headwater streams of the east side of the Santa Lucia Mountains.

As of 2002, the species was still widespread in coastal creeks from San Francisco to Cambria (and beyond), according to surveys we conducted in that year. (See Figure 1.) In Big Sur we found the species in all the coastal basins in which it had been recorded historically, even the tiniest systems such as Partington and Plaskett Creeks. This fact hints at the idea that small populations, usually thought to be extinction-prone, may be unusually resilient in Big Sur. Meanwhile, steelhead numbers have rebounded in the Carmel River, believed to be partly a result of changes in water and fisheries management and partly a result of the Pacific Decadal Oscillation, which has apparently improved ocean survival of salmonids throughout the West Coast. Clearly, the species is quite resilient under the right conditions. Nevertheless, nowhere
is there evidence for the steelhead numbers of a century ago.

Figure 1. Occurrence (by county) of anadromous Oncorhynchus mykiss as of 2002, in coastal basins (sub-basins for the Pajaro and Salinas systems) in which the species had been recorded historically. 'Barrier exclusions' refer to systems in which impassable dams or other human-made barriers block access to spawning or rearing habitat. Many of these basins have extant non-anadromous populations of O. mykiss above the barrier.

Rainbow trout, which stay in fresh water their entire lives, have steelhead as progeny and vice versa. We suspect that environmental cues may influence which of the two strategies a juvenile fish adopts -- a hypothesis currently being tested experimentally by Sue Sogard of the National Oceanic and Atmospheric Administration (NOAA), Rob Titus of California Department of Fish and Game and Marc Mangel of UCSC. The rainbow trout 'option' clearly confers resilience on steelhead populations, allowing, for example, the species to persist above impassable dams such as those on the San Antonio and Nacimiento Rivers near Camp Roberts. Genetic studies we conducted in collaboration with Anthony Clemento and Eric Anderson of NOAA and Derek Girman of Sonoma State University indicate that the fish above these dams are not descendents of hatchery fish but are as closely related to existing steelhead populations as the latter are to each other.

Coastal estuaries also appear to confer resilience. Some years ago, Jerry Smith of San Jose State University showed that oversummering juveniles grew very fast in certain lagoon estuaries. In a recent study of Scott Creek steelhead, Sean Hayes of NOAA confirmed this result and suggested that it confers improved ocean survival. By analyzing scales, Hayes found that 'early fast growers' were disproportionately over-represented in the adult steelhead returning to Scott Creek during his four-year study.

These results suggest that the species has the capacity to respond rapidly and positively to the appropriate recovery actions, such as improvements in lagoon condition and restoration of migration corridors. Still, the climate trends are quite worrisome, because they are so overarching. Further south, geologists Lee Harrison and Ed Keller of the University of California Santa Barbara have begun to find that juvenile steelhead are often limited to stream reaches where geologic faulting forces cool, reliable underground base flows to the surface. These are stream reaches that defy climate, so to speak, and we do not yet know if they are widespread or common.

David Boughton
NOAA Fisheries, Southwest Fisheries Science Center