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Managing for Resilience:
Case Study:
Salmon River watershed and estuary provide a case study of the relationship between natural history and anthropogenic development and the resilience of estuarine habitats and salmon populations in the basin. Conditions in the estuary reset following a 9.0 magnitude earthquake on January 26, 1700 when the estuary subsided approximately 0.5 m. A layer of sand surged into the estuary and covered the wetland vegetation and seasonal encampments of Native Americans. The Native Americans have a long history in the basin. They constructed weirs, fish baskets, gill nets, gaffs, and scaffolds to trap, net, and spear fish. In the 1800s and 1900s, Euro Americans moved into the basin, starting a long history of resource extraction – logging, grazing, and fishing. Significant changes occurred in the upper basin because of logging practices including splash dams. Although cattle were grazed in the valley since 1851, when settlers annually drove cattle from the Willamette Valley for summer pasture, dikes were not built in the estuary until the early 1960s following the availability for government grants and subsidies to increase agricultural land. The Salmon River Hatchery was authorized by the Oregon Legislature, constructed in 1975, and the first Chinook salmon were released in 1977. Highway 101 was routed through the upper estuary, blocking the normal outlet of Salmon Creek into a large wetland complex. By 1975, the historic complexity of the watershed and estuary were greatly reduced, although the watershed still supported native runs of Chinook, coho, and chum salmon, and steelhead and cutthroat trout.
Protection of segments of the watershed and estuary was initiated in 1935 with the purchase of a corridor of land along upper Salmon River, the H.B. Van Duzer State Park. The park includes 20 kilometers of streams, with some second growth forest as well as 200+ year-old trees. In 1966, volunteers organized an effort to purchase Cascade Head and turn it over to the Nature Conservancy. Congress, in 1974, also authorized the establishment of The Cascade Head Scenic Research Area in the lower basin to be managed by the USFS, including the entire estuary. The Cascade Head and surrounding area were recognized as a National Scenic Research Area and a United Nations Biosphere Reserve. As part of the USFS management plan, estuary dikes were removed and tidal flow re-established in much of the estuarine wetlands in 1976, 1987, and 1996. Land use rules were also enacted that protected riparian zones along streams, tidal areas, and wetlands.
The sequential restoration of tidal wetlands in the Salmon River estuary provided the opportunity to examine the response of salmon populations to increased habitat in the estuary. We examined juvenile life history diversity of fall-spawning Chinook salmon, Oncorhynchus tshawytscha, for evidence of change in estuarine residency and migration patterns following the removal of dikes from 145 ha of former salt-marsh habitat in the Salmon River estuary ( Oregon). The absence of fry migrants in the estuary during spring and early summer in 1975-77—a period that precedes restoration of any of the diked marshes—and the extensive use of marsh habitats by fry and fingerlings April–July, 2000-02 indicate that wetland restoration has increased estuarine rearing opportunities for juvenile Chinook salmon. Persistent changes in spawner distribution since 1975-77, including the concentration of hatchery strays in the lower river, and the restoration of upper estuary wetlands, may account for the large proportion of fry that now disperse into the estuary soon after emergence in the spring. Although few of these earliest migrants appear to survive to the river mouth, many fry and fingerlings from mid- and upper-basin spawning areas distribute throughout a greater portion of the estuary during the spring and summer and migrate to the ocean over a broader range of sizes and time periods than thirty years ago. The proportions of juvenile life history types represented among subyearling outmigrants and those among adults returning to spawn were similar. Both juvenile outmigrants and returning adults were dominated by individuals that entered the estuary during the summer at sizes from 60-95 mm. The findings support the linkage between time and location of spawning adults and juvenile life histories. The results suggest that wetland recovery has expanded life history variation in the Salmon River population by allowing greater expression of estuarine-resident behaviors.
By some ecological indicators, the expansion of estuarine rearing habitat has increased resilience of the ecosystem. The estuarine habitat responded with an immediate return of tidal energy and rapid regeneration of emergent vegetation formerly removed from the ecosystem. Insect production expanded, and invertebrate and fish use of the marsh channels quickly returned. Juvenile Chinook salmon responded by making use the estuary in greater abundance over space and time. Growth rates in the newly restored marsh were similar to that in the reference marsh, and estuarine residence time increased compared with the pre-diking period. Juvenile fish that reared in the estuary contributed disproportionately to the returning adult population, although all life history types contributed to adult returns. Resilience of the population to interannual variations in river flow was apparent by the fish population responses to both drought and historic high flows. The contributions of different segments of the adult population (spawning time and location in the watershed) and diverse life history strategies of the juvenile Chinook salmon may increase the resilience of the chinook salmon population to disturbance and natural climate cycles.
Restoration of wetland habitat in the estuary has increased the life history diversity of Chinook salmon in the Salmon River watershed. Theses benefits are tempered by the fact that the Chinook salmon population is dominated by the escapement of hatchery fish to the spawning grounds, most of the watershed is managed as commercial forest, and Highway 101 affects the hydrology of Salmon Creek and the upper estuary wetland. Although the majority of hatchery fish spawn in the lower river, and contribute little in the way of adult returns, they do nevertheless interbreed with naturally spawned fish in the upper river and tributaries. Hatchery practices attempt to minimize the impact on wild populations, but as the hatchery is managed as a production facility, the number and timing of hatchery fish is high and punctuated relative to the wild salmon population. Since we have no data to compare population responses before hatchery influence AND before wetland diking, we cannot distinguish whether the present hatchery influence may affect the breadth of life history expression by naturally spawning Chinook salmon in the basin. Yet wild fish are under increased risk because of competition, disease, and harvest associated with a large-scale hatchery program. Hatchery salmon benefit the angling public and commercial fishing guides, but sometimes create social conflicts with landowners or among anglers.
The commitment of the USFS to restore estuarine wetlands and protect forest lands bounding the estuary, and the resources and willingness of local individuals and The Nature Conservancy to advocate for and manage aquatic and terrestrial resources in the watershed has biological and social implications for long-term resilience.. Long-term protection of the riparian forest in the H.B. Van Duzer State Park also provides resilience to flood and drought, and no doubt improves the survival of juvenile fish as they rear and migrate in the upper river. Individuals and non-profit groups (YMCA and Sitka Center for Ecology and Art) enjoy the aesthetic and economic benefits of the natural area, taking advantage of opportunities to learn, hike, bird-watch, beachcomb, and sea-kayak.
The overall improvement in resilience of the habitat and populations in the estuary may not be fully realized because of competing interests in the watershed. The dichotomy of management philosophies concerning wild and hatchery fish, and upriver versus estuarine habitat highlights the challenges of managing an entire salmon ecosystem (fish and habitat) for resilience.
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