Availability of organic matter and nutrients transported from the marine environment to streams by spawning salmon was increased in two small streams in southwestern Washington, U.S.A., by adding salmon carcasses from a nearby hatchery. Response of juvenile coho salmon (Oncorhynchus kisutch) and steelhead (Oncorhynchus mykiss) rearing at these sites was compared with nearby streams where few salmon spawned. Densities of age 0+ coho salmon and age 0+ and age 1+ steelhead increased following carcass additions to the treated streams. A similar increase in density was not observed at the reference sites. Condition factors in treated streams increased following carcass addition and remained at high levels while carcasses were present whereas no similar pattern was observed at the reference sites. Stomach contents of fish in streams to which carcasses had been added consisted primarily of salmon eggs and carcass flesh when carcasses were present in the stream. Stable isotope analysis indicated that the proportion of marine-derived nitrogen in the muscle tissue of juvenile salmonids increased as much as 39% following carcass placement. Results suggest that eggs and carcasses of adult salmon provide a very important resource during a period when other food items are often scarce.

The extent to which nutrients from Pacific salmon are transported to riparian areas may be influenced by differences in spawning behavior among species. Chum salmon Oncorhynchus keta, pink salmon O. gorbuscha, and sockeye salmon O. nerka typically spawn in dense aggregations, while species like steelhead O. mykiss and coho salmon O. kisutch spawn at lower densities. The contribution of nutrients to riparian vegetation was compared at two watersheds in western Washington, Griffin Creek (used by coho salmon) and Kennedy Creek (used by chum salmon). Salmonberry Rubus spectabilis foliage was collected at the channel edge above and below barriers to spawning salmon and at 20, 50, and 100 m upslope from the stream and analyzed for nitrogen stable isotope ratio (δ15N, an indicator of salmon‐derived nitrogen), total nitrogen (N), and phosphorus (P) content. Cover, plant density, and the species richness of shrub and understory vegetation were compared between sites with and without salmon. The δ15N values in salmonberry leaves were higher at sites with salmon than at corresponding distances from the channel at sites without salmon at Kennedy Creek but not Griffin Creek. Salmonberry foliage adjacent to salmon spawning reaches possessed significantly higher levels of total N and P in both watersheds. Nitrogen content was positively associated with δ15N values at the Kennedy Creek sites but not at the Griffin Creek sites. At Kennedy Creek, shrub species diversity and understory plant density and species diversity were higher at sites with salmon than at sites without salmon. These results suggest that areas bordering streams utilized by high‐density‐spawning species like chum salmon receive a substantial nutrient contribution from the fish and that this subsidy influences the vegetation. We did not see clear evidence for a similar nutrient contribution from coho salmon.

No abstract

Recent improvements in tagging technology allow for the examination of the migration of individual fish, the detection of previously unidentified life histories, and the detailed examination of factors affecting growth, migration, and survival. Using passive integrated transponder tags and instream readers installed near tidewater, we examined the migration, growth, and survival of 18,642 juvenile coho salmon Oncorhynchus kisutch in two small western Washington rivers from 2005 to 2009. In most years, more than 50% of the juvenile coho salmon from a given brood year migrated to sea between 1 October and 31 December (fall migrants). These fall migrants were significantly smaller at tagging than fish that migrated between 1 January and 30 June (spring migrants) but were similar in size to fish that were never detected after tagging and assumed to have died. Annual coho salmon survival estimates from tagging to out‐migration ranged from 31% to 40% for fall and spring migrants combined but from 5% to 15% for spring migrants only. The best fitting regression models indicated that survival differed by river and year and was negatively correlated with tagging location (river kilometer) and positively correlated with fish length: larger fish and those tagged lower in the watershed were more likely to survive. The number of days juvenile coho salmon spent in freshwater before migrating to sea was positively correlated with tagging location, fish length (mm), and habitat depth (m) and negatively with density (coho salmon/m2). Our results suggest that fall or early winter migration is a common life history for juvenile coho salmon that is driven in part by fish size and location in the watershed. The exclusion of fall migrants may lead to underestimates of the total number of migrants and parr‐to‐smolt survival.

Regulations governing human activities in streams and riparian zones frequently differ depending on whether or not a stream reach supports fish. Fish presence or absence is usually determined by sampling or by assuming the presence of fish if the stream exhibits certain physical characteristics. Field surveys of fish occurrence in streams are time consuming and expensive. Inference of fish presence from simple thresholds of physical attributes, such as gradient or channel width alone, is inaccurate. We attempted to improve the accuracy and efficiency of this determination by developing a geographical information systems (GIS)‐based predictive model. A 10‐m digital elevation model incorporated field data on fish distribution from 517 streams in western Washington State and GIS‐derived representations of the physical characteristics of stream networks. A model predicting the upstream extent of fish occurrence was derived using logistic regression models coupled with a heuristic “stopping rule.” Candidate variables included stream gradient, upstream basin area, elevation, and mean annual precipitation. When assessed against independent survey data, 91.9% of the occupied fish habitat was correctly classified by the model. Errors were generally small, but occasional large errors did occur and were most frequently associated with barriers to fish movement. Smaller errors occurred in marginal habitats, streams of low topographic relief, and streams that originated from headwater ponds. Use of this type of model, coupled with targeted field survey in areas most likely to be associated with model error, would greatly improve the efficiency and accuracy of current classification schemes.