Trophic pathways supporting juvenile Chinook and coho salmon in the glacial Susitna River, Alaska: patterns of freshwater, marine, and terrestrial food resource use across a seasonally dynamic habitat mosaic

Contributions of terrestrial-, freshwater-, and marine-derived prey resources to stream fishes vary over time and space, altering the energy pathways that regulate production. In this study, we determined large-scale use of these resources by juvenile Chinook and coho salmon (Oncorhynchus tshawytscha and Oncorhynchus kisutch, respectively) in the glacial Susitna River, Alaska. We resolved spatial and temporal trophic patterns among multiple macrohabitat types along a 97 km segment of the river corridor via stable isotope and stomach content analyses. Juvenile salmon were supported primarily by freshwater-derived resources and secondarily by marine and terrestrial sources. The relative contribution of marine-derived prey to rearing salmon was greatest in the fall within off-channel macrohabitats, whereas the contributions of terrestrial invertebrate prey were generally greatest during midsummer, across all macrohabitats. No longitudinal (upstream–downstream) diet pattern was discernable. These results highlight large-scale spatial and seasonal patterns of energy flow and the dynamic interplay of pulsed marine and terrestrial prey subsidies to juvenile Chinook and coho salmon in a large, complex, and relatively pristine glacial river.

A once in 10 year drought alters the magnitude and quality of a floodplain prey subsidy to coastal river fishes

Disturbances that alter cross-habitat food web linkages can lead to whole-scale changes to aquatic systems. In coastal rivers of the Everglades (Florida, U.S.A.), increases in rainfall inundate adjacent floodplains, providing habitat for floodplain fish and macroinvertebrate species. In the dry season, rainfall decreases and floodplains dry, forcing floodplain prey into these river systems. These prey provide a prey subsidy for an estuarine predator, the common snook (Centropomus undecimalis). In 2011, severe drought impacted the region, likely affecting this prey subsidy. In this study, we ask (i) did the 2011 drought affect the magnitude and composition of floodplain prey subsidies to the common snook? and (ii) if species composition changed, were there energetic differences between the pre- and post-disturbance prey species? Results showed that 1 year after the drought, subsidies to the common snook decreased by 75%. On top of that decrease in overall flux, diet composition of the common snook switched from floodplain fishes to drought-tolerant floodplain macroinvertebrates. Lastly, energetic analyses showed that these postdrought macroinvertebrate prey subsidies had 43% less calories than floodplain fishes. Our findings illustrate the importance of considering not only the biomass that transfers from one food web to the next, but also how the species composition of the subsidy may affect incorporation into recipient food webs.

Invasive European bird cherry (Prunus padus) reduces terrestrial prey subsidies to urban Alaskan salmon streams

The spread of invasive species in riparian forests has the potential to affect both terrestrial and aquatic organisms linked through cross-ecosystem resource subsidies. However, this potential had not been explored in regards to terrestrial prey subsidies for stream fishes. To address this, we examined the effects of an invasive riparian tree, European bird cherry (EBC, Prunus padus), spreading along urban Alaskan salmon streams, by collecting terrestrial invertebrates present on the foliage of riparian trees, their subsidies to streams, and their consumption by juvenile coho salmon (Oncorhynchus kisutch). Riparian EBC supported four to six times less terrestrial invertebrate biomass on its foliage and contributed two to three times lower subsidies relative to native deciduous trees. This reduction in terrestrial invertebrate biomass was consistent between two watersheds over 2 years. In spite of this reduction in terrestrial prey resource input, juvenile coho salmon consumed similar levels of terrestrial invertebrates in stream reaches bordered by EBC. Although we did not see ecological effects extending to stream salmonids, reduced terrestrial prey subsidies to streams are likely to have negative consequences as EBC continues to spread.