Using strontium isotopes to determine critical spawning habitat for chinook and sockeye salmon

Production patterns of highly mobile species, such as anadromous salmon, often exhibit high spatial and temporal heterogeneity across landscapes. Such variability often varies over time among habitats, which stabilizes production at aggregate scales of complexity. Reconstructing production patterns explicitly in space and time across multiple scales, however, remains difficult, but is important for prioritizing habitat conservation. 

Former postdoctoral researcher, Sean Brennan, spent much of his scientific career focused on developing tools and methods that would have broad application in reconstructing salmonid habitat production patterns.

Brennan and his colleagues first developed a new mapping approach that would allow researchers to overlay patterns in fish otolith Strontium isotopes with matching isotope ratios present in the geographic landscape.  Next they constructed a model to demonstrate how isotope data from otoliths can be used to infer habitat use by juvenile Pacific salmon during their freshwater residency.  They then extend the application of that model over space and time, in combination with the mapping tool for visualization, to provide insight on the range of habitat critical for sockeye and Chinook salmon in the Nushagak basin.  Results indicate that the relative productivity of locations across the river network, as both natal- and juvenile-rearing habitat, varies widely among years and that this variability is expressed across a broad range of spatial scales.  In other words, different locations in the drainage are important rearing habitat in different years.  The variability over space and time ultimately stabilizes the interannual production of fish at the scale of the entire basin. (see also Schindler et al. Portfolio effects)  Researchers in Alaska Salmon Program are currently working on applying these new methods and tools to offer insight into potentially productive chinook spawning are rearing locations in the Yukon River watershed.

Brennan, S.R., D.E. Schindler, T.J. Cline, T.E. Walsworth, G. Buck, and D.P. Fernandez.  2019.  Shifting habitat mosaics and fish production across river basins.  Science.  364(6442): 783-786.  DOI: 10.1126/science.aav4313

Brennan, S.R., T.J. Cline, and D.E. Schindler. 2019. Quantifying habitat use of migratory fish across riverscapes using space-time isotope models. Methods in Ecology & Evolution 10(7):1036-1047.  https://doi.org/10.1111/2041-210X.13191

Brennan, S.R., and D.E. Schindler.  2017.  Linking otolith microchemistry and dendritic isoscapes to map heterogeneous production of fish across river basins. Ecological Applications. 27:363-377.


Patterns in riparian soil marine-derived nitrogen (MDN) in response to a long-term salmon carcass manipulation experiment

Pacific salmon acquire most of their biomass in the ocean before returning to spawn and die in coastal streams and lakes, thus providing subsidies of marine-derived nitrogen (MDN) to freshwater and terrestrial ecosystems. Recent declines in salmon abundance have raised questions of whether managers should mitigate for losses of salmon MDN subsidies. To test the long-term importance of salmon subsidies to riparian ecosystems, we measured soil nitrogen cycling in response to a 20-yr manipulation where salmon carcasses were systematically removed from one bank and deposited on the opposite bank along Hansen Creek, in the Wood River System. The objective of this work was to determine whether prolonged enhancement and reduction of salmon subsidies altered long-term soil N cycling, similar to that documented in forests receiving N fertilizer additions. If long-term changes in N availability due to salmon enhancement or reduction were observed, compensatory nutrient subsidies may be valuable for maintaining critical ecosystem functions in riparian areas with reduced salmon returns. If not, then the addition of nutrients as a management response to low salmon returns may have unintended negative consequences.

Soil samples were taken at different distances from the stream bank along nine paired transects and measured for organic and inorganic nitrogen concentrations, and nitrogen transformation rates. Marine-derived nitrogen was measured using 15N/14N for bulk soils, and NH4 and NO3 soil pools. Stable isotope analyses confirmed 15N/14N was elevated on the salmon-enhanced bank compared to the salmon-depleted bank. However, 15N/14N values of plant-available inorganic nitrogen exceeded the 15N/14N of salmon inputs, highlighting nitrogen isotope fractionation in soils that raises significant methodological issues with standard MDN assessments in riparian systems. Surprisingly, despite 20 yr of salmon supplementation, the presence of MDN did not cause a long-term increase in soil nitrogen availability. This study confirmed that MDN was both present in soils and increased on the bank enhanced with salmon carcasses for 20 yr. However, plant available inorganic N pools and N transformation rates measured in soil during the peak growing season immediately prior to the annual return of salmon were largely unaffected by salmon enhancement. Even though the salmon-enhanced bank had increased net nitrification compared to the salmon-depleted bank, our analysis found no pattern with distance from the stream, suggesting that elevated nitrification was caused by bank characteristics unrelated to salmon carcass density. This finding indicates the importance of MDN to ecosystem nitrogen biogeochemistry, and riparian vegetation may be overestimated for some systems.

Feddern et al. 2019. Riparian soil nitrogen cycling and isotopic enrichment in response to a long‐term salmon carcass manipulation experiment. Ecosphere 10(11):e02958. 10.1002/ecs2.2958c

Juvenile fish exploit spatial heterogeneity in thermal habitat and trophic resources

Vertical heterogeneity in the physical characteristics of lakes and oceans is ecologically salient and exploited by a wide range of taxa through diel vertical migration to enhance their growth and survival. Whether analogous behaviors exploit horizontal habitat heterogeneity in streams is largely unknown. We investigated fish movement behavior at daily timescales to explore how individuals integrated across spatial variation in food abundance and water temperature. Juvenile coho salmon made feeding forays into cold habitats with abundant food, and then moved long distances (350–1300 m) to warmer habitats that accelerated their metabolism and increased their assimilative capacity. This behavioral thermoregulation enabled fish to mitigate trade-offs between trophic and thermal resources by exploiting thermal heterogeneity. Fish that exploited thermal heterogeneity grew at substantially faster rates than did individuals that assumed other behaviors. Our results provide empirical support for the importance of thermal diversity in lotic systems, and emphasize the importance of considering interactions between animal behavior and habitat heterogeneity when managing and restoring ecosystems.

Armstrong et al. 2013. Diel horizontal migration in streams: juvenile fish exploit spatial heterogeneity in thermal and trophic resources. Ecology, 94(9), 2066–2075.