Marine-nutrient assimilation in rearing coho and Chinook salmon in the Unalakleet River Philip Joy 1, Wes Jones 2, Craig Stricker 3, and Mark S. Wipfli 4 1 Alaska Department of Fish and Game - Sport Fish Division, Fairbanks, Alaska USA; and Alaska Cooperative Fish and Wildlife Research Unit, School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, Alaska USA; ; 2 Norton Sound Economic Development Corporation, Norton Sound Fisheries Research and Development Department, Unalakleet, Alaska USA. 3 U.S. Geological Survey, Fort Collins Science Center, Denver Field Station, Denver, Colorado USA 4 U.S. Geological Survey, Alaska Cooperative Fish and Wildlife Research Unit, Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska, USA Introduction Marine nutrients imported to freshwater systems by migrating salmon, or marine-derived nutrients (MDN), have been identified as a significant variable affecting growth and survival of juvenile salmon. The effects on stock productivity, however, have not been assessed directly. Given that larger smolt are associated with higher marine survival, understanding the impacts of MDN on juvenile growth, size and abundance may ultimately improve manager’s ability to forecast return rates of adult salmon. Understanding the effect of MDN on the freshwater productivity of a salmon stock requires an understanding of the timing, route and source of MDN being utilized by rearing salmon. This poster reports on preliminary results from the 2011 season. Objectives The objectives of this study are to identify the source and timing of MDN assimilation in rearing salmon parr and migrating smolt and quantify MDN assimilation using stable isotope and stomach content analysis. Methods To examine MDN assimilation at the watershed scale the stable isotope ratio of 15 N and 13 C in rearing salmon tissue will is being tracked during the freshwater portion of the salmon life cycle from 2011 through The  15 N and  13 C values of marine organic material are typically higher than those in terrestrial and freshwater environments. Given that spawning salmon cease feeding upon migrating into freshwater environments, they retain marine isotopic signatures. Thus, stable isotopes can be used to trace MDN through freshwater ecosystems. MDN endpoints for the isotope models were derived from samples of adult salmon tissue and eggs and from pink and chum salmon fry caught during the spring smolt migration. Non-MDN endpoints were derived from juvenile coho salmon sampled in reaches of the Unalakleet River drainage where little to no salmon spawning is known to occur (Figure 1). In 2011 juvenile Chinook and coho salmon were sampled during the spring smolt migration, mid-summer, and fall. The smolt populations migrating from two sub-drainages (North and mainstem Unalakleet Rivers) were examined in the spring while rearing parr were sampled at 5 study areas within each sub- drainage before (Late June/early July) and after (late August/early September) salmon spawning. Study sites were selected to reflect the range of salmon species composition and MDN deposition occurring in the watershed. MDN deposition is being modeled using a combination of ADF&G radio-telemetry data on spawning, carcass surveys and escapement estimates. Diet analysis is being conducted to determine the route through which MDN is being incorporated and the relative importance of MDN related diet items (such as carcass tissue, salmon eggs, invertebrates or salmon fry). The %MDN enrichment in fish tissue was estimated using stable isotope analysis of fin tissue. Results Non-MDN and MDN endpoint samples demonstrated a significant gradient (Figure 2) in their isotopic signatures thus making it possible to model the amount of MDN in sampled fish. While still preliminary, results from the 2011 field season demonstrated seasonal increases in marine-nutrients in rearing juvenile salmon after pink, chum and Chinook salmon spawned in the drainage (Figure 2). Diet analysis is currently occurring to identify the route of MDN uptake (egg, carcass or invertebrate). The seasonal increase in marine signatures was most noticeable in the North River and minimal in the Unalakleet River study areas. Results (continued) Migrating Chinook and coho salmon smolt had even stronger marine signatures (Figure 3). We currently believe this is indicative of substantial predation on chum and pink salmon smolt, which occur regularly in diet samples. Strong marine signals may be related to carryover in the marine signal from the previous fall, although the results of the parr sampling suggest otherwise. In Unalakleet River parr and smolt there were also numerous samples that had stable isotope signatures that fell outside of the range of values described by non-MDN and MDN endpoints (Figure 2 and 3, green circles). Figure 2. Stable isotope signatures of juvenile coho salmon sampled in the summer and fall from Unalakleet River drainage study areas with reference values of control and MDN samples. Red circles highlight samples with marine signals present. Green circles represent low-carbon values associated with lentic habitat. Results (continued ) Parr samples with these values were taken either directly from lentic habitat such as ox-bows and sloughs or from areas immediately adjacent to such habitat. Similar isotopic signatures are referenced in the literature as being linked to phytoplankton blooms in lentic water bodies. The mainstem of the Unalakleet River has an abundance of such habitat while the North River has very little and no such signals were seen in either parr or smolt samples from the North River. Although not displayed here, Chinook salmon showed a similar pattern to that seen in coho salmon, although the amplitude of the effect was less pronounced Summary While these results are preliminary, they do demonstrate significant assimilation of marine nutrients by rearing salmon both during the summer (from eggs, carcasses and invertebrates) and the spring (from consumption of pink and chum salmon smolt). Summer uptake was most pronounced in the North River, although greater numbers of spawners are found in the Unalakleet River mainstem. This may demonstrate an interaction between the number of spawners and other factors that affect the availability of MDN to juvenile salmon such as the size of the river. Conversely, the large amount of lentic habitat available in the mainstem Unalakleet River and the use of this habitat indicated by the stable isotope data may offer an alternative explanation. This data suggests that MDN may be a smaller component of the salmon’s food web in systems with considerable lentic habitat available. MDN may make up a larger portion of juvenile salmon diets in systems such as the North River where lentic habitat is limited. During the 2012 field season, the sampling protocol was modified in several ways. To determine the degree of carry-over in marine signals from fall to spring, juvenile coho salmon were sampled from MDN rich study sites in March, before salmon fry emerge from the gravel. Sculpins, sticklebacks and blackfish were sampled in the summer to provide more endpoints for the stable isotope models such that sculpins will be used to reference the lotic, benthic niche and sticklebacks and blackfish will be used to reference lentic habitat. Ultimately, Bayesian mixing models will be used to estimate the proportion of the diet comprised of MDN for individuals and populations and will be compared to growth rates (being measured with RNA:DNA ratios) as well as condition (Ricker’s condition factor) and size. Acknowledgments We thank AYK-SSI, ADF&G,NSEDC and BLM for funding. Special thanks to field technicians extraordinaire; Jacob Ivanoff, Renée Ivanoff, Clayton Mixsooke, Jessie Dunshie, John Ivanoff, Jenny Dill, Yosty Storms, Allison Martin, Will Tompkins, Matt Robinson, and Maya Uranishi. We would also like to thank BLM staff Merlyn Schelske, Jeff Beyersdorff, and Jeff Kowalczk. Figure 1. Map of the Unalakleet River drainage and sampling sites (1-10) and control sites (C). North Unalakleet Summer samplesAdult salmon (MDN) Fall samplesControl (non-MDN)  13 C  15 N Lower River Head- waters Coho salmon smolt Pink salmon smolt Chum salmon smolt Control (non-MDN) Figure 3. Stable isotope signatures of coho salmon smolt sampled in the North and Unalakleet Rivers with reference values of control samples and pink and chum salmon samples. North Unalakleet