The ecological niche of a population is dynamic and will be affected by changes in the ecosystem and as a population migrates. An ontogenetic niche shift can also occur as organisms grow and can include changes in morphology, habitat, and feeding behaviour. Although they are the two most abundant salmon species, and are further augmented through hatchery stocking, it is unclear the degree to which the niches of juvenile pink salmon (Oncorhynchus gorbuscha) and chum salmon (O. keta) overlap. Furthermore, juvenile pink salmon and chum salmon undergo a period of rapid growth during their first summer at sea and it is unclear how their ecological niche changes with their ontogeny. Understanding the foodweb dynamics of juvenile salmon in the coastal marine environment is important because a large proportion of the overall mortality of salmon is thought to occur during their first summer at sea. The purpose of this study is to determine the degree to which the niches of juvenile pink salmon and chum salmon overlap, how their trophic position and food source changes as they grow into a new ontogenetic niche, and how these processes are affected by ocean conditions. I expected that years of poorer feeding conditions and increased competition would result in reduced trophic position and greater overlap of the niches of juvenile pink salmon and chum salmon. I hypothesized that juvenile salmon would shift their diet to a more offshore-based foodweb as they grew and that their trophic position would increase with size, but that the shift would be stronger when feeding conditions were improved. Statistical analysis showed evidence that the overlap of the niches of pink salmon and chum salmon increased when the abundance of salmon was high. Contrary to expectations, the trophic position of salmon appeared to decrease under favourable conditions. The trophic position of both pink salmon and chum salmon was higher in the southern portion of the study area, and increased when juvenile abundance was high. I suggest that the higher trophic position among juvenile salmon when there is more competition might result from increased reliance on gelatinous zooplankton, which are carnivorous, but a nutritionally poor food choice compared to other common prey items. The ontogenetic shift from summer to fall among juvenile salmon included a shift to a more offshore-based diet and a higher trophic position. In the northern portion of the study area, which was comprised of the southern reaches of the Alaska Coastal Current (ACC), the shift to an offshore-based food source was more pronounced than the trophic shift. In the southern portion of the study area, which included the Transition Domain (TD) between the ACC and the California Current System (CCS), the shift to a higher trophic position was more pronounced than the shift in food source. The results of this study suggest that if climate change leads to poorer feeding conditions, the niches of pink salmon and chum salmon may increasingly overlap when the abundance of these species is high. Hatchery stocking of these species may also contribute to this trend if it leads to a greater abundance of juvenile salmon in the coastal marine environment. There is evidence that the structure of the food web and the nature of the ontogenetic niche shift are very different in the ACC and the TD, and climate change and hatchery stocking will most likely affect these regions differently. / Graduate

In response to the increasing need for ecosystem services throughout the Southeast Alaska region, decision makers are tasked with balancing the need for natural resources with salmon conservation. However, accurate historical and current information on salmonid population abundance, freshwater distribution, and habitat quality are sparse with limited resolution for large portions of this remote and rugged landscape. Here, I created Intrinsic Potential (IP) models for chum and pink salmon to predict the potential for portions of coastal rivers to provide high-quality spawning habitat. I developed IP models for both species from field redd surveys and synthetic habitat variables derived from 1-m resolution digital elevation models. The surveys were performed at 49 study reaches in five coastal drainage basins on the north end of Chichagof Island, Southeast Alaska. I used a spatially balanced random sampling design that included field surveys for redds during two field seasons with contrasting precipitation patterns and disparate adult salmon escapements. The IP models predict probable spawning habitat for both species based on persistent landform characteristics and hydrologic processes that control the formation and distribution of spawning habitat across the landscape. Selection of persistent reach variables for both species IP models was informed by principal component analysis (PCA), resource selection ratios, random forest modeling, and regression models of field and synthetic variable comparisons. I observed primarily one spawning strategy by chum salmon associated with mainstem channels, and two distinct spawning strategies for pink salmon related to small moderate-gradient channels and tributaries, and lower drainage basin mainstem channels. The relationships suggest that chum and pink salmon primarily selected for unconstrained channel types in large-and small-size channels, with chum salmon being more selective toward the larger mainstem channels, and pink salmon selecting for smaller channels and tributaries. The prediction of chum salmon redd presence within a specific reach for both high and low streamflow regimes was explained by channel gradient, floodplain width, and mean annual flow in order of importance. In general, chum salmon redds were observed in larger unconstrained low-gradient floodplain reaches where accumulation of deposited gravels and adequate flow produce habitat heterogeneity suitable for spawning. Pink salmon redd presence for both survey years was explained by channel gradient, reach elevation, and mean annual flow, in order of importance. Specifically, when flows allowed upstream access, spawning pink salmon utilized smaller moderate-gradient channels where substrate size and flows were better suited to their smaller body size. Remotely sensed persistent fish habitat data is valuable information for helping understand fish population distributions across the landscape. These synthetic metrics enabled the identification and evaluation of persistent landscape features as probable predictors of IP. Validation of LiDAR-derived channel characteristics indicated channel lengths measured from the DEM were 12% longer than field measured channel length, primarily for channels wider than 10 meters. Thus, understanding the limitations of the data is important so that decision makers do not unintentionally set unrealistic objectives. This research highlights the utility of using IP models with high resolution remote sensing to expand known distributions and quality of spawning habitat for these two species in Southeast Alaska coastal streams.