Conclusions and Implications

Over the course of a year Susitna River basin trout moved long distances and occasionally utilized multiple tributaries within and among seasons

Inter-tributary movements suggest a basin-wide metapopulation and heterogeneity in trout movements

Habitat use by trout varied across seasons, with individuals selecting stream reaches with characteristics supporting refuge from harsh overwinter conditions, spawning in the spring, and feeding over summer months

In general, individuals exhibited a novel seasonal movement pattern where they overwintered in glacially-influenced mainstem reaches during the long ice-covered season from October through May, moved into upper reaches of clearwater tributaries during the spawning season from mid-May to early June, and remained in tributaries to feed from mid-June through September. Movements varied among seasons, with the longest mean distance traveled being from overwintering to spawning

Rainbow trout exhibit differential habitat selection over multiple spatial scales. In the case of Willow Creek Rainbow Trout, fish responded to finer-scale (channel unit) characteristics rather than more coarse-scale (stream reach) variables. The presence of salmon, increasing mean depth, and longer reach length were particularly useful positive predictors of habitat use, while trout selected against increased salmon richness (possibly due to avoidance of large aggregations of aggressive pink salmon)

There was no evidence at either the weekly or seasonal (early and late feeding) temporal scales of sex-biased movements. However, weekly movements averaged over individuals were significantly longer during the late feeding season when spawning salmon were present

The proportion of females deemed less mobile was high across both early and late feeding seasons, while males were more sedentary pre-salmon arrival and equally split between sedentary and mobile strategies in the late feeding season

The Kashwitna River Rainbow Trout subpopulation may be an important vector of gene flow in the Susitna River basin owing to the prevalence of long-distance and out-of-tributary movements by trout from this drainage. Additionally, the propensity of Kashwitna River trout to undertake movements away from their tributary of capture may indicate low-quality or limited availability of seasonal habitats in this drainage, potentially owing to the substantial glacial influence (turbidity) in this tributary. Higher turbidity may result in reduced sight distance for trout, lowering feeding efficiency and increasing the difficulty in locating conspecifics for mating, which may prompt movements to less-turbid tributaries

High tributary fidelity in Willow, Montana, and Chunilna Creeks suggests there is an abundance of quality habitats available for trout in these drainages across seasons

As a result of possible metapopulation dynamics, it is critical to manage Rainbow Trout at a basin-wide scale and maintain riverscape connectivity in order to account for the migratory nature of the diverse subpopulations

Significant harvest of Rainbow Trout still occurs in the Susitna River basin, and the effect of heavy catch-and-release angling, as evidenced by the presence of hooking scars on about one-third of all captured fish, may increase stress on fish, cause exhaustion, and result in higher risk of mortality

Spawning salmon presence influences habitat use and movements. Trout likely rely on high-calorie salmon-derived food subsidies to gain fat and energy reserves in order for survival during the winter months and gonadal development the following spring (Scheuerell et al. 2007; Armstrong and Bond 2013). Because of the potential importance of spawning salmon to Rainbow Trout population health, it is important that salmon are not overharvested and are kept at population levels high enough to avoid detrimental effects on nonanadromous salmonids

In order to foster ecologically and economically important trout populations while still allowing for a reasonable commercial catch of salmon, all resource users must make equal compromises to ensure the preservation of healthy salmon populations (Eastman 1996, Denton et al. 2009). This may include decreased salmon and trout quotas and bag limits for commercial and sport fisherman and the utilization of substitute food items for subsistence users during times of low salmon abundance

In addition, protection of salmon spawning and rearing habitats in freshwater in Alaska will be important in light of future climate change and human development such as urbanization and installation of hydropower dams, which may have cascading effects on salmon (and also trout) freshwater habitats

Another factor that may influence Rainbow Trout in the Susitna River basin is increased anthropogenic impacts in the region. This includes accelerated global change in the sub-Arctic, the proposed Susitna-Watana Hydropower Project, human population expansion, road building, clearing of riparian vegetation, and fish passage issues. Road building and riparian alteration could cause siltation and higher water temperatures in tributaries containing Rainbow Trout. This could be particularly harmful during spawning if siltation smothered eggs in redds or increased temperatures caused deleterious changes in egg incubation and juvenile trout growth

The Susitna-Watana hydropower dam, currently in the planning and environmental assessment stage, would likely cause far-reaching effects in the Susitna River from dam operations that may include altered flow and thermal regimes and changes in turbidity, with effects on aquatic habitat in the mainstem Susitna River downstream of the site (Devil’s Canyon in the upper basin). Overwintering habitat is of particular concern for Rainbow Trout downstream of the dam site because a high proportion of tagged fish from all tributaries used the mainstem Susitna River during this season. Our research showed that overwintering trout select areas with increased sinuosity and mean annual flow (stream size), habitat characteristics that likely would be affected by the alteration of flows and loss of seasonal flood events that shape channel geometry (Ligon et al. 1995; Gordon and Meentemeyer 2006). If winter base flows and water temperatures increase following dam construction, overwintering Rainbow Trout may be affected through increased energy expenditure from higher metabolic rates

Future Rainbow Trout research in the Susitna River basin could examine the influences of seasonal and daily flows and water temperatures recorded from USGS streamgage or using a set of field-installed temperature loggers on trout movements and habitat use across tributaries. These data were available for some of the Susitna River basin study area, but the analysis was outside the scope of our effort and objectives. Additionally, the suggested metapopulation dynamics and female biased sex ratio could be further studied by conducting a genetics survey of juvenile Rainbow Trout from various Susitna, Talkeetna, and Chulitna River tributaries. This approach could quantify genetic difference and mixing as well as at-birth sex ratios throughout the basin. The results of these possible avenues of research would likely be very valuable for understanding and management of Susitna Rainbow Trout.