Figure 15. Changes in relative reproductive success in year 10 in response to simulated changes in flow (cfs) during summer and fall in three different sites (Kittitas, Taneum, and Toppenish Mid).
Temperature changes did not appear to affect relative reproductive success of anadromous O. mykiss in the main stem Kittitas site, but did affect results in the Taneum and Toppenish Mid sites (Figure 16). Increasing temperature in the Taneum site reduced resident abundance and increased anadromous relative reproductive success up to about 2oC warmer than baseline, at which point anadromous relative reproductive success begins to decline. Surprisingly, the same effect was not observed in the Toppenish Mid site across the range of temperatures tested. This result was likely caused by the warmer baseline temperatures in Toppenish Mid and subsequent reduction in growth at temperatures that exceeded the optimum of 17oC. Growth reduction in the Toppenish Mid site resulted in a shift in age-at-smoltification toward older age smolts and a reduction in smolt-to-adult survival.
Figure 16. Changes in relative reproductive success in year 10 in response to simulated changes in stream temperature (°C) during summer and fall in three different sites (Kittitas, Taneum, and Toppenish Mid).
Model results were highly sensitive to changes in smolt-to-adult survival, moderately sensitive to changes in cross-ecotype production, spawner fidelity, and growth rate (i.e. food availability), and minimally sensitive to changes in winter dispersal rates, smolt size threshold, age-at-maturity, and maximum freshwater survival during spring through fall (Appendix D). Sensitivity results generally varied by channel type, with changes in relative reproductive success being greatest in tributary reaches and less pronounced in mainstem reaches.
Model estimates of relative reproductive success were most sensitive to changes in smolt-to-adult survival, particularly for tributary reaches. Relative reproductive success in year 10 for Taneum and Toppenish Mid reaches increased by approximately six fold as SAR was increased from 0 to 6%, compared with only a 2-fold increase for the main stem Kittitas site (Figure 17). These results suggest that small to moderate increases in SAR can tip the balance of total egg production in favor of anadromy, especially in tributary reaches. For example, relative reproductive success in the Taneum Creek site was estimated to be 0.65 under baseline conditions (i.e., mean SAR = 1.25%). However, increasing the mean SAR for this population to only 2.88%, as was estimated for lower basin populations, resulted in a relative reproductive success value greater than one.
Figure 17. Relative reproductive success in simulation year 10 as a function of smolt-to-adult survival.
Relative reproductive success was moderately sensitive to changes in cross-ecotype smolt production. For Toppenish Mid, a lower basin tributary reach, relative reproductive success in simulation year 10 increased considerably in response to increases in the proportion of offspring from anadromous female and resident male crosses that smolted (Figure 18). Specifically, relative reproductive success increased from approximately 0.80 to 7.5 as the proportion of offspring that smolted was increased from 30 to 90%. In contrast, increases in cross-ecotype smolt production had little effect on relative reproductive success in Kittitas and Taneum reaches. The difference in sensitivity among reaches is likely due to location within the basin and associated differences in smolt-to-adult survival. For the Toppenish Mid population, where SAR is relatively high, moderate increases in smolt production can yield substantial gains in anadromous adult returns and associated egg production. On the other hand, increasing smolt production from upper basin sites, where SAR is quite low, has less of an effect on the reproductive success of anadromous fish.
Figure 18. Relative reproductive success in simulation year 10 as a function of the percentage of offspring that smolt from mating crosses between anadromous females and resident males.
Discussion and Conclusions
Model results were generally consistent with observed patterns of O. mykiss ecotype distribution in the Yakima Basin and suggest that the spatial distribution of different ecotypes is largely determined by environmental conditions such as flow and temperature. The migratory life history type (steelhead) was dominant in tributary sites in the lower Yakima Basin where summer flows were relatively low and stream temperatures were comparatively warm. In contrast, residents (rainbow trout) were the predominate life history form in the upper Yakima Basin and other mainstem locations where stream channels generally maintained higher summer flows and cooler temperatures. Important mechanisms influencing life history diversity in the Yakima Basin identified by the model included rearing capacity (as moderated by flow, temperature, and channel type), smolt-to-adult survival (as influenced by migration distance), and growth conditions in freshwater.
Available rearing capacity for adult-sized resident fish (e.g., > 150 mm) appeared to be an important bottleneck for production of resident O. mykiss, particularly in tributary sites. Due to the requirements of adult fish for habitats with greater depth and velocity, sharp declines in flow during summer resulted in substantial reductions in rearing capacity for adult resident fish, favoring a migratory life history strategy. Tributary sites in the lower Yakima Basin continued to favor an anadromous life history under a variety of simulated flow conditions, although the relative reproductive success of anadromous fish decreased somewhat as simulated summer flows increased. In contrast, the volume of water in mainstem sites tended to remain high enough to sustain adult resident carrying capacity year-round, even after simulating large summer flow reductions.
Model simulations for the Taneum Creek tributary site in the upper Yakima Basin highlighted the important influence of population location within the Basin on life history diversity. Reproductive success was slightly higher for the resident population in Taneum Creek under baseline environmental conditions. However, increasing the smolt-to-adult survival rate for smolts originating from this site to a value equal to that of lower Yakima Basin smolts yielded a relative reproductive success ratio (A:R) greater than one. These results suggest that mortality costs associated with greater migration distance can have an important influence on the balance of O. mykiss ecotype distribution within a basin. That is, habitats located further from the ocean are less likely to support anadromy, even when local environmental conditions promote a migratory life history strategy. These results are supported by a recent study examining the influence of landscape on O. mykiss life history diversity in the Klickitat River basin which demonstrated that genetic diversity was significantly negatively correlated with elevation and upstream distance (Narum et al. 2008).
Variability in growth conditions between mainstem and tributary habitats had important implications for the balance between resident and anadromous O. mykiss in the Yakima Basin. In mainstem locations, where growth was relatively fast and resident adults were able to achieve lengths of 400 mm and greater, reproductive success for resident spawners substantially outweighed that of anadromous spawners. Resident rainbow trout grow to a larger size in mainstem reaches than tributary reaches (Figure 8), which resulted in higher fecundity of resident females in the main stem compared to tributaries. In contrast, growth of resident spawners in tributary sites was predicted to plateau between 160 and 200 mm, resulting in much lower average fecundity for tributary spawners.
Studies throughout the Pacific Rim corroborate our findings. O. mykiss populations in the Kamchatka Peninsula, far eastern Russia provide an excellent opportunity to compare ecotype distribution patterns predicted by the LHRM to those in pristine river systems. For example, the Kol and Sedanka Rivers support predominantly resident rainbow trout in spite of no migration barriers and few anthropogenic influences. Flow and temperature regimes in the spring-fed Kol and Sedanka Rivers also differ from neighboring rivers that support predominantly anadromous O. mykiss (Augerot and Foley 2005). They have cooler temperatures and more consistent, year-round flow conditions with less variability between wet and dry seasons. More stable environmental conditions are likely to improve survival and maintain adult resident carrying capacity, particularly through the summer and fall. Genetic analysis of pristine populations throughout the Kamchatka Peninsula confirm observations in the Pacific Northwest that resident and anadromous O. mykiss from the same basin typically function as an interdependent population, and genetic differences are more closely correlated to geographic separation than ecotype (McPhee et al. 2007).
Our model results were consistent with broad scale patterns in flow variability and observed distributions of adult steelhead in the Yakima Basin. Calculating an index of seasonal flow variability for each population provided a course method of stratifying the Yakima Basin into spatial units that were likely to provide habitat conditions favorable to either residency or anadromy (Figure 19). Flow variability for the Metolius River, Oregon, a spring-fed stream with consistent year-round flows, was provided for reference. Accounting for watershed area, populations in the Yakima Basin experiencing the most variable flows, Satus and Toppenish Creeks, sustain the largest abundance of anadromous O. mykiss, and the upper Yakima main stem, having the most stable flows, produces the fewest steelhead but is known to have a very large population of resident O. mykiss (Figure 19; Pearsons et al. 2008). This simple approach suggests that flow is strongly correlated to O. mykiss ecotypic distribution, and that LHRM is parameterized in a manner that produces results consistent with empirical observation.
Figure 19. Index of flow variability for the four Yakima independent steelhead populations. The spring-fed Metolius River, central Oregon is provided as an example of a stream with extremely low flow variability. Numbers above each box indicate the geometric mean of estimated steelhead spawner abundance from 1985-2004 expressed in numbers of fish per 100 square km of watershed area.
The rainbow-steelhead typology has obstructed adequate evaluation of O. mykiss population performance and restrained proper management of the species. Our findings are consistent with the conclusion of McPhee et al. (2007) that there is sufficient scientific evidence to, “…abandon the typological thinking (‘steelhead’ and ‘rainbow trout’ as biologically independent units) that has pervaded the biology and management of this species...” Given clear evidence of genetically stable life history plasticity and demonstrable environmental influence on life history response, a biologically justifiable method of examining viability or developing restoration goals for steelhead populations must include efforts to quantify the effects of codependent resident rainbow trout populations. We recommend using approaches that quantify abundance and productivity of both anadromous and non-anadromous O. mykiss when evaluating long term viability of either ecotype.
Inadequate understanding of the factors driving ecotype abundance in facultatively anadromous fish populations continues to deter researchers from properly evaluating effects of watershed management on steelhead viability. Federal regulations prepared by the National Oceanic and Atmospheric administration, categorically excluded resident O. mykiss from steelhead Distinct Population Segments (DPSs) (71 FR 848); however, in most cases, these two ecotypes are expressions of different life history strategies within a single population and function in a manner consistent the definition of an Independent Population described by McElhany et al. (2000).
Observation of codependent anadromous and non-anadromous O. mykiss ecotypes throughout the Pacific Northwest should not be regarded as unique or unusual. The overwhelming conclusion from studies of rainbow trout, cutthroat trout, brown trout, sockeye salmon, Atlantic salmon and charr life history diversity is that partial anadromy, a strategy beneficial to adaptability and versatility in uncertain river environments, is common among species of the family Salmonidae (Rounsefell 1958, Jonsson and Jonsson 1993 and Hendry et al. 2004).
Our analysis suggests that alterations to river discharge regimes due to irrigation and hydropower projects changes the flow and temperature conditions throughout the summer and fall, making the habitat more suitable for a resident life history strategy. Regulated river hydrographs throughout Washington, Oregon and California more closely resemble spring-fed systems that traditionally support large populations of resident rainbow trout. We suspect this is because adult carrying capacity is maintained throughout the dry season. Therefore, it is reasonable to conclude that reduction in steelhead abundance and increase in resident rainbow trout abundance in regulated rivers is in part a response to environmental conditions.
One of the primary benefits of a life-cycle modeling project is the ability to identify key data gaps and study needs. Assembly of the complete life cycle forces researchers to synthesize the best available information. Key functions and parameters driving population performance are identified, and components of the life cycle that were developed with uncertain data can be examined through sensitivity analysis. We identified several studies that would improve our understanding of O. mykiss life history drivers:
The spatial structure of the model is limited to specific reaches within each Independent Population. Expanding the spatial structure of the model to the population level would greatly improve the utility of the model for evaluating population responses to environmental conditions and would likely yield important and potentially unanticipated insights into the factors driving life history diversity. A population level analysis would likely require an alternative approach to modeling the relationship between discharge and habitat area, as PHABSIM and 2D modeling methods are costly and highly site specific. On alternative is an empirically based estimate of flow effects on carrying capacity similar to the Unit Characteristic Method described in Cramer and Ackerman (In Press). A combination of fish abundance surveys and measurements of stream channel type, mesohabitat composition, substrate, wood complexity, temperature and flow would be required for this type of analysis.
Migratory juveniles may residualize upon entry into suitable habitat for continued rearing, such as conditions found in the upper Yakima River main stem. Our model did not account for life history switching after initiation of migration, but the likelihood of that behavior could have a significant effect on ecotype distribution. Laboratory experiments could provide important insights into the factors influencing residualization behavior.
Movement of juveniles and adults between mainstem and tributary habitats was not adequately accounted for in the LHRM due to data limitations. Field studies of these behaviors are needed. We also recommend an individual based modeling approach to address these dynamics.
Hydrodynamic modeling would create a link between flow and temperature making it possible to construct a life-cycle model with continuous spatial and temporal structure throughout the main stem Yakima River.
Juvenile tagging studies or outmigrant sampling at Roza Dam and the Chandler Juvenile Fish Facility accompanied by otolith microchemistry or equivalent technique to link outmigrants back to resident and anadromous parents would address important uncertainties regarding the contribution of resident spawners to the anadromous populations.
The LHRM provided a framework that linked together life history parameters and environmental drivers of O. mykiss ecotype distribution. Moreover, the model offered a sufficiently detailed reconstruction of the mechanistic relationships between environmental conditions and ecotypic dominance such that water management impacts could be evaluated and alternatives tested. High summer flow conditions typical of regulated rivers improve survival of residents and increase reproductive success of resident O. mykiss relative to anadromous spawners; however, the ability of water managers to alter flow conditions in mainstem habitats in a manner that results in greater reproductive success of anadromous individuals relative to residents appears limited. Our modeling demonstrated that tributary habitats were most likely to support an anadromous ecotype, and management actions that protect or improve tributary habitats have the greatest potential to increase abundance of steelhead in the Yakima Basin. We recommend controlled field experiments or adaptive management to examine O. mykiss life history response to altered flow regimes.
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