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Draft TRT Document – for Discussion Purposes – OK to circulatemorphology or life history characteristics may also be useful as expressions of underlyinggenetic differences depending on the mechanism of expression. Adaptive life historydifferences between presumptive populations likely reflective ecological differences inthe natal streams and are, in part, indicative of underlying genetic differences. Since thedegree of isolation necessary to maintain genetic independence is much higher than thatfor demographic independence, genetic information will tend to give a more conservativemeasure of demographic population structure. That is, populations that are geneticallysignificantly different are almost certainly demographically independent; alternatively,some populations that do not appear to be genetically distinct may still be largelyindependent demographically.Our knowledge of steelhead population genetics in Puget Sound is based on anumber of older allozyme-based studies (e.g. Phelps et al. 1997) and several recent, butmore geographically limited, studies using microsatellite DNA markers (e.g. Kassler etal. 2008). In some cases, interpretation of results from these studies may be limited byuncertainty in estimating the degree of introgression by non-native hatchery fish intopopulations. We do not have genetic data for populations prior to the large, widespread,and sustained releases of hatchery stocks. Thus we cannot directly estimate geneticimpacts to population structure from hatchery fish spawning naturally over the timeperiod of interest. Phelps et al. (1997) suggested that there was little evidence forhatchery introgression in most basins sampled. Kassler et al. (2008) found evidence ofinterbreeding between native N.F. Skykomish River steelhead and the non-nativesummer-run hatchery stock (Columbia Basin-origin) released in the Skykomish River.Presently, there are a number of steelhead genetics studies underway throughout PugetSound, and we used preliminary results from some of these. Final results from projectsare pending. Although the state of knowledge of Puget Sound steelhead populationgenetics is growing, it is clear that much more work is needed. The TRT used geneticdata and results that were available and best met the needs of identifying DIPs andMPGs. As appropriate data become available it will be important to re-evaluatepopulation genetic relationships and DIP designations.GeographyThe boundaries of a steelhead population are influenced, in part, by the spatialconfines of its spawning habitat. Physical features such as a river basin’s topographical,hydrological, and temperature characteristics dictate to a large degree where and whensteelhead can spawn and delimit the spatial area over which a single group of fish can beexpected to interact. For example, because of potential differences in homing fidelity theTRT distinguished between streams draining directly to Puget Sound and those that weretributaries to larger river system in the assessment of population independence.Geographic features such as elevation, geology, and precipitation will determine flowdistribution, riverbed characteristics (substrate size, stream width and depth) and waterconditions. Geographic constraints on population boundaries (such as distance betweenstreams) can provide a useful starting point, but geographic constraints will not generallysupport strong inferences at a fine scale (e.g., distinguishing separate populations withintributaries of a sub-basin). In addition, biogeographical characteristics and historical10
Draft TRT Document – for Discussion Purposes – OK to circulateconnections between river basins on geological time scales can be informative in definingpopulation boundaries.Patterns of Life History and Phenotypic CharacteristicsPhenotypic traits based on underlying genetic variation (rather thanenvironmentally induced variation) are useful in identifying distinct populations (definedon the basis of reproductive isolation and demographic independence). Variation inspawning time, age at juvenile emigration, age at maturation, and ocean distribution are,to some degree, genetically influenced (Busby et al. 1996, Hard et al. 2007). Differencesin the expression of those traits that influence fitness are generally thought to beindicative of long-term selection for local conditions, although depending on the trait, asubstantial portion of most variation observed is still due to purely environmental effects.Hydrological conditions (i.e., water temperature, times of peak and low flows, etc.)influence the time of emigration and return migration and spawning, and over time(several generations) will influence life history traits best adapted to local conditions.While a population may be genetically adapted to general conditions in its natal basin,individual fish within the population will still vary in their life history traits due togenetic variability and in their individual response to environmental cues. In the face ofdramatic ecological fluctuations (e.g., El Niños, Pacific decadal oscillations (PDO)) eachpopulation is expected to strike a balance between being highly adapted to localconditions and maintaining multiple life-history strategies (bet-hedging).Observed variation in life history traits can be used to infer genetic variation, andmay indicate similarities in the selective environments experienced by salmonids indifferent streams. In some cases, similarities in phenotype may arise independently indistinct populations, and the absence of phenotypic differences does not preclude thatpopulations are distinct. The TRT accepted the premise that phenotypic differences inlife history traits between populations (especially those that have recently diverged) doprovide a strong level of support for geographic separation and the presence of distinctpopulations.Population DynamicsAbundance data can be used to explore the degree to which demographictrajectories of two fish groups are independent of one another. All else being equal, theless correlated two time series of abundance are between two fish groups, the less likelythey are to be part of the same population. For steelhead, however, the majority ofpopulation abundance estimates are based on index area redd counts taken in the spring,during periods of relatively high flow and poor visibility and there is considerableuncertainty in the accuracy of these data. Further complicating the interpretation ofcorrelations in abundance are the potentially confounding influences of correlatedenvironmental characteristics, such as shared estuarine and ocean conditions or regionwidedrought. Harvest effects also may result in correlations of abundance when distinctpopulations share oceanic and inshore migratory routes or simply share harvestmanagement goals. However, the majority of Puget Sound steelhead sport and tribalharvest takes place in freshwater and shared harvest effects would predominately only11
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Appendix 12. TRT score sheet for id
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