Distribution of Steelhead / Redband Trout In Response to Biotic Interactions
(Pattern detection, experimentation and epistemology)

Seth monitoring underwater fish interactions
Seth monitoring underwater fish interactions

I examined the relative roles of biotic and abiotic factors in structuring redband trout Oncorhynchus mykiss distributions in the South Fork John Day. I first examined the relationship between the biological traits of the fish assemblage and riparian-geomorphic features in context of prevailing theories of stream ecology stemming from the river continuum concept (Chapter 2). I found that fish traits and habitat characteristics were related to one another in a different manner according to the spatial scale of observation (watershed, tributary, and reach scales) and that mode of species coexistence (niche overlap, niche partitioning, and non-associative) was correlated more strongly with the distribution of traits than with environmental features, suggesting that biotic interactions play an important role in structuring the fish assemblage.

Next, I described the ‘behaviorscape’ of redband trout foraging and aggression in context of prevailing foraging theory (Chapter 3, Part I). I found that redband trout behaviors could be described in terms of expected energetic gains and losses and potentially predation risk, with ‘risky’ behaviors yielding higher expected energetic gains in habitats with abundant structural refugia. These findings raised the questions of how fish growth and foraging behavior would be affected by increased food resources, questions which I addressed via a supplemental feeding experiment in two streams (Chapter 3, Part II). I observed nearly an order of magnitude increase in instantaneous growth rates of redband trout O. mykiss and a potential competitor, juvenile Chinook salmon O. tshawytscha, upon feeding, demonstrating that food is a limiting factor for salmonid growth in this system. Also upon feeding, redband trout foraging and aggression increased in a stream with no heterospecifics, lower discharge, and colder water temperature, but not in a stream with the opposite characteristics.

Fish abundance estimation through snorkeling
Fish abundance estimation through snorkeling

These findings suggested that O. mykiss and O. tshawytscha may coexist in a state of interactive segregation, contrary to the common wisdom that the two species are selectively segregated and have therefore evolved fully-distinct habitat preferences. To acquire further evidence for this hypothesis and to elucidate the role of competitor densities on habitat selection at local scales (which would in turn affect fish distribution patterns at larger scales), I employed a multi-species habitat selection model (Chapter 4). Redband trout habitat selection of enriched patches was negatively affected by conspecific densities as expected, but positively affected by juvenile Chinook densities, a finding that I attribute to habitat effects or heterospecific attraction. Redband trout foraging and aggression decreased in the presence of both conspecifics and heterospecifics, and aggression appeared to decrease with supplemental feeding. These contrary results between the former and latter experiments testing for aggression as a response to feeding are explained via the manner in which food was delivered to the stream, pulsed at dawn and dusk versus delivered evenly over time, respectively. In the concluding chapter (Chapter 5), I make the case that restoration practices acknowledging a broad range of potential drivers for species distributions-including behaviorally mediated biotic interactions among the fish assemblage-will better serve a broader range of restoration goals.

Additional Research Information:

A ‘behaviorscape’ perspective on stream fish ecology and conservation: linking fish behavior to riverscapes
Seth M. White, Guillermo Giannico and Hiram Li

The Influence of Release Strategy and Migration History on Capture Rate of Oncorhynchus mykiss in a Rotary Screw Trap
Ian A. Tattam , James R. Ruzycki , Peter B. Bayley , Hiram W. Li and Guillermo R. Giannico

Body Size and Growth Rate Influence Emigration Timing of Oncorhynchus mykiss
Ian A. Tattam , James R. Ruzycki , Hiram W. Li and Guillermo R. Giannico

Landscape Level Integration of John Day River Steelhead / Redband Trout Distribution

Understanding the distribution patterns, habitat associations, and the influence of the landscape on the factors is crucial to salmonid management and habitat restoration. The research presented in this dissertation describes a set of independent models as a mean to analyze how a complex system affects the distribution patterns of redband/steelhead trout (Oncorhynchus mykiss gairdneri) in the South Fork of the John Day River, a semi-arid, mid elevation basin with naturally reproducing populations of summer redband/steelhead trout. Summer redband-steelhead trout populations in this basin have declined from historic abundances, and are protected under the Endangered Species Act.

Tributary to the South Fork John Day River
Tributary to the South Fork John Day River

In Chapter 2, an exploratory method to identify the most appropriate scales to study the distribution patterns of redband trout was developed using an information theoretic approach based on the Akaike Information Criteria (AIC) following the laws of parsimony. The objectives of this chapter are the following: (1) define the principal habitat associations for redband trout and (2) find the most appropriate spatial scales in which trout distribution should be studied.

Based on the findings of chapter 2 that acted as a “blind model” using physical parameters, chapter 3 is a more “informed model” that includes more biological realism using a multi-scale spatial analysis of factors affecting the distribution patterns of redband trout. The working hypothesis for this chapter is that at cold water temperatures (< 18 C) an increase in pool area will be positively correlated with an increase in redband/steelhead trout biomass, and the null hypothesis is that there is not relation between water temperature and pool area, predicting that in stream reaches with lower water temperatures an increase carrying capacity per pool will be observed.

Finally, with the finding that water temperature seems to be a persistent factor that explains the distribution patterns of trout at different spatial scales, chapter 4 examines whether longitudinal-summer-stream temperature profiles in arid-environment streams can be used to index carrying capacities of threatened populations of redband/steelhead trout (Oncorhynchis mykiss gairdneri) in the mid-Columbia Basin. The specific objectives of this chapter are the following: a) to determine if water temperature can be used as a carrying capacity indicator for redband trout, b) to locate the sites with the highest redband trout biomass, and c) to identify the factors that contribute to high biomass in those sites during summer base flow.