High latitude oceans are strongly seasonal ecosystems where winter conditions are marked by periods of low primary productivity. These oceans tend to have shortened food webs with relatively few species linking primary production to upper trophic levels. In the case of the Southern Ocean, a single species, Euphausia superba, is thought to be this link between trophic levels. The polar regions in both hemispheres are also among the ecosystems heavily impacted by climate change. For example, the western Antarctic Peninsula is experiencing some of the most rapidly changing climate on the planet with changes in temperature, wind, and sea ice durations and extent (Vaughn et al 2003, Stammerjohn et al 2008). Understanding how climate change will affect these ecosystems requires knowledge of trophic structure, its key species, their life history, and their plasticity to environmental variability. In this thesis, I explore the seasonal life history strategies of Antarctic euphausiids. In chapter one, I introduce the high latitude marine ecosystem of the Southern Ocean, Antarctic euphausiids, and climate change and its impact on the ecosystem in this region. In chapter two, I fill in gaps in our knowledge of the life history strategy of the highly abundant, but relatively understudied, Thysanoessa macrura, by synthesizing distribution, maturity, and diet data from summer and winter surveys in the Antarctic Peninsula region. I find that krill show knife edge maturity, can spawn in their first year, are more dispersed and offshore in winter, and are more predatory with increasing size and in winter. In the third chapter I investigate E. superba from the perspective of optimal foraging theory to examine when krill generalize or specialize to unify seasonal and regional differences in the diet and feeding behavior of E. superba under the Trophic Wave Hypothesis. I predict that krill have a more specialized yet higher mortality risk diet in summer and more of a generalist diet in winter. In chapter four, I ask whether E. superba found in benthic and mesopelagic habitats are an aberration or reflect an important part of their life history strategy. In this chapter, I use a Stochastic Dynamic Programming model (VertiKrill) to explore how food, predation, and respiration drive vertical habitat selection across a range body conditions throughout the year. I find that for both juveniles and adults, deep water habitats provide important refuges for avoiding predation and starvation in winter as well as during the transition between seasons. In chapter five I summarize the main findings of each chapter, discuss their implications, and offer future directions for this research.

Euphausiids, particularly the Antarctic krill Euphausia superba, are a crucial part of the Southern Ocean ecosystem. As the major link between primary production and higher trophic levels, variation in euphausiid abundance has important implications for predators, nutrient cycling, and ecosystem functioning. Climate variability influences euphausiids through multiple pathways, including direct effects on growth and indirect effects on primary production. Increasing ocean temperatures, declining sea ice, and other effects of climate change are likely to negatively affect E. superba. In contrast, little is known about the potential impacts of climate change on Thysanoessa macrura, a euphausiid with an abundance that likely rivals that of E. superba. In this thesis, I explore multiple aspects underlying the spatial and temporal variability of Antarctic euphausiids. In Chapter 2, I use state-dependent life history theory and stochastic dynamic programming to challenge the traditional paradigm of E. superba as a passive drifter, and find that accounting for active behavior in this species has important implications for spatial distribution, growth, and survival. In Chapter 3, I use a data-driven approach to quantifying spatial and temporal E. superba abundances near the North Antarctic Peninsula. I find that fluctuations in

abundance are tied to lagged indices of climate variability, and that abundance and measures of spatial aggregation are linked in some habitats and seasons. Finally, in Chapter 4, I use temperature-dependent growth models to explore how changing temperatures may differentially affect growth in E. superba and T. macrura. I find that as ocean temperatures increase, the biomass per recruit of the stenothermic E. superba is likely to decline over much of the temperature range in the Southwest Atlantic. In contrast, the eurythermic T. macrura is expected to have enhanced growth over much of this range, and increases in T. macrura biomass could potentially compensate for some loss of E. superba biomass. However, this biomass may not be energetically equivalent from the perspective of a euphausiid predator.

Steelhead trout exhibit highly variable population dynamics that are driven by the vast life history variability of individuals that results in a complex system with highly non-linear dynamics for both individuals and populations. In this dissertation, I consider three different projects with the aim of understanding both individual processes and population dynamics using mathematical and statistical tools. In each chapter I develop new mathematical and statistical methodology that incorporates the biological knowledge about steelhead life history in order to answer different questions about the individual and population dynamics observed in nature.

The first project focuses on the development and application of methodology to better explain the observed adult returns in the Carmel River. To do this, I incorporate knowledge about conditional migration strategies into a life history model. In this work, I discover a decreasing trend in mean stream temperature that is coupled with a decreasing trend in mean length of individuals. One of the primary goals of this chapter is to test the scientific hypothesis that the inclusion of life history attributes would improve the predictions of adult returns. I demonstrate that the mechanistic inclusion of life history attributes into the mathematical model does significantly improve the ability to predict adult returns.

In the second project, I develop a Physiologically Structured Population Model (PSPM) for steelhead trout. The PSPM framework allows me to account for the biological processes driving the life history of steelhead through their entire lifetime. A benefit of this framework is that the resultant population dynamics arise solely from the interactions between individuals and their environment. The model captures the wide variability of life histories that are observed in nature. With this model, I explore the effects that size-dependent mortality and competition have on the population dynamics. I also explore the effects of three different temperature regimes on the population dynamics, highlighting the non-linear nature of the system.

The last project is the development of a Bayesian Semi-parametric model that describes the relationship between temperature and individual consumption that determines growth. Developing a state-space model that incorporates a Gaussian Process prior for the temperature-consumption relationship allows the data to determine the shape of the relationship and account for both measurement error and process stochasticity. I first test the model with simulated data with different levels of data availability, measurement error and process stochasticity. This application demonstrates that the total number of temporal measurements affects the performance of the model more than the number of individuals. I then apply the model to experimental data from a growth experiment of steelhead trout. The results demonstrate the ability of the model to describe the growth of individuals as well as to capture individual consumption. The model shows agreement between the shape of the temperature-consumption function that I predict and the relationship that is commonly used for steelhead.

The overall objectives of this project were to: 1. Establish the nature and magnitude of the link among the physical dynamics of coastal upwelling, primary production, krill production, and productivity of the most important commercial fish species in California (market squid, Loligo opalescens); 2. Using the information from the first objective, construct models to explore various scenarios of the impacts of global climate change on nearshore krill and squid productivity.