Graduate Thesis Or Dissertation
 

Maternal effects, the environment, and age structure: A mechanistic fine-scale approach to describe a complex set of factors contributing to larval growth and survival

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  • The relationship between population characteristics and population productivity is fundamental to sustainable fisheries management, but predicting productivity remains a challenging task. Proposed mechanisms driving the variability in productivity at a given population size have included environmental and demographic factors related to the age structure of the population, but the broad-scale modeling approaches used to evaluate these factors have not identified consistent relationships to inform predictive management models. However, recent finer-scale approaches capable of integrating a number of possible recruitment drivers at spatial and temporal scales relevant to larval survival have helped illuminate the complexity of the dynamics driving recruitment variability. Considering the environmental context at scales relevant to the prey environments experienced by fish larvae enables evaluation of mechanisms contributing to larval survival when mortality is governed by density-independent processes as well as mechanisms contributing to larval competency that may carry over as advantages determining survival when density-dependent mortality dominates. In the original research presented in the following chapters, I seek to first identify and then evaluate the impact of two maternal effects on productivity in a long-lived rockfish, the Pacific Ocean Perch (Sebastes alutus, POP), in the environmental context of the Gulf of Alaska (GOA). With a spatially-explicit larval dispersal model, our overall goal is to describe the interaction of the environment and maternal effects at scales relevant to larval growth and development. In Chapter 2, our goal was to determine if maternal effects operated on offspring provisioning in a long-lived rockfish (genus Sebastes), and to evaluate any such effects as an intrinsic function of maternal age or a context-dependent effect of the offspring release environment. I found that offspring provisioning is a function of both maternal age and the timing of offspring release; older females exhibit more provisioning (oil globule volume) than younger females throughout the spawning season despite a decrease in provisioning across all maternal ages as the season progresses. These findings suggested a role for both maternal age effects and a potential context-dependent maternal effect in population productivity that may be important in models of population persistence and resilience. In Chapter 3, our goal was to evaluate the maternal effects of parturition timing and parturition location on the fine scale prey availability experienced by early dispersal larvae. I used the output from a linked hydrodynamic-nutrient-phytoplankton-zooplankton (ROMS-NPZ) model to inform a larval individual-based model (DisMELS) that tracked dispersing particles through a seascape populated with potential prey items (the “preyscape”), from parturition through the pelagic juvenile stage. I found that both parturition timing and location determined distinct early larval preyscapes for individual dispersing larvae, but the differences in preyscape related to parturition timing were more consistent across years than the differences that emerged among parturition locations. Relative to the mid and late release cohorts, early parturition cohorts experienced higher concentrations of microzooplankton and lower concentrations of copepods, particularly in the initial thirty days of the dispersal period. In addition, the 16-year ROMS-NPZ model output included both a positive and negative phase of the Pacific Decadal Oscillation (PDO), a multidecadal phenomenon that affects climate and productivity in the North Pacific. The signature of the PDO switch in 2007 was clearly evident in the copepod preyscapes predicted by the models. However, the impact of the PDO switch was consistent across preyscapes defined by parturition location, and unique to the preyscapes defined by parturition timing. This suggests that maternal effects on release timing and provisioning of POP larvae may have differential advantages in low and high PDO years. In Chapter 4, I further explored the potential interactions of these changing environments and maternal effects by modeling larval growth as a function of both the daily prey concentration and maternal age within an individual-based modeling framework. I applied a growth rate function to larvae representing three distinct maternal-age populations: age 12, age 15, or age 20. These “same-age” populations revealed that larval growth and development during the earliest stages of dispersal were more sensitive to maternal age than prey concentration in our seascapes, but notable shifts to smaller lengths and later flexion ages occurred across all maternal ages, coincident with the shift in the Pacific Decadal Oscillation in 2008. To investigate the additional maternal effect of age-dependent parturition timing, I applied the growth rate function to three populations representing different proportions of age-12, age-15, and age-20 females. These “age-structured” populations represented larval cohorts impacted by both a maternal effect on the maximum growth rate and a maternal effect on parturition timing. The “age-structured” populations showed that the annual distributions of larval length and development lagged those of the same-age populations. Our approach describes both the maximum potential of a maternal-age dependent larval growth rate at a given maternal age within a varying environment, and the realized potential of this maternal effect on larval growth in the context of an age-structured population with maternal-age dependent parturition timing. Our findings highlight the complexity of the impacts of maternal effects on productivity in a variable environment, which may confer advantages to older or younger females spawning in different locations in some years and locations, but not others. This complexity could be the reason why modeling approaches that “average” over these processes fail to capture the role of maternal effects in fisheries management reference points. Further work is needed to relate these findings to empirical data, such as larval growth rates in the field and recruitment signals that match model predictions.
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  • The research presented in Chapter 2 was funded by a grant to Scott Heppell and Selina Heppell by the North Pacific Research Board, project number 629. The research presented in Chapters 3 and 4 was supported by a grant from the National Oceanic and Atmospheric Administration, Fisheries and the Environment (FATE) program to Dr.’s Cianelli, Heppell, Spencer, and Stephens.
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