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Biological and Oceanographic Drivers of Larval Growth, Settlement, and Recruitment of Rockfishes (Sebastes spp.)
Wheeler, Sarah Grace
Anderson, ToddMorgan, StevenRussell, Ann
xi, 119 pages : illustrations.
Recruitment of marine fishes is largely determined by biological and oceanographic factors acting on early life stages. Coastal upwelling has long been recognized as a critical factor influencing the survival of larvae and recruitment to adult populations. Dynamics in regional upwelling influence the magnitude and timing of primary productivity, affecting the availability of critical food sources for larval fish. In addition, upwelling-relaxation cycles affect the dispersal of marine larvae and their onshore delivery prior to settlement. Challenges with tracking larvae, however, have limited our understanding of how oceanography influences the early life stages of fishes. The objective of this dissertation is to evaluate the biological and oceanographic drivers of larval growth, settlement, and recruitment, using rockfishes (Sebastes spp.) as model organisms. Overlap of larval production and favorable feeding conditions may drive recruitment for many temperate marine fishes, as small changes in larval growth can result in order-of-magnitude differences in year-class-strength. In Chapter 1, I assess the influence of regional productivity, temperature, and larval condition in explaining growth in rockfishes. I employ a combination of otolith microstructure and satellite imagery to measure initial larval growth and estimate the productivity and temperature experienced by individuals to determine their relative importance in subsequent growth at metamorphosis. I compare model performance using indexed environmental conditions scaled over three different regions. In both years of study, net primary productivity explained the most variation in pre-metamorphic growth relative to temperature and initial growth. This relationship was consistent across spatial regions. Recent settlement, juvenile recruitment, and individual growth were significantly higher in a year when productivity bloomed earlier and individual larvae experienced higher levels of productivity. These results support the hypothesis that large-scale oceanographic processes that stimulate upwelling and secondary production are primary drivers of larval growth and subsequent year-class strength in rockfishes. Characterizing the behavior of larvae prior to settlement is integral to understanding population dynamics because coastal oceanography may facilitate or limit settlement. Otolith microchemistry can be used to determine patterns of fish movement, although there is a limited understanding of how this tool can be applied in coastal marine systems. My goal in Chapter 2 is to evaluate the application of otolith microchemistry to characterize water mass associations of settlement-stage marine fish in a coastal upwelling region using a three-step approach. First, I characterize seawater chemistry of coastal water masses across multiple years, finding significant differences in the chemical signatures of strong upwelling, weak upwelling, and relaxation. Second, I experimentally determine the effect of temperature on the partitioning of trace elements in otoliths for two rockfishes to find that the effect of temperature on otolith partition coefficients was element- and species-specific. Finally, I compare the synchrony in seawater and otolith chemistry of settlement-stage rockfishes that were exposed to naturally variable conditions over an upwelling-relaxation cycle. I subsequently evaluate whether laser ablation inductively coupled plasma mass spectrometry effectively measures otolith chemistry over ecologically relevant time scales. I discovered that elemental concentrations in otoliths respond rapidly to changes in seawater chemistry and reflect equivalent proportional changes. This study provides evidence that elemental signatures are valuable tools for reconstructing larval histories of marine fish. In Chapter 3, I use otolith chemistry to examine water mass associations of two juvenile rockfishes during onshore transport and settlement in an upwelling region. I develop a chemical proxy for upwelling and relaxation by characterizing Sr/Ca and Ba/Ca signatures of otoliths collected during these oceanographic conditions. Otolith chemistry differed between rockfishes collected during upwelling and relaxation, with signatures unique to each year. I subsequently compare otolith signatures of rockfishes collected during high and low settlement periods to determine whether specific water masses affect settlement. I provide evidence that copper rockfish associate with upwelling currents during periods of high settlement, suggesting that upwelling may facilitate settlement for these species. Conversely, I found evidence that the closely related gopher rockfish associate with relaxation events during peak settlement periods. This research takes an important first step at in evaluating the utility of trace element signatures to characterize larval fish movement during onshore delivery and settlement in marine systems. Together, these studies improve our understanding of how coastal upwelling impacts larval growth, settlement, and recruitment, which provides important information for understanding population dynamics in marine ecosystems.
Doctor of Philosophy (Ph.D.) University of California, Davis and San Diego State University, 2015
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