Graduate Thesis Or Dissertation


Modeling Transport, Retention, and Growth of Early Life Stages of Small Pelagic Fishes in the Southern Humboldt System Public Deposited

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  • Small pelagic fishes (SPF), such as anchovies and sardines, are ecologically important due to their large abundance and intermediate trophic position that links plankton production to upper trophic levels. They are also economically important, supporting large fisheries that contribute to one fourth of the world fish landings. Reproductive success in SPF strongly depends on environmental factors acting on early life stages (ELS). Among these factors are ocean circulation, habitat temperature, prey and predators. Understanding the coupling between ocean circulation and ELS is relevant to obtain insights on the processes that control SPF abundance. Using a numerical modeling approach, this dissertation examines spatiotemporal patterns in dispersal, growth and survival of ELS of anchovy (Engraulis ringens) and common sardine (Strangomera bentincki) in the southern Humboldt System (off central Chile). Three model components are integrated: 1) a realistically driven three-dimensional high-resolution ocean circulation model, coupled to an eight-component lower trophic level model (LTLM), which reproduces the main physics and plankton dynamics experienced by fish ELS; 2) a particle-tracking model (PTM) to simulate ELS dispersal; and 3) an Individual Based Model (IBM) of bioenergetics to simulate ELS development, growth, and survival. A combination of remotely sensed observations and the outputs of the ocean model are used in Chapter 2 to document the impact of intraseasonal upwelling variability on plankton abundance and structure. Intraseasonal changes in phytoplankton are consistent with alongshore wind disturbances due to Madden-Julian oscillations. The phytoplankton intraseasonal variability exhibits largest amplitude in spring-summer, a feature related to the background seasonal conditions in ocean stratification and light. Chapter 3 documents variability in ELS dispersal using the PTM. We examine if patterns in anchovy and sardine spawning promote shelf retention and prey availability for ELS. We show that the timing of spawning (Aug-Oct) is only partially linked to high retention, but also demonstrate that the main spawning locations along the coast favor high coastal retention year-round. Experiments with ontogenetic diel vertical migration (ODVM) schemes increase retention and spawning-to-nursery connectivity. We suggest that the main spawning period is mostly explained by enhanced prey availability and connectivity, whereas inner shelf spawning and ODVM are the main strategies to increase retention of larvae nearshore. Chapter 4 examines anchovy ELS development, growth, and survival derived from an IBM. Mortality rate is assumed to decline with individual development and size, and increases with temperature. Dispersal patterns strongly determine individual growth and survival. Passive individuals initialized at 5 m depth growth fast near the coast, but they experience strong food limitation when advected far offshore. Passive individuals initialized at 20 m depth have the largest nearshore retention, but slow growth and survival due to low temperature and prey abundance. A favorable trade-off between high retention and growth was derived from individuals initialized at 10 m. Ontogenetic and diel vertical migration increased retention and survival over the shelf, but decreased larval growth. This study advances understanding of the interplay of multiple processes that modulate the timing and intensity of spawning, distribution, growth and survival of small pelagic fish in coastal upwelling ecosystems. It also documents the need for more and better field studies to represent better crucial early life stage traits and processes, such as vertical distribution, larval foraging, mortality, and bioenergetics.
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