PhD Graduate
Year Graduated
2025
Faculty Advisor(s)
Steve Gaines and Ben Halpern
Committee
Cody Szuwalski, Steve Gaines
Dissertation Title & Abstract
An empirical assessment of the role of large-sized phytoplankton, sardine predation, and fishing on anchovy fluctuations during a period of changes in productivity regimes, in the mid-high Humboldt Current System off Chile
Collapses in anchovy (Engraulis ringens) populations have been recorded in the Humboldt Current System (HCS) since paleontological times, yet the mechanisms driving these fluctuations remain incompletely understood. The anchovy is the predominant small pelagic species in the HCS, represented by four stocks, with only the stock in the mid-high latitudes HCS recently collapsing.
In my first chapter, I focus on large-sized phytoplankton, the preferred prey of anchovy early larvae and the foundation of short food chains that sustain meso-zooplankton, the primary food source for adult anchovies. I empirically examined the fluctuations of anchovy stock in the mid-high latitudes of the HCS off Chile, analyzing associations with various factors, such as prey-size availability, predation, fishing, climate, and local hydrology during periods of productivity regime changes. We identified significant delayed relationships between anchovy relative biomass and each factor, incorporating them as covariates within non-linear modeling approaches. The analysis revealed a positive association between large-sized phytoplankton and anchovy early life stages, emphasizing that delayed relationships involving large-sized phytoplankton and fishing play critical roles in anchovy fluctuations. My second chapter leverages the climate-driven large-sized phytoplankton and fishing scenarios from Chapter 1 to project anchovy dynamics under warming and historical scenarios. We found that anchovy decrease under high fishing and increase under low fishing scenarios. Large-sized phytoplankton appear to benefit anchovy stock dynamics under warming conditions and high fishing pressure, although warming also amplifies uncertainties in both high and low fishing scenarios. In my third chapter, I incorporate sardine (Strangomera bentincki) predation on anchovy early life stages to investigate its association with adult anchovy fluctuations and to assess its relative importance compared to other factors. The results revealed a significant negative association between sardine predation and adult anchovy fluctuations. Delayed relationships involving sardine predation, large-sized phytoplankton, and fishing were identified as key contributors to anchovy fluctuations.
Together, these chapters provide empirical evidence supporting the theory that environmental shifts in the HCS reduce large-sized plankton and increase sardine predation, which collectively impact anchovy early life stages, leading to reduced recruitment and subsequent collapses in anchovy spawning biomass. Fishing exacerbates this decline by intensifying when reductions in anchovy recruitment diminish adult biomass. Under persistent unfavorable conditions and continuous high fishing pressure, anchovy depleted state is maintained. This dissertation concludes that localized shifts in environmental conditions in the mid-high HCS, driven by climate change, are negatively impacting anchovy stocks, offering key insights for management strategies.
Education
BS, Marine Science, University of Concepcion, Chile