Award: OCE-2053719

Northern Anchovy is an important constituent of the marine food web, supporting numerous species of marine mammals, seabirds and a diverse group of marine fishes inhabiting the US west coast. During an extended period of unusually high sea surface temperatures that began with the 2014 warm-water anomaly, anchovy, previously thought to thrive only under high-upwelling cold-water conditions, boomed to a population size far exceeding historical records. Like many other small coastal pelagic fishes worldwide, anchovy exhibit massive boom and bust fluctuations in population size, but this recent event raised new questions as to what specific factors drive the fluctuations. We had linked anchovy booms and busts to changes in larval food chain length (FCL) that affect energy transfer efficiency from the food chain base to larvae, but the exact mechanisms behind FCL regulation were unknown. Fish are most vulnerable to starvation and predation during the larval stage, and survival is crucial to recruitment of young fish to the adult populations. The current project capitalized on persistent warm-water conditions in the Southern Californian Bight and high numbers of anchovy and other species spawning there to gain a better understanding of what, where and how changes in larval food chain length occur, how that relates to fitness, growth and survival, and other factors, such as maternal investment, that may be important for larval survival. Larval anchovy, abundant co-occuring species of larval rockfish and their plankton prey were sampled at multiple locations throughout the Southern Californian Bight in fall 2020, and winter and spring 2021. In addition, we also used larval anchovy and shortbelly rockfish (a commonly caught species) collected by the California Cooperative Oceanic Fisheries Investigations (CalCOFI) over the past 2 decades and larval anchovy size data from 1960 to 2005. We used microscopic and imaging techniques to identify fish larval, prey communities in the environment, diet and prey selection by the larvae. We also employed otolith analysis to measure larval otolith core size, a proxy for maternal investment, otolith growth ring widths to age and reconstruct the growth history of the larvae, and Compound Specific Isotopic Analysis of nitrogen isotopes in Amino Acids to determine FCL. We found that larval anchovy as well as rockfish are highly selective in their feeding, and mostly rely on specific copepod species and development stages, suggesting that they could be actively selecting for prey that result in a shorter more efficient food chain. In larval shortbelly rockfishes, the ingestion of these preferred prey resulted in higher body condition and faster growth. Shorter FCL led to significantly faster growth rate and a higher body fitness. Development of a larval size ratio index as a proxy for larval survival also revealed that during the historical time periods of short FCL to larval anchovy, the larval size ratio index was low, indicating high survival of young larvae which subsequently resulted in large adult population size. From these three tightly linked parameters, we proposed the "Trophic Efficiency in Early Life (TEEL)" Hypothesis for fisheries recruitment. High survival of larvae to recruit to the adult population depends on high energy transfer efficiency from the base of the food chain to the larvae. We also found that a larger otolith core size correlated with improved foraging of larval rockfish and higher survival and body fitness of both larval anchovy and rockfish, suggesting that maternal investment may be another important determinant of recruitment success. The findings of this project have generated new understanding of the principal mechanisms driving growth and survival of young fishes and their recruitment to adult populations. This information along with the FCL and larval size ratio indexes has the potential to improve the assessment and short-term predictions of population sizes and inform management strategies of these species and possibly other species as well. In sum, our research suggests that the combination of being born large (maternal investment) and short FCL to preferred prey (trophic efficiency) are keystone factors that facilitate rapid growth and survival of very different fishes in the California Current. Temporal variability in these factors may apply as a general mechanism that drives the boom-bust dynamics of small pelagic species worldwide. Last Modified: 03/22/2023 Submitted by: Brice Semmens