Larval condition helps explain the recent anchovy boom in an extremely warm California Current Ecosystem

Abstract

Discerning the causes of boom-and-bust cycles for coastal pelagic fishes (CPF) has been a critical element in fisheries oceanography research for over a century. Although the biological and environmental conditions that drive CPF dynamics are difficult to pin down, it is clear that year-class strength (the “endpoint” when mortality rates stabilize and adult population sizes are set) is contingent on larval survival to the “pre-recruit” life history stage. Here, we test and verify whether two ideas, the Maternal Effects Hypothesis and the Growth-Survival Paradigm, can explain, at least to some degree, the exponential rise of northern anchovy (Engraulis mordax) off California from 2009–2019. We extracted otoliths from 127 anchovy larvae collected by the California Cooperative Oceanic Fisheries Investigation (CalCOFI) program and measured otolith core diameter (an index of size-at-hatch), larval age, size-at-age and growth rate-at-age (indexed by otolith increment width). Both hypotheses were synergistically supported. Larvae that hatched large (a likely Maternal Effect) grew faster, were larger at age, and were more likely to survive to three weeks. In addition, in years when larvae grew faster and were more robust at age, recruitment was higher (Growth-Survival Paradigm). The oceanographic conditions that facilitated high recruitment (2015–2016) were unexpected because anchovy were previously believed to thrive when the ocean was cold and 2014–2016 was the warmest 3-year period in the California Current Ecosystem in at least a hundred years. In spite of the overall warm conditions, the anchovy larvae we examined were in better condition and had higher survival in patches of ocean with moderate temperatures and low chlorophyll in the southern portion of the sampling region. By simultaneously testing multiple hypotheses, we have come closer to discerning the mechanisms driving the abundance of an ecologically important CPF.