Upwelling of cold, nutrient-rich water from the oxygen minimum zone (OMZ) off Peru sustains the world's highest production of forage fish, mostly composed of anchovy (Engraulis ringens). However, the potential impacts of climate change on upwelling dynamics and thus fish productivity in the near future are uncertain. Here, we reconstruct past changes in fish populations during the last 25,000 years to unravel their response to changes in OMZ intensity and productivity. We quantified and identified fish scales and bones deposited in laminated sediments from Pisco (Peru) with an average sampling resolution of 20.4 years (+/- 7.1). The records span the Last Glacial Maximum to the recent Holocene and thus encompass a variety of combinations of productivity, oxygen, and global temperature. Our results reveal that productivity appears to be the main factor controlling small pelagic fish abundance, while sub-surface oxygenation affects mainly anchovy and likely sardine populations. Lower productivity and higher oxygen concentrations during the glacial resulted in lower total fish productivity, whereas higher productivity and a stronger OMZ in some time intervals during the Holocene resulted in higher fish abundances. A variety of different conditions between these two oceanographic end members indicate preferred environmental conditions for a variety of small pelagic fishes. There is no evidence in our record for an out of phase relationship between anchovy and sardine at the timescales examined in the present study. Anchovy have been the predominant small pelagic fish throughout the record, at least over centennial to millennial timescales. Its abundance reached a maximum during the Current Warm Period, an era characterized by high productivity and intense OMZ conditions. Thus, industrial fisheries developed during a period of exceptional productivity in relation to that of the last 25 kyr. The records reveal that dramatic decreases in pelagic fish abundances have occurred in response to past large-scale climate changes than those observed in the instrumental period, which suggests that future climate change may result in substantial changes in ecosystem structure.

The Humboldt Current System (HCS) has the highest production of forage fish in the world, although it is highly variable and the future of the primary component, anchovy, is uncertain in the context of global warming. Paradigms based on late 20th century observations suggest that large-scale forcing controls decadal-scale fluctuations of anchovy and sardine across different boundary currents of the Pacific. We develop records of anchovy and sardine fluctuations since 1860 AD using fish scales from multiple sites containing laminated sediments and compare them with Pacific basin-scale and regional indices of ocean climate variability. Our records reveal two main anchovy and sardine phases with a timescale that is not consistent with previously proposed periodicities. Rather, the regime shifts in the HCS are related to 3D habitat changes driven by changes in upwelling intensity from both regional and large-scale forcing. Moreover, we show that a long-term increase in coastal upwelling translates via a bottom-up mechanism to top predators suggesting that the warming climate, at least up to the start of the 21st century, was favorable for fishery productivity in the HCS.

Marine laminated sediments in dysoxic areas of the ocean floor are an excellent archive for high-resolution climate reconstructions. While the existence of discontinuities produced by natural events, such as underwater landslides (slumps), strong bottom currents, and/or bioturbation is usually acknowledged for long records, the extent of their influence on high-resolution sequences is usually not considered. In the present work we show strong evidence for multiple stratigraphic discontinuities in different gravity and box-cores retrieved off Pisco (Peru) covering the last 600years. Chronostratigraphies are largely based on cross-correlation of distinct sedimentary structures (determined by X-ray image analysis) and validated using 210Pb, 241Am, and 14C profiles, as well as proxy records. The cross-correlation of distinct stratigraphic layers allows for chronostratigraphic tie points and clearly shows that some sedimentary sequences are continuous across scales of tens of kilometers, indicating that regional processes often determine laminae formation. Some differences in laminae thickness were found among cores, which could be explained by different sedimentation rates, spatially variable deposition of diatom blooms, changes in silica dissolution and partial deposition/erosion caused by bottom currents. Using multiple stratigraphic tie points provides clear evidence for laminated sequences present in some cores to be missing in other cores. Moreover, instantaneous depositions from upslope were identified in all the cores disrupting the continuity of the sediment records. These discontinuities (instantaneous deposits and missing sequences) may be due to slumps, possibly triggered by earthquakes and/or erosion by strong bottom currents. In spite of the missing sequences in some cores, a continuous composite record of the last six centuries was reconstructed from spliced sequences of the different cores, which provides a well-constrained temporal framework to develop further high-resolution proxies in this region. The present work shows that paleoreconstructions developed from single cores, particularly in areas with strong seismic activity and/or strong bottom currents, are subject to both temporal gaps and instantaneous depositions from upslope, both of which could be misinterpreted as abrupt climate changes or anomalous climate events. We stress the need for multiple cores to determine the stratigraphic continuity and chronologies for high-resolution records.

The habitats of populations of small, pelagic fish, especially anchovy and sardine, in the Benguela, California, Humboldt, and Kuroshio-Oyashio current systems, and in the NE Atlantic, are described and discussed in regard to future climate change. These stocks have been the primary concern of the Small Pelagic Fish and Climate Change (SPACC) program of International GLOBEC. Each of these regions and stocks has a unique set of climate and ocean conditions and their variability. However, they also share common characteristics. Spawning and development occurs within broad ranges of temperature (12-26 ̊C) and salinity (<30-36) and in regions of high plankton production, associated with either upwelling or freshwater. Often, sardine are more oceanic and anchovy more coastal, often associated with wind-driven upwelling and rivers. Sardine tend to make longer migrations between spawning and feeding regions than do anchovy. The habitat of most populations of small, pelagic fish expands when the population size is large and contracts when it is small, often into refugia. Climate change may affect populations of small, pelagic fish by causing poleward shifts in distribution due to warming, some of which have already occurred. Other potential effects are due to changes in winds, hydrology, currents, stratification, acidification, and phenology.