just see them now through the rather distorting lens of fishery data. Shorter time series
from sediment under anoxic waters off Peru and Chile (e.g. Díaz-Ochoa et al. 2009)
also show alternations, with anchoveta scales usually dominant, seconded by
myctophid, sardine, or hake scales.
(^) Key factors in all regional cases appear to be differences between sardines and
anchovy in respect to temperature preferences and best food-particle sizes. In the
Humboldt and California Current systems, the high sardine intervals have been
relatively warmer and higher in salinity, likely with less equatorward flow and less
coastal upwelling, while periods of anchovy dominance have been colder inshore with
stronger upwelling. Both genera feed primarily on zooplankton, but sardines have
more closely spaced gill rakers and filter the smaller animals typical of warmer, more
offshore waters. Anchovy are primarily visual particle feeders, thriving on somewhat
larger zooplankton typical in colder, more productive nearshore upwelling ecosystems
(van der Lingen 2006). Thus they are distinctive in both temperature optima and
preferred food-types. The Japanese comparison is similar in respect to food type, but
reversed in respect to temperature. That is likely to represent distinctive adaptation
schemes matching eastern and western boundary current systems.
(^) For the Californian case, Rykaczewski and Checkley (2008) have suggested that the
food-size preference combines with the relative strength of two distinct modes of
upwelling (Fig. 11.39) to generate regimes favorable and unfavorable to sardines.
Coastal upwelling forced by equatorward nearshore winds generates cold, nutrient-
rich ecosystems with diatoms supporting abundant larger zooplankton (Calanus,
Euphausia, ...). If that is the only prevalent upwelling effect, then anchovy are more
likely to thrive. In sardine-favorable intervals, winds are stronger well offshore than
close inshore, such that there is strong wind-stress curl (a spatial gradient in wind
speed). Then, in addition to coastal upwelling, slower, more dispersed upwelling
occurs by different physics, by “Ekman pumping” offshore of the main upwelling
(Fig. 11.39). That extends the zone of enhanced primary production seaward, but at a
less-intense level. This supports greater production of smaller zooplankton (possibly
Paracalanus and Oithona) without strong cooling, setting the scene for increases in
sardines. Rykaczewski and Checkley show the effect by comparing levels of curl-
driven upwelling (calculated from monthly mean wind-speed maps) for the May–July
season of sardine larval development with sardine “surplus production”, basically a
measure from fisheries and survey data of sardine recruitment, a statistic primarily
influenced each year by the abundance of 0-age sardines (Fig. 17.22). The
resemblance is close enough that they may well be right. A similar comparison for the
Peru–Chile region will be enlightening.
Fig. 17.22 Time-series of relative recruitment rate of California sardine compared to
coastal upwelling and curl-driven upwelling May through July off Southern
California. Recruitment is uncorrelated with coastal upwelling but significantly (r =