0.62, P ∼ 0.005) correlated with the curl effect.
(^) (After Rykaczewski & Checkley 2008.)
The timing of Humboldt Current switches between anchovies and sardines has been
attributed by Alheit and Bakun (2010) directly to the distinctions between the species
in temperature preference and feeding ecology. The rise in sardines starting about
1972 corresponded with a strong and prolonged rise in near-coast SST and salinity.
Conditions shifted from predominance of cold coastal water (14–18°C, S = 35) to
more prevalent subtropical surface conditions (18–27°C, S = 35.1–35.7). Large
zooplankton became diminished, anchovy recruitment (already hammered by fishery
removal of older spawners) fell, and stock abundance plummeted and stayed down.
Sardines were favored by the new conditions, their stock coming to support fisheries
of 6 Mt yr−1. In the late 1980s, conditions shifted back toward lower temperatures and
salinity. Coastal sardines declined and anchovy finally recovered and were again
fished at a 12 Mt yr−1 rate in 1993. They dropped from that peak, but stocks and
yields have remained high, except for 1998. Fishing was actually stopped for a
significant interval during the El Niño of that year, and then resumed.
(^) Synchrony of the Humboldt and Japanese sardine “outbursts” of 1973–1993 appears
to have to do with at least Pacific-scale atmospheric “teleconnections”, but the
Japanese case is quite different. The Japanese sardine migrates to spawn in winter
where the Kuroshio approaches Japan. The larvae are carried north as they grow, the
adults migrating also into the Kuroshio–Oyashio confluence where both juveniles and
adults feed and grow. Recruitment is strongest during cooler periods in the spawning
grounds, which tend to correspond to greater zooplankton stocks in the spring feeding
grounds. This combination appears to be associated with the strength of the Asian
winter monsoon, for which of course there is an index, the MOI (high from 1970 to
1990), and low winter values of the Arctic Oscillation (AO, low in the same interval).
These are, however, closely enough associated with periods of decadal variations
indexed by the PDO and even SOI that the exact atmospheric effects controlling
variations remain to be resolved. Unlike the Peru case, the 1980s burst of the Japanese
sardine did not greatly diminish Japanese anchovy catches, but more anchovies were
taken as the sardine declined. That may have been a stock increase or simply