414
Chapter 11: The Pacific Ocean
It was the variable and sometimes disastrous changes in weather patterns on the coast
of Peru that first attracted the attention of oceanographers and fisheries scientists to the
ENSO phenomenon. Obviously, we cannot end the discussion of the Humboldt province
without some consideration of how an ENSO event is manifested. There is a sufficient
number of easily available accounts of the phenomenon not to require another full-blown
description here: if you are not familiar with the issue, I recommend starting with Tomczak and Godfrey (1994) for the physics and with Pauly and Tsukuyama (1987) or Pauly
et al. (1989) for the ecology and for the disastrous fisheries consequences that may ensue.
Briefly to recapitulate the effects of the ENSO cycle in the HUMB province, the onset
of an ENSO event is heralded by the slackening of upwelling intensity and the reduction
of the number and extent of upwelling centers as indicated by surface water temperatures.
This most often occurs in the autumn or winter (see Tomczak and Godfrey, 1994, p. 364,
for an interesting account of how this process was misnamed by oceanographers for El
Niño, the Christ child) and accompanied by progressive warming of the surface water,
in which Chile lags Peru. For the years 1948–1985, three major and three minor ENSO
events were correlated by Chavez et al. (1989) with deepening of the 14
C isotherm
(from 80 m to 100–175 m) by reduction in 60-m nitrate (from 25 to 14–22 M) and by
a decrease in new, nitrate-based primary production rate (by about 10 g C m
−2 day
−1 ).
Curiously, the rate of upwelling at the coast may even increase during ENSO periods
because the coastal wind system is enhanced by lessened cloud cover even as the offshore
trades, which regulate the depth of the thermocline, are at their weakest: what is upwelled
to the surface is, of course, warm tropical or subtropical surface water, already fully
depleted of nutrients, rather than nutrient-replete subthermocline water.
A multivariate ENSO index has been devised recently by Wolff et al. (2003) that
brings together six variables: sea-level pressure, components of wind direction, SST, air
temperature, and cloudiness. Positive values of this index represent the warm ENSO
phase. Comparing the seven strongest such events since 1950, it is found that three
events between 1957 and 1973 each featured an early warming in the western Pacific
and matured during their first year. Three events between 1982 and 1992 took longer
to develop and matured only in spring of their second year. The unusual 1997–98 event
developed very fast, and within only 50 days had major impact at depths down to 150 m,
and in relative temperature increase in the HUMB province.
The consequences of the 20-fold decrease in productivity and phytoplankton biomass
that may occur during El Niño events are catastrophic and cascade all the way from
the phytoplankton through herbivores to the disappearance of pelagic fish stocks and
great perturbations in the fishmeal trade and in soybean futures. The details of this
collapse have been sufficiently described that I do not need to provide a litany of loss
of reproduction in seabirds, starvation of marine mammals, and the disruption of the
hake, jack mackerel, shrimp, and sardine fisheries because the changed distribution of
the stocks put them beyond the traditional ambit of the fishery, whereas the Peruvian
anchoveta stock, already stressed by heavy exploitation, may collapse.
This region lacks a CalCOFI-like data base so that relatively little certitude has emerged
from studies of the pelagic ecosystem here; instead, what we must work with are fishery
landings, conflicting inferences, best guesses, and insights. Some things have become
clearer in recent years, however, despite these difficulties: the different manner in which
sardine (Sardinops sagax) and anchoveta (Engraulis ringers) populations react to the same
environmental forcing seems to hold the key for understanding changes in their stock
abundance. Some interaction also between these and the pelagic predators Trachurus
trechae (horse mackerel) and Scomber japonicus (mackerel) is now seen to contribute to
interspecific balances.
It is clear that here, as off California, anchoveta is the dominant planktivorous species,
and that the population is subject to natural fluctuations: the stock appears to be under
Chapter 11: The Pacific Ocean
It was the variable and sometimes disastrous changes in weather patterns on the coast
of Peru that first attracted the attention of oceanographers and fisheries scientists to the
ENSO phenomenon. Obviously, we cannot end the discussion of the Humboldt province
without some consideration of how an ENSO event is manifested. There is a sufficient
number of easily available accounts of the phenomenon not to require another full-blown
description here: if you are not familiar with the issue, I recommend starting with Tomczak and Godfrey (1994) for the physics and with Pauly and Tsukuyama (1987) or Pauly
et al. (1989) for the ecology and for the disastrous fisheries consequences that may ensue.
Briefly to recapitulate the effects of the ENSO cycle in the HUMB province, the onset
of an ENSO event is heralded by the slackening of upwelling intensity and the reduction
of the number and extent of upwelling centers as indicated by surface water temperatures.
This most often occurs in the autumn or winter (see Tomczak and Godfrey, 1994, p. 364,
for an interesting account of how this process was misnamed by oceanographers for El
Niño, the Christ child) and accompanied by progressive warming of the surface water,
in which Chile lags Peru. For the years 1948–1985, three major and three minor ENSO
events were correlated by Chavez et al. (1989) with deepening of the 14
C isotherm
(from 80 m to 100–175 m) by reduction in 60-m nitrate (from 25 to 14–22 M) and by
a decrease in new, nitrate-based primary production rate (by about 10 g C m
−2 day
−1 ).
Curiously, the rate of upwelling at the coast may even increase during ENSO periods
because the coastal wind system is enhanced by lessened cloud cover even as the offshore
trades, which regulate the depth of the thermocline, are at their weakest: what is upwelled
to the surface is, of course, warm tropical or subtropical surface water, already fully
depleted of nutrients, rather than nutrient-replete subthermocline water.
A multivariate ENSO index has been devised recently by Wolff et al. (2003) that
brings together six variables: sea-level pressure, components of wind direction, SST, air
temperature, and cloudiness. Positive values of this index represent the warm ENSO
phase. Comparing the seven strongest such events since 1950, it is found that three
events between 1957 and 1973 each featured an early warming in the western Pacific
and matured during their first year. Three events between 1982 and 1992 took longer
to develop and matured only in spring of their second year. The unusual 1997–98 event
developed very fast, and within only 50 days had major impact at depths down to 150 m,
and in relative temperature increase in the HUMB province.
The consequences of the 20-fold decrease in productivity and phytoplankton biomass
that may occur during El Niño events are catastrophic and cascade all the way from
the phytoplankton through herbivores to the disappearance of pelagic fish stocks and
great perturbations in the fishmeal trade and in soybean futures. The details of this
collapse have been sufficiently described that I do not need to provide a litany of loss
of reproduction in seabirds, starvation of marine mammals, and the disruption of the
hake, jack mackerel, shrimp, and sardine fisheries because the changed distribution of
the stocks put them beyond the traditional ambit of the fishery, whereas the Peruvian
anchoveta stock, already stressed by heavy exploitation, may collapse.
This region lacks a CalCOFI-like data base so that relatively little certitude has emerged
from studies of the pelagic ecosystem here; instead, what we must work with are fishery
landings, conflicting inferences, best guesses, and insights. Some things have become
clearer in recent years, however, despite these difficulties: the different manner in which
sardine (Sardinops sagax) and anchoveta (Engraulis ringers) populations react to the same
environmental forcing seems to hold the key for understanding changes in their stock
abundance. Some interaction also between these and the pelagic predators Trachurus
trechae (horse mackerel) and Scomber japonicus (mackerel) is now seen to contribute to
interspecific balances.
It is clear that here, as off California, anchoveta is the dominant planktivorous species,
and that the population is subject to natural fluctuations: the stock appears to be under