402
Chapter 11: The Pacific Ocean
the basin (Osgood and Checkley, 1997). This situation resembles that of C. finmarchicus
in the deep basins of the Scotia Shelf in the Northwest Atlantic Shelves Province.
A few species of diatoms (Coscinodiscus, Nitzschia, and Tripodonesis) may form >80%
of large algal cell volume in these upwelling cells; as well as by large copepods, these
diatoms are utilized by a very unusual organism: the bright red, swimming, galatheid
crab Pleuroncodes planipes. In their pelagic phase, these crowd into the surface layer off
Baja California where they tail-flip up to the surface and then parachute down again with
outstretched legs, filtering actively with their maxillipeds; this is a remarkable sight against
the rich olive-green upwelled water. These crabs (at one per 3 m
−3 , each capable of clearing
diatoms from 3–4 liter
−1 hr
−1 ) may comprise 90% of the total zooplankton/nekton
biomass in upwelling cells and contribute 85% of all zooplankton/nekton grazing pressure.
Pleuroncodes is directly preyed on, and a preferred food of, yellowfin tuna in the same
region, so this is a remarkably direct link from diatoms to your table.
The three-dimensional differential distribution of the five most abundant copepod
species was used by Peterson et al. (1979) to clarify how these organisms exploit
water movement during upwelling off Oregon. Over a very narrow continental shelf
(the 100-m isobath is only 10 km from the coast) the pycnocline lies at 20–50 m, sloping up toward the beach: in an upwelling episode the pycnocline intersects the surface
5–10 km seaward and nitrate-replete deep water is brought to the surface. Within this
system each copepod has a narrowly defined and specialized distribution, in which it is
maintained by details of the circulation pattern and its reproductive behavior: Acartia
clausii is restricted to the upper 5–10 m and within 5 km of the shore; Acartia longiremis
occurs 10 km offshore and similarly near the surface; Pseudocalanus occurs out to 15 km
from shore but also only within the pycnocline at 10–20 m; Oithona similis occurs at
similar depths but not in the first 10 km offshore; and C. pacificus has wider ranges for
both depth and distance offshore. Off California the endemic Calanus has a life history
like that of Calanoides elsewhere; during winter and other periods when upwelling is not
active, it descends to 400–600 m as a population of C5s and remains dormant in the
oxygen minimum layer that underlies the California Current.
The California Current is, of course, home to what is perhaps the best known of all
fluctuating fishery resources: sardines and anchovies. This is not the place for a major
discourse on this classical case, the outlines of which have become clearer in recent
years after decades of confusion. The collapse of the great California sardine (Sardinops
sagax) fishery in 1950, and the subsequent building of an equally large stock of anchovies
(Engraulis mordax) that likewise crashed in 1990, is too well known to require retelling.
A similar pattern of relative abundance has occurred between sardines and anchovies
off Japan, Peru, and South Africa, and competitive population dynamics, forced by
environmental change, is the explanation that comes most readily to mind.
Unfortunately, it is not so simple as that and the relative consequences of fisheryinduced changes in population structure and environmental forcing have not yet been
satisfactorily untangled. The evidence of scales counted from unperturbed, varved cores
in the Santa Barbara basin off Southern California shows that such changes in abundance
have occurred naturally throughout the last 1700 years. But, and it’s a large “but,” there
is no evidence whatever of the short-term alternation observed in modern data; instead,
both species fluctuate in abundance at a 60-year period, with an additional 100-year
frequency for anchovies—whose biomass, during these 1700 years, was approximately
three times that of sardines (Baumgartner et al. 1992). Sardine biomass appears to have
been significantly more variable than that of anchovies, and some spectacular episodes
of very high abundance were recorded in the core samples. Thus, in the 16th century,
over a period of around 25 years, sardine biomass reached 15 million tonnes, an order
of magnitude greater than otherwise occurred in the 400 years from the 14th to the 18th
centuries. At present, we have no way of knowing to what extent the pattern observed
Chapter 11: The Pacific Ocean
the basin (Osgood and Checkley, 1997). This situation resembles that of C. finmarchicus
in the deep basins of the Scotia Shelf in the Northwest Atlantic Shelves Province.
A few species of diatoms (Coscinodiscus, Nitzschia, and Tripodonesis) may form >80%
of large algal cell volume in these upwelling cells; as well as by large copepods, these
diatoms are utilized by a very unusual organism: the bright red, swimming, galatheid
crab Pleuroncodes planipes. In their pelagic phase, these crowd into the surface layer off
Baja California where they tail-flip up to the surface and then parachute down again with
outstretched legs, filtering actively with their maxillipeds; this is a remarkable sight against
the rich olive-green upwelled water. These crabs (at one per 3 m
−3 , each capable of clearing
diatoms from 3–4 liter
−1 hr
−1 ) may comprise 90% of the total zooplankton/nekton
biomass in upwelling cells and contribute 85% of all zooplankton/nekton grazing pressure.
Pleuroncodes is directly preyed on, and a preferred food of, yellowfin tuna in the same
region, so this is a remarkably direct link from diatoms to your table.
The three-dimensional differential distribution of the five most abundant copepod
species was used by Peterson et al. (1979) to clarify how these organisms exploit
water movement during upwelling off Oregon. Over a very narrow continental shelf
(the 100-m isobath is only 10 km from the coast) the pycnocline lies at 20–50 m, sloping up toward the beach: in an upwelling episode the pycnocline intersects the surface
5–10 km seaward and nitrate-replete deep water is brought to the surface. Within this
system each copepod has a narrowly defined and specialized distribution, in which it is
maintained by details of the circulation pattern and its reproductive behavior: Acartia
clausii is restricted to the upper 5–10 m and within 5 km of the shore; Acartia longiremis
occurs 10 km offshore and similarly near the surface; Pseudocalanus occurs out to 15 km
from shore but also only within the pycnocline at 10–20 m; Oithona similis occurs at
similar depths but not in the first 10 km offshore; and C. pacificus has wider ranges for
both depth and distance offshore. Off California the endemic Calanus has a life history
like that of Calanoides elsewhere; during winter and other periods when upwelling is not
active, it descends to 400–600 m as a population of C5s and remains dormant in the
oxygen minimum layer that underlies the California Current.
The California Current is, of course, home to what is perhaps the best known of all
fluctuating fishery resources: sardines and anchovies. This is not the place for a major
discourse on this classical case, the outlines of which have become clearer in recent
years after decades of confusion. The collapse of the great California sardine (Sardinops
sagax) fishery in 1950, and the subsequent building of an equally large stock of anchovies
(Engraulis mordax) that likewise crashed in 1990, is too well known to require retelling.
A similar pattern of relative abundance has occurred between sardines and anchovies
off Japan, Peru, and South Africa, and competitive population dynamics, forced by
environmental change, is the explanation that comes most readily to mind.
Unfortunately, it is not so simple as that and the relative consequences of fisheryinduced changes in population structure and environmental forcing have not yet been
satisfactorily untangled. The evidence of scales counted from unperturbed, varved cores
in the Santa Barbara basin off Southern California shows that such changes in abundance
have occurred naturally throughout the last 1700 years. But, and it’s a large “but,” there
is no evidence whatever of the short-term alternation observed in modern data; instead,
both species fluctuate in abundance at a 60-year period, with an additional 100-year
frequency for anchovies—whose biomass, during these 1700 years, was approximately
three times that of sardines (Baumgartner et al. 1992). Sardine biomass appears to have
been significantly more variable than that of anchovies, and some spectacular episodes
of very high abundance were recorded in the core samples. Thus, in the 16th century,
over a period of around 25 years, sardine biomass reached 15 million tonnes, an order
of magnitude greater than otherwise occurred in the 400 years from the 14th to the 18th
centuries. At present, we have no way of knowing to what extent the pattern observed