348
Chapter 11: The Pacific Ocean
was internally driven. However, this sequence may equally well have been imposed by
mortality due to consumption by sardines: Sardinops melanostictus was involved in the
Pacific-wide population explosion of these fish that occurred between 1970 and 1998,
when catches rose from very small numbers to 1.5 million tons. This was the latest in
a series of major shifts in population strength of this species. I note that the period of
peak sardine catches, 1985–1990, exactly matches that of greatest decline in large copepod
biomass in the Kuroshio. This species spawns preferentially in the path of the Tsushima
Current off southern Honshu and in the eastern Sea of Japan.
The annual and decadal shifts in the core flow of the Kuroshio off the east coast
of Honshu have long been known to have important biological consequences that can
be traced through their effects on the recruitment of the small clupeid fish that collectively comprise the Iwashi fishery: principally Sardinops melanostictus, Engraulis japonicus,
and Etrumeus micropus. Populations of these three planktivorous fish have undergone
major fluctuations since the historical fishery was started in the 1500s and, as elsewhere, their relative abundances do not change simultaneously, or in the same sense.
The post-1970 upswing in sardine population size represented a return to the large
population that produced catches of about 11 × 10
6 tons in the 1930s, reduced to very
small population size subsequently. Sardinops is a typical sardine in its food requirements, being able to eat diatoms in the larval and postlarval stages but not to subsist
on them later: progressively larger zooplankton is utilized during its growth. Engraulis
can utilize much smaller plankton organisms by active gill-raker filtration. It has long
been known that these two biota tend to be mutually exclusive: many sardine and few
anchovies, or vice versa. It has also long been known that a major population of sardines would only occur when warm oceanographic conditions obtained on the coast of
Honshu.
The path taken by the Kuroshio loop off Honshu appears to control the relative
abundance of species of pelagic fish. During the 1964–1971 period of rapidly declining
Sardinops abundance, the Kuroshio meander to the southeast of Honshu formed a strong
arc, enclosing a cold cyclonic eddy at the coast where conditions for the survival of
sardine larvae were poor during almost a decade. Related to these observations is the fact
that the conjunction between warm Kuroshio and cold subarctic water is also a boundary
zone for the distributions of many pelagic species; the same must occur at the north wall
of the Gulf Stream off eastern Canada. However, off Japan, the regional cultural interest
in pelagic fisheries has led to a much greater knowledge of the distribution of pelagic
invertebrates than in the Atlantic. For instance, maps of the distribution of the eggs and
larvae of the Pacific saury Cololabris saura show that this species is restricted strictly to
the warm Kuroshio water. Similarly, two species of chaetognath show specialization to
cold and warm water: Pterosagitta draco in Kuroshio water and Sagitta nagae in coastal
water.
Synopsis
Case 2—Nutrient-limited spring production peak—The pycnocline undergoes boreal winter
excursion (10–15 m June–September, 120 m February–March) and so the thermocline
is illuminated for a relatively long period in boreal summer from May to October.
Productivity increases in February and reaches its annual maximum in May, followed
by a steep decline (nutrient-limited?) in June as Z m shoals above Z eu , leading to an
early seasonal minimum sustained from September to January (Fig. 11.5). Chlorophyll
biomass consistently fails to track P rate in the second half of the year, even though most
accumulation does occur in the period of maximum P-rate increase (February–April).
Relations between P and chlorophyll are inconsistent with close matching of production
and consumption. There is a strong apparent effect of seasonal vertical migrant biomass
descending to overwintering depths.
Chapter 11: The Pacific Ocean
was internally driven. However, this sequence may equally well have been imposed by
mortality due to consumption by sardines: Sardinops melanostictus was involved in the
Pacific-wide population explosion of these fish that occurred between 1970 and 1998,
when catches rose from very small numbers to 1.5 million tons. This was the latest in
a series of major shifts in population strength of this species. I note that the period of
peak sardine catches, 1985–1990, exactly matches that of greatest decline in large copepod
biomass in the Kuroshio. This species spawns preferentially in the path of the Tsushima
Current off southern Honshu and in the eastern Sea of Japan.
The annual and decadal shifts in the core flow of the Kuroshio off the east coast
of Honshu have long been known to have important biological consequences that can
be traced through their effects on the recruitment of the small clupeid fish that collectively comprise the Iwashi fishery: principally Sardinops melanostictus, Engraulis japonicus,
and Etrumeus micropus. Populations of these three planktivorous fish have undergone
major fluctuations since the historical fishery was started in the 1500s and, as elsewhere, their relative abundances do not change simultaneously, or in the same sense.
The post-1970 upswing in sardine population size represented a return to the large
population that produced catches of about 11 × 10
6 tons in the 1930s, reduced to very
small population size subsequently. Sardinops is a typical sardine in its food requirements, being able to eat diatoms in the larval and postlarval stages but not to subsist
on them later: progressively larger zooplankton is utilized during its growth. Engraulis
can utilize much smaller plankton organisms by active gill-raker filtration. It has long
been known that these two biota tend to be mutually exclusive: many sardine and few
anchovies, or vice versa. It has also long been known that a major population of sardines would only occur when warm oceanographic conditions obtained on the coast of
Honshu.
The path taken by the Kuroshio loop off Honshu appears to control the relative
abundance of species of pelagic fish. During the 1964–1971 period of rapidly declining
Sardinops abundance, the Kuroshio meander to the southeast of Honshu formed a strong
arc, enclosing a cold cyclonic eddy at the coast where conditions for the survival of
sardine larvae were poor during almost a decade. Related to these observations is the fact
that the conjunction between warm Kuroshio and cold subarctic water is also a boundary
zone for the distributions of many pelagic species; the same must occur at the north wall
of the Gulf Stream off eastern Canada. However, off Japan, the regional cultural interest
in pelagic fisheries has led to a much greater knowledge of the distribution of pelagic
invertebrates than in the Atlantic. For instance, maps of the distribution of the eggs and
larvae of the Pacific saury Cololabris saura show that this species is restricted strictly to
the warm Kuroshio water. Similarly, two species of chaetognath show specialization to
cold and warm water: Pterosagitta draco in Kuroshio water and Sagitta nagae in coastal
water.
Synopsis
Case 2—Nutrient-limited spring production peak—The pycnocline undergoes boreal winter
excursion (10–15 m June–September, 120 m February–March) and so the thermocline
is illuminated for a relatively long period in boreal summer from May to October.
Productivity increases in February and reaches its annual maximum in May, followed
by a steep decline (nutrient-limited?) in June as Z m shoals above Z eu , leading to an
early seasonal minimum sustained from September to January (Fig. 11.5). Chlorophyll
biomass consistently fails to track P rate in the second half of the year, even though most
accumulation does occur in the period of maximum P-rate increase (February–April).
Relations between P and chlorophyll are inconsistent with close matching of production
and consumption. There is a strong apparent effect of seasonal vertical migrant biomass
descending to overwintering depths.