Pacific Trade Winds Biome
377
There are therefore, as discussed by Le Borge and Rodier, significant differences in the
functioning of the biological pump in the WARM and PEQD regimes. The active flux,
contributed by diel migrants, represents 40% of the passive flux of sinking POM in the
WARM province and only 9% in PEQD.
As would be anticipated in such clear water, several genera of euphausiids
(e.g., Thysanopoda, Euphausia, and Nematoscelis) migrate between great depths of around
400–500 m by day, and the DCM at 100 m at night (Hirota, 1987). Stylocheiron remains
within the DCM by day and disperses both up and down at night. These results, species
for species, follow the same pattern as those described by Sameoto et al. (1986) and
Brinton (1962) in the eastern tropical Pacific.
An enigma associated with this province is that it should be so productive of tunas of
several species. The distribution of these open-ocean predatory fish is probably better known
globally than that for any other pelagic organisms: modern Japanese, Korean, and U.S. long
liners, bait boats, and purse seiners have efficiently explored all tropical and subtropical
oceans and several international fishery commissions have meticulously recorded their catch
rates. Much better than any comparable ocean basin-scale maps of the distribution of plankton organisms, the maps of the catch rates of skipjack, yellowfin, albacore, and bluefin tuna
give confidence that the distributions they show are real. Even these must, of course, be read
with caution—is the edge of a species distribution natural, or is it the effect of fishery regulations, as occurs in the eastern tropical Pacific, or is it yet the effect of a deepening thermocline
on the efficiency of purse seines, as may occur toward the west of the ocean?
Nevertheless, quite consistently in all Pacific maps of tuna distribution the boundaries
of the WARM province enclose the region of greatest Pacific abundance of skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares), which are the characteristic species
of the trade wind zone (e.g., Bayliff, 1980; Sund et al., 1980). For both these species, and
also for bigeye (Thunnus obesus), the greatest concentrations of larvae occur in WARM,
though they are also widely distributed across the other trade wind biome provinces.
Though adult yellowfin and skipjack are also widely distributed in PNEC, PEQD, and
the warmer parts of South Pacific Subtropical Gyre Province (SPSG) and NPSG, it is in
WARM Province that the most persistent high concentrations occur; for skipjack, this
population center extends seasonally into KURO as far as the Japanese Islands.
There seems to be no simple explanation for the paradox that such anomalously high
concentrations of tuna should occur in such an oligotrophic province. It has been noted
that skipjack occur preferentially in very warm water, but it seems most unlikely that a
difference of <2
C in surface water temperature in this region compared with other Pacific
tropical regions could be physiologically of such advantage as to select for this otherwise
apparently unsuitable area, at least in terms of food supply. I suggest that the explanation
may perhaps lie in the unique character of the province and its extraordinarily strongly
stratified water column with a boundary layer. Such a feature may have two consequences
for tuna: the multiple pycnoclines may serve to aggregate layers of food organisms, thus
simplifying their location by foraging tuna (which are able to tolerate cold temperatures
during very deep hunting forays during daylight hours to at least 300 m), or perhaps the
very stable water column provides invariant and predictable conditions for first-feeding
tuna larvae. The latter suggestion is made in the light of well-known studies of the
differential survival of fish larvae when their prey abundance matches their hatching date
and when concentrated layers of prey organisms are disrupted by wind-mixing episodes.
Synopsis
Case 4—Small-amplitude response to trade-wind seasonality—The integrated climatological
data illustrate this anomalous case in which the photic depth coincides rather closely with
the thermocline, but the halocline lies shoaler; all are seasonally invariant (Fig. 11.13).
377
There are therefore, as discussed by Le Borge and Rodier, significant differences in the
functioning of the biological pump in the WARM and PEQD regimes. The active flux,
contributed by diel migrants, represents 40% of the passive flux of sinking POM in the
WARM province and only 9% in PEQD.
As would be anticipated in such clear water, several genera of euphausiids
(e.g., Thysanopoda, Euphausia, and Nematoscelis) migrate between great depths of around
400–500 m by day, and the DCM at 100 m at night (Hirota, 1987). Stylocheiron remains
within the DCM by day and disperses both up and down at night. These results, species
for species, follow the same pattern as those described by Sameoto et al. (1986) and
Brinton (1962) in the eastern tropical Pacific.
An enigma associated with this province is that it should be so productive of tunas of
several species. The distribution of these open-ocean predatory fish is probably better known
globally than that for any other pelagic organisms: modern Japanese, Korean, and U.S. long
liners, bait boats, and purse seiners have efficiently explored all tropical and subtropical
oceans and several international fishery commissions have meticulously recorded their catch
rates. Much better than any comparable ocean basin-scale maps of the distribution of plankton organisms, the maps of the catch rates of skipjack, yellowfin, albacore, and bluefin tuna
give confidence that the distributions they show are real. Even these must, of course, be read
with caution—is the edge of a species distribution natural, or is it the effect of fishery regulations, as occurs in the eastern tropical Pacific, or is it yet the effect of a deepening thermocline
on the efficiency of purse seines, as may occur toward the west of the ocean?
Nevertheless, quite consistently in all Pacific maps of tuna distribution the boundaries
of the WARM province enclose the region of greatest Pacific abundance of skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares), which are the characteristic species
of the trade wind zone (e.g., Bayliff, 1980; Sund et al., 1980). For both these species, and
also for bigeye (Thunnus obesus), the greatest concentrations of larvae occur in WARM,
though they are also widely distributed across the other trade wind biome provinces.
Though adult yellowfin and skipjack are also widely distributed in PNEC, PEQD, and
the warmer parts of South Pacific Subtropical Gyre Province (SPSG) and NPSG, it is in
WARM Province that the most persistent high concentrations occur; for skipjack, this
population center extends seasonally into KURO as far as the Japanese Islands.
There seems to be no simple explanation for the paradox that such anomalously high
concentrations of tuna should occur in such an oligotrophic province. It has been noted
that skipjack occur preferentially in very warm water, but it seems most unlikely that a
difference of <2
C in surface water temperature in this region compared with other Pacific
tropical regions could be physiologically of such advantage as to select for this otherwise
apparently unsuitable area, at least in terms of food supply. I suggest that the explanation
may perhaps lie in the unique character of the province and its extraordinarily strongly
stratified water column with a boundary layer. Such a feature may have two consequences
for tuna: the multiple pycnoclines may serve to aggregate layers of food organisms, thus
simplifying their location by foraging tuna (which are able to tolerate cold temperatures
during very deep hunting forays during daylight hours to at least 300 m), or perhaps the
very stable water column provides invariant and predictable conditions for first-feeding
tuna larvae. The latter suggestion is made in the light of well-known studies of the
differential survival of fish larvae when their prey abundance matches their hatching date
and when concentrated layers of prey organisms are disrupted by wind-mixing episodes.
Synopsis
Case 4—Small-amplitude response to trade-wind seasonality—The integrated climatological
data illustrate this anomalous case in which the photic depth coincides rather closely with
the thermocline, but the halocline lies shoaler; all are seasonally invariant (Fig. 11.13).