Indian Ocean Coastal Biome
303
investigations did make some preliminary assessment of the consumption of copepods by
myctophid fish, for which they form the principal item of diet; it has been suggested that
this province harbors the largest single stock of mesopelagic fish anywhere in the oceans.
It is likely that the dominant species (Diaphus arabicus and Benthosema pterotum) are
major predators of Calanoides and the other large copepods. Their biomass is relatively
invariant through the year, but with some increase in summer months (<400 mg C m
−2 ),
and they have an annual life cycle. These fish feed principally at night and so probably fill
their guts once each 24 hours. Their computed daily consumption is quite close to the
potential daily growth of zooplankton, assuming a growth rate of 24% d
−1 for the latter.
They are themselves probably a major component of the diet of the yellowfin tuna that
migrate to the coastal region of Oman, while being essentially absent in the oligotrophic
regions of the Arabian Sea (Fontenau, 1997).
It is also curious that the accumulated literature appears to be quite silent on the place
of shoaling clupeid fish in this upwelling province, a silence all the stranger because of
the abundant studies and documentation of the role of sardine and anchovies in other
upwelling provinces: California, Benguela and so on. Raja (1969) records the presence
of the oil sardine (Sardinella longiceps) from central Somalia eastward into the Bay of
Bengal. I can personally vouch for its presence at Aden, because I watched them feeding
from the deck of a troop ship in 1946. This species is unusual in its habit of swimming
with its mouth agape, filter-feeding on large diatoms, resembling an animated plankton
net. It will be discussed in the account of the INDW province because it is very abundant
there, but FAO fisheries statistics suggest only very small catches west of Pakistan. It
may well be, as has been suggested, that a lack of markets and of fishing traditions
along the Omani and Somali coasts conceals its true abundance. It would certainly be
very surprising if, as would be deduced from evidence presently available, this upwelling
region were to prove to be unique in not being exploited by sardines and anchovies
en masse.
Synopsis
Case 5—Large-amplitude response to monsoonlike reversal of trade winds (Fig. 10.5)—From
an intermonsoon depth of 20 m, the mixed layer undergoes two weak excursions, to 50 m
by the seasonal cooling of the surface water mass in the NE Monsoon (January–February)
and to 35 m by spin-up of Arabian Sea gyre in the SW Monsoon (July–August). Z eu
is related to chlorophyll seasonality (50 m April–June, 30 m September), so thermocline
illumination occurs principally during the intermonsoon period of March–July. The
seasonal cycle of P is dominated by the massive response to coastal upwelling during the
SW Monsoon with a sustained rate increase from the intermonsoon minimum (May,
800 mgC m
−2 d
−1 ) to the brief annual maximum (September, 1500 mgC m
−2 d
−1 ). There
is a second rate increase (to 1200 mgC m
−2 d
−1
) during the NE Monsoon and when
winter cooling occurs in the north of the province. Chlorophyll biomass tracks P rate,
accumulating 10 mgC m
−3 (NE Monsoon) to 19 mgC m
−3 (SW Monsoon). This cycle
is consistent with a close match between consumer biomass and production rate, and
also with the ascent of seasonal vertical migrant herbivores at the onset of the SW
Monsoon.
Western India Coastal Province (INDW)
Extent of the Province
The INDW province extends from the mouth of the Indus at 25
N to the Gulf of Manar
at about 7
N, comprising the continental shelf of western India and southern Pakistan
303
investigations did make some preliminary assessment of the consumption of copepods by
myctophid fish, for which they form the principal item of diet; it has been suggested that
this province harbors the largest single stock of mesopelagic fish anywhere in the oceans.
It is likely that the dominant species (Diaphus arabicus and Benthosema pterotum) are
major predators of Calanoides and the other large copepods. Their biomass is relatively
invariant through the year, but with some increase in summer months (<400 mg C m
−2 ),
and they have an annual life cycle. These fish feed principally at night and so probably fill
their guts once each 24 hours. Their computed daily consumption is quite close to the
potential daily growth of zooplankton, assuming a growth rate of 24% d
−1 for the latter.
They are themselves probably a major component of the diet of the yellowfin tuna that
migrate to the coastal region of Oman, while being essentially absent in the oligotrophic
regions of the Arabian Sea (Fontenau, 1997).
It is also curious that the accumulated literature appears to be quite silent on the place
of shoaling clupeid fish in this upwelling province, a silence all the stranger because of
the abundant studies and documentation of the role of sardine and anchovies in other
upwelling provinces: California, Benguela and so on. Raja (1969) records the presence
of the oil sardine (Sardinella longiceps) from central Somalia eastward into the Bay of
Bengal. I can personally vouch for its presence at Aden, because I watched them feeding
from the deck of a troop ship in 1946. This species is unusual in its habit of swimming
with its mouth agape, filter-feeding on large diatoms, resembling an animated plankton
net. It will be discussed in the account of the INDW province because it is very abundant
there, but FAO fisheries statistics suggest only very small catches west of Pakistan. It
may well be, as has been suggested, that a lack of markets and of fishing traditions
along the Omani and Somali coasts conceals its true abundance. It would certainly be
very surprising if, as would be deduced from evidence presently available, this upwelling
region were to prove to be unique in not being exploited by sardines and anchovies
en masse.
Synopsis
Case 5—Large-amplitude response to monsoonlike reversal of trade winds (Fig. 10.5)—From
an intermonsoon depth of 20 m, the mixed layer undergoes two weak excursions, to 50 m
by the seasonal cooling of the surface water mass in the NE Monsoon (January–February)
and to 35 m by spin-up of Arabian Sea gyre in the SW Monsoon (July–August). Z eu
is related to chlorophyll seasonality (50 m April–June, 30 m September), so thermocline
illumination occurs principally during the intermonsoon period of March–July. The
seasonal cycle of P is dominated by the massive response to coastal upwelling during the
SW Monsoon with a sustained rate increase from the intermonsoon minimum (May,
800 mgC m
−2 d
−1 ) to the brief annual maximum (September, 1500 mgC m
−2 d
−1 ). There
is a second rate increase (to 1200 mgC m
−2 d
−1
) during the NE Monsoon and when
winter cooling occurs in the north of the province. Chlorophyll biomass tracks P rate,
accumulating 10 mgC m
−3 (NE Monsoon) to 19 mgC m
−3 (SW Monsoon). This cycle
is consistent with a close match between consumer biomass and production rate, and
also with the ascent of seasonal vertical migrant herbivores at the onset of the SW
Monsoon.
Western India Coastal Province (INDW)
Extent of the Province
The INDW province extends from the mouth of the Indus at 25
N to the Gulf of Manar
at about 7
N, comprising the continental shelf of western India and southern Pakistan