50
R. 1. CURRIE, A. E. FISHER and P. M. HARGREAVES:
(entirely within the cold surface water area) showed the highest values of P04 3 - -P and
chlorophyll a accompanied by an increase in the amount of zooplankton. On section VI
all stations appeared to lie within the warmer "Gulf of Aden" surface water and yet the
values of P04 3 --P and chlorophyll were still comparatively high.
The "Discovery" returned to the area after an interval of about 8 days and found
that very active convection was taking place between the upwelling water and the warmer
"Gulf of Aden" surface water. A cyclonic displacement of the surface isotherms had
taken place in the interval and the boundary between the two water masses had sharpened
considerably. At one station (5557) 2 sets of observations were made while the ship
drifted between the warm and the cold water. The warmer water had a well-mixed layer
at the surface, some 50 m thick, whereas the cold water showed pronounced stability in
this layer. The line of demarcation between the two was clearly visible because of a tenfold
difference in chlorophyll a between the 2 water masses. A considerable degree of mixing
must have been taking place between the upwelled water and the adjacent flow of "Gulf
of Aden" surface water.
II. The Significance of the Upwelling to Fisheries
It is not practicable for an oceanographical expedition to obtain any proper evaluation
of the abundance of fish in a region, and statistical data from the Arabian Sea area under
consideration are so sparse that little guidance can be derived from that source.
During these cruises of RRS "Discovery", however, continuous echo-soundings were
taken with a 10 kc sec- 1 echo-sounder. These showed the presence of diffuse midwater
acoustic scattering, single mid-water targets and discrete fish shoals. The fish shoals
which almost invariably occurred in the 110 - 310 m layer could be counted and it was
felt that the occurrence of these shoals might give some indication of the effect of the
upwelling off the Arabian and Somali coasts on the abundance of fish .
1. Characteristics of Shoals and Assessment of their Abundance
The beam of the precision echo-sounder used can be likened to a cone some 28
0
wide,
and the fish shoals show up on the records as typically crescent-shaped marks (Fig. 13).
While these vary in size by a factor of 2 or 3, and in the denser regions may even coalesce,
simple enumeration at least provides a crude measure of fish abundance. While the
records were being taken, the ship's speed varied from zero to a maximum of 11-12
knots. Therefore, in the first place the counts which are a function of depth and time
on the echo-sounder records had to be converted into a sensible measure of their spatial
abundance. This was done by dividing the records into 5-minute periods and counting
all the identifia:ble shoals. Knowing the speed of the ship over the ground, the counts
could then be expressed in terms of their geographical abundance, by dividing the average
rate of incidence of fish shoals per hour by the ship's speed over the ground determined
from the appropriate part of the ship's track chart. The resultant figures represent the
concentration of fish shoals per mile of the ship's track.
Frequently, the echo-sounder transmission was gated, generally when weather conditions or the nature of the sea bed were such that bottom soundings became difficult to
record. This leads to a record on which it is impossible to identify fish shoals and so the
corresponding parts of the ship's track when the sounder was gated had to be distin-
R. 1. CURRIE, A. E. FISHER and P. M. HARGREAVES:
(entirely within the cold surface water area) showed the highest values of P04 3 - -P and
chlorophyll a accompanied by an increase in the amount of zooplankton. On section VI
all stations appeared to lie within the warmer "Gulf of Aden" surface water and yet the
values of P04 3 --P and chlorophyll were still comparatively high.
The "Discovery" returned to the area after an interval of about 8 days and found
that very active convection was taking place between the upwelling water and the warmer
"Gulf of Aden" surface water. A cyclonic displacement of the surface isotherms had
taken place in the interval and the boundary between the two water masses had sharpened
considerably. At one station (5557) 2 sets of observations were made while the ship
drifted between the warm and the cold water. The warmer water had a well-mixed layer
at the surface, some 50 m thick, whereas the cold water showed pronounced stability in
this layer. The line of demarcation between the two was clearly visible because of a tenfold
difference in chlorophyll a between the 2 water masses. A considerable degree of mixing
must have been taking place between the upwelled water and the adjacent flow of "Gulf
of Aden" surface water.
II. The Significance of the Upwelling to Fisheries
It is not practicable for an oceanographical expedition to obtain any proper evaluation
of the abundance of fish in a region, and statistical data from the Arabian Sea area under
consideration are so sparse that little guidance can be derived from that source.
During these cruises of RRS "Discovery", however, continuous echo-soundings were
taken with a 10 kc sec- 1 echo-sounder. These showed the presence of diffuse midwater
acoustic scattering, single mid-water targets and discrete fish shoals. The fish shoals
which almost invariably occurred in the 110 - 310 m layer could be counted and it was
felt that the occurrence of these shoals might give some indication of the effect of the
upwelling off the Arabian and Somali coasts on the abundance of fish .
1. Characteristics of Shoals and Assessment of their Abundance
The beam of the precision echo-sounder used can be likened to a cone some 28
0
wide,
and the fish shoals show up on the records as typically crescent-shaped marks (Fig. 13).
While these vary in size by a factor of 2 or 3, and in the denser regions may even coalesce,
simple enumeration at least provides a crude measure of fish abundance. While the
records were being taken, the ship's speed varied from zero to a maximum of 11-12
knots. Therefore, in the first place the counts which are a function of depth and time
on the echo-sounder records had to be converted into a sensible measure of their spatial
abundance. This was done by dividing the records into 5-minute periods and counting
all the identifia:ble shoals. Knowing the speed of the ship over the ground, the counts
could then be expressed in terms of their geographical abundance, by dividing the average
rate of incidence of fish shoals per hour by the ship's speed over the ground determined
from the appropriate part of the ship's track chart. The resultant figures represent the
concentration of fish shoals per mile of the ship's track.
Frequently, the echo-sounder transmission was gated, generally when weather conditions or the nature of the sea bed were such that bottom soundings became difficult to
record. This leads to a record on which it is impossible to identify fish shoals and so the
corresponding parts of the ship's track when the sounder was gated had to be distin-