Distribution and Abundance of Planktonic Amphipods in the Indian Ocean
355
contributed by 2 collections which sampled through a dense patch of amphipods, while
for the latter area night samples were generally lower.
Longitudinal variations show their highest peak between 40° - 50° E for both seasons
(Figs. 8 and 9). Other peaks occur between 90° -100° E (Fig. 9) and 110° -120° E
(Fig. 8) and are due to the areas of upwelling off the Somali coast, and rich areas in the
Bay of Bengal and off south of Java. During the southern summer, minimum values are
noted between 20° and 30° E for both day and night, while the same area shows fairly
high values during the southern winter (Fig. 9), the low-density areas being shifted to
30° _40 ° E. These 2 areas are under the dominance of the Agulhas Current and the
changes may be related to seasonal displacement of the areas of upwelling in this area.
Night values were slightly lower than day values (Fig. 9) between 30 ° -40 ° E,
40° - 50° E, 60° -70° E and 80° - 90 ° E; this may be due to chance.
Table 1 shows the percentage composition by families represented in the catches and
the number of stations from which they were collected. The family Hyperiidae contributed
the most individuals and Dairellidae the least, being recorded from only 2 stations at
17° 30'N, 68° 00' E (central Arabia Sea) and 06° 51' N, 75° 02' E (to the southwest of
Cape Comorin). The family comprises only one genus of 2 species and of these only
Dairella latissima had been recorded from the Indian Ocean (BARNARD, 1937).
III. Discussion
The total data, irrespective of seasons and times of collection, show a higher concentration of pelagic amphipods towards the northern part of the Indian Ocean, especially
in areas enriched by upwelling and land drainage, or regions adjacent to them. Pelagic
amphipods were thought to be more abundant in high latitudes (BOWMAN, 1960) but our
data indicate high concentrations towards the tropical and sub-tropical regions as well,
associated with highly productive waters.
The overall picture of distribution and abundance of pelagic amphipods (Fig. 5) on the
scale ofthe IOBC maps (1970 a, b) closely resembles that of copepods and fish larvae with
minor variations here and there. Amphipods are abundant (> 501) in the northern part
of the Arabian Sea, whereas the copepod maxima extend south to the Gulf of Aden and
along the Somali coast. The high values of copepods over a wide area may be due to their
varied feeding habits. Fairly high concentrations of amphipods noted towards the
northern part of the Bay of Bengal also coincide with the great abundance of copepods and
decapod larvae. Areas of minimum concentration of copepods also show low values of
amphipods.
A comparison of amphipod distribution in relation to zooplankton hiomass distribution (lOBC, 1968 a, b), in the Indian Ocean for the whole year reveals thatthepeaks of both
are located towards the northern part of the Arabian Sea and head of the Bay of Bengal.
However, at the station level no clear correlation was found between the areas of high
primary production and the areas of abundance of amphipods (see maps on the inside of
the front and back cover).
Studies of the gut content of members of the family Phronimidae show that they are
omnivorous (SHIH, 1969). This is probably true for other hyperiid amphipods as well.
Young hyperiids generally occur towards the surface, probably due to the abundance of
food in the surface layers. Vertical migrations are known in hyperiid amphipods (SHIH,
355
contributed by 2 collections which sampled through a dense patch of amphipods, while
for the latter area night samples were generally lower.
Longitudinal variations show their highest peak between 40° - 50° E for both seasons
(Figs. 8 and 9). Other peaks occur between 90° -100° E (Fig. 9) and 110° -120° E
(Fig. 8) and are due to the areas of upwelling off the Somali coast, and rich areas in the
Bay of Bengal and off south of Java. During the southern summer, minimum values are
noted between 20° and 30° E for both day and night, while the same area shows fairly
high values during the southern winter (Fig. 9), the low-density areas being shifted to
30° _40 ° E. These 2 areas are under the dominance of the Agulhas Current and the
changes may be related to seasonal displacement of the areas of upwelling in this area.
Night values were slightly lower than day values (Fig. 9) between 30 ° -40 ° E,
40° - 50° E, 60° -70° E and 80° - 90 ° E; this may be due to chance.
Table 1 shows the percentage composition by families represented in the catches and
the number of stations from which they were collected. The family Hyperiidae contributed
the most individuals and Dairellidae the least, being recorded from only 2 stations at
17° 30'N, 68° 00' E (central Arabia Sea) and 06° 51' N, 75° 02' E (to the southwest of
Cape Comorin). The family comprises only one genus of 2 species and of these only
Dairella latissima had been recorded from the Indian Ocean (BARNARD, 1937).
III. Discussion
The total data, irrespective of seasons and times of collection, show a higher concentration of pelagic amphipods towards the northern part of the Indian Ocean, especially
in areas enriched by upwelling and land drainage, or regions adjacent to them. Pelagic
amphipods were thought to be more abundant in high latitudes (BOWMAN, 1960) but our
data indicate high concentrations towards the tropical and sub-tropical regions as well,
associated with highly productive waters.
The overall picture of distribution and abundance of pelagic amphipods (Fig. 5) on the
scale ofthe IOBC maps (1970 a, b) closely resembles that of copepods and fish larvae with
minor variations here and there. Amphipods are abundant (> 501) in the northern part
of the Arabian Sea, whereas the copepod maxima extend south to the Gulf of Aden and
along the Somali coast. The high values of copepods over a wide area may be due to their
varied feeding habits. Fairly high concentrations of amphipods noted towards the
northern part of the Bay of Bengal also coincide with the great abundance of copepods and
decapod larvae. Areas of minimum concentration of copepods also show low values of
amphipods.
A comparison of amphipod distribution in relation to zooplankton hiomass distribution (lOBC, 1968 a, b), in the Indian Ocean for the whole year reveals thatthepeaks of both
are located towards the northern part of the Arabian Sea and head of the Bay of Bengal.
However, at the station level no clear correlation was found between the areas of high
primary production and the areas of abundance of amphipods (see maps on the inside of
the front and back cover).
Studies of the gut content of members of the family Phronimidae show that they are
omnivorous (SHIH, 1969). This is probably true for other hyperiid amphipods as well.
Young hyperiids generally occur towards the surface, probably due to the abundance of
food in the surface layers. Vertical migrations are known in hyperiid amphipods (SHIH,