Seasonal Studies of a Pelagic Ecosystem (Meridian 110° E)
511
A conspicuous feature of the distribution of these fish is their tendency to drift long
distances from one latitude to another in synchrony with their breeding habits. A conspicuous example is the myctophid Scope/opsis multipunctatus. The adults spawn in the
subtropics in mid-winter, and the young develop gonads in the tropics about 9 months
later (May). Gonostoma? rhodadenia, on the other hand, spawns in the tropic zone.
It is a fairly general characteristic that, whatever the distributional focus of the
species, the displacement from south to north takes place between October and March,
and the displacement towards the south between April and September. This raises the
question whether there is some mass transport of water with such a pattern.
No observations were made at higher trophic levels than micronekton during the
present study. However there is information about pelagic schools of southern bluefin
tuna (Thunnus thynnus maccoyii) and sperm whales (Physeter catodon) in the general
study area. Sperm whales feed principally on squid (CHITILEBOROUGH, personal communication). Tuna feed on squid and smaller carnivores (ROBINS, 1963), though not
necessarily the same components taken with the midwater trawl (KING and IVERSON,
1962).
The southern bluefin tuna is a cool-water species living mainly in the West Wind
Drift (SHINGU, 1967). The species makes a remarkable northward migration into warm
water to spawn in the waters south of Indonesia. On their way to and from their spawning
grounds, the populations aggregate in two particular areas that have come to be known
as the "Oka" and "Oki" fishing grounds (MIMURA and WARASHlNA, 1962). The first lies
to the north of 20° S. and extends from our study section towards the east. The second
lies to the south fo 20° S. and extends from our study section to the west. Sperm whale
records spanning nearly 200 years (TOWNSEND, 1935) show that these are the same
areas where that species also aggregates. The first area lies within our tropical zone, the
second within our subtropical zone.
Of the 2 tuna fishing grounds, the richer are the subtropical grounds, the principal
fishing seasons being September-October (northward migration) and January- February
(return migration) (KIKAWA, 1964; WARASHINA and HISADA, 1970). The annual catch
on the subtropical grounds in 1960 - 61 (immediately before our study period) was
40 000 tons. Taken over an area 10 degrees square, this is equivalent to approximately
4x 10- 3 g C m- 2 year-I. This is an order of magnitude greater than would be expected
on the basis of the endemic primary production assuming a 6-stage food chain with
transfer coefficients of 10- 1 per link.
These seasonal concentrations of sperm whales and tuna bear such a striking similarity
to the seasonal distribution of micronekton (Fig. 10), that one can safely conclude they
are all part of the same pelagic food chain. In general the nekton concentration occurs
immediately before the micro nekton maximum. It is important to remember that there is
a connecting link for which we have no observations, viz. pelagic squid.
VI. Species Diversity
Species diversity is an important parameter of the ecosystem (MACARTHUR, 1965;
MARGALEF, 1968). It is a measure of ecological structure. Systems with high diversity
have more trophic pathways through which energy may flow (MACARTHUR, 1955).
There is a food web rather than a food chain. MARGALEF (1968) believes that diversity
is also an index of maturity, the mature ecosystem producing less entropy than the
511
A conspicuous feature of the distribution of these fish is their tendency to drift long
distances from one latitude to another in synchrony with their breeding habits. A conspicuous example is the myctophid Scope/opsis multipunctatus. The adults spawn in the
subtropics in mid-winter, and the young develop gonads in the tropics about 9 months
later (May). Gonostoma? rhodadenia, on the other hand, spawns in the tropic zone.
It is a fairly general characteristic that, whatever the distributional focus of the
species, the displacement from south to north takes place between October and March,
and the displacement towards the south between April and September. This raises the
question whether there is some mass transport of water with such a pattern.
No observations were made at higher trophic levels than micronekton during the
present study. However there is information about pelagic schools of southern bluefin
tuna (Thunnus thynnus maccoyii) and sperm whales (Physeter catodon) in the general
study area. Sperm whales feed principally on squid (CHITILEBOROUGH, personal communication). Tuna feed on squid and smaller carnivores (ROBINS, 1963), though not
necessarily the same components taken with the midwater trawl (KING and IVERSON,
1962).
The southern bluefin tuna is a cool-water species living mainly in the West Wind
Drift (SHINGU, 1967). The species makes a remarkable northward migration into warm
water to spawn in the waters south of Indonesia. On their way to and from their spawning
grounds, the populations aggregate in two particular areas that have come to be known
as the "Oka" and "Oki" fishing grounds (MIMURA and WARASHlNA, 1962). The first lies
to the north of 20° S. and extends from our study section towards the east. The second
lies to the south fo 20° S. and extends from our study section to the west. Sperm whale
records spanning nearly 200 years (TOWNSEND, 1935) show that these are the same
areas where that species also aggregates. The first area lies within our tropical zone, the
second within our subtropical zone.
Of the 2 tuna fishing grounds, the richer are the subtropical grounds, the principal
fishing seasons being September-October (northward migration) and January- February
(return migration) (KIKAWA, 1964; WARASHINA and HISADA, 1970). The annual catch
on the subtropical grounds in 1960 - 61 (immediately before our study period) was
40 000 tons. Taken over an area 10 degrees square, this is equivalent to approximately
4x 10- 3 g C m- 2 year-I. This is an order of magnitude greater than would be expected
on the basis of the endemic primary production assuming a 6-stage food chain with
transfer coefficients of 10- 1 per link.
These seasonal concentrations of sperm whales and tuna bear such a striking similarity
to the seasonal distribution of micronekton (Fig. 10), that one can safely conclude they
are all part of the same pelagic food chain. In general the nekton concentration occurs
immediately before the micro nekton maximum. It is important to remember that there is
a connecting link for which we have no observations, viz. pelagic squid.
VI. Species Diversity
Species diversity is an important parameter of the ecosystem (MACARTHUR, 1965;
MARGALEF, 1968). It is a measure of ecological structure. Systems with high diversity
have more trophic pathways through which energy may flow (MACARTHUR, 1955).
There is a food web rather than a food chain. MARGALEF (1968) believes that diversity
is also an index of maturity, the mature ecosystem producing less entropy than the