516
D. ]. TRANTER:
The mesopelagic fish populations studied by LEGAND (1967) are a valuable source
of information about horizontal translation. Although these fish (principally myctophids
and stomiatids) are powerful migrants from one stratum to another, their capacity for
horizontal migration is limited, except by way of the prevailing currents. Consequently
as a result of their low reproductive rate (relative to lower trophic levels), their appearance
at fixed observation points in space (stations) is discontinuous, the duration of their
presence being determined by the length of the breeding season of the parent stock
located further upstream (LEGAND, 1967; LEGAND and RIVATON, 1969). Populations
of the myctophid Scopelapsis multipunctatus, for example, appear in sequence along the
study section, at consecutive stages of developmental maturity (LEGAND, 1967; LEGAND
et aI., 1971).
These micronekton populations are the main food of oceanic squid (NESIS, 1970)
which, in turn, are the principal diet of such predatory nekton as sperm whales and
southern bluefin tuna. These large predators are the nomads of the pelagic ecosystem,
maintaining their presence at fixed points in space not by their rapid reproduction as at
low trophic levels, but by their powers of independent movement.
Other components of the pelagic ecosystem whose history is dictated by the forces of
translation rather than succession are the long-lived larvae of eels (leptocephala), rock
lobsters (phyllosoma), and mantis shrimps whose distribution in the eastern Indian Ocean
have been the subject of special studies by CASTLE (1969), CHITTLEBOROUGH and
THOMAS (1969), and MICHEL (1968). Whereas eels spawn in the open ocean, rock
lobsters spawn only near the coast; their larvae are taken first out to sea and then brought
back again some 12 months later. The latter are unique in 3 respects: reproduction of the
species is located at a site peripheral to the pelagic ecosystem of which they form a part;
a proportion of the pelagic larvae must eventually return for the species to survive; and
the duration of larval life is of the order of 12 months. The larvae may be regarded as
upstream "additives" to the ecosystem whose presence or absence at any observation point
downstream is a function of the continuity or discontinuity of the breeding season of the
coastal stocks from which they arise.
The physical requirements for viable recruitment to rock lobster populations are so
unique that spawning of the adults and the subsequent larval drift must surely be
synchronized with seasonal changes in the water movements. Some form of closed
physical system must exist-either a two-way translation with a seasonal frequency, or
a closed gyre with an orbital period of approximately 12 months. It is of considerable
practical and theoretical importance to determine the nature of this physical mechanism.
This is relevant not only to rock lobster recruitment but also to the presence of subtropical concentrations of micronekton and nekton, and to the stability anomalies already
noted.
ROCHFORD (1969) has shown that there are seasonal intrusions of low-salinity water
within the subtropical region in autumn and winter, and seasonal intrusions of highsalinity water within the tropical zone in spring and summer. These results, and those of
NEYMAN, FILJUSHKIN and SCHERBININ(1966) on the prevailing currents, suggest a seasonal
interaction between the waters of the tropical and subtropical zones. MICHEL (1968),
GUEREDRAT (1971), and LEGAND et al. (1971) accept this as the physical mechanism
responsible for the characteristic distribution of the midwater trawl components which
they studied.
Further information is available in the Oceanographic Atlas of the Indian Ocean (WYRTKI,
D. ]. TRANTER:
The mesopelagic fish populations studied by LEGAND (1967) are a valuable source
of information about horizontal translation. Although these fish (principally myctophids
and stomiatids) are powerful migrants from one stratum to another, their capacity for
horizontal migration is limited, except by way of the prevailing currents. Consequently
as a result of their low reproductive rate (relative to lower trophic levels), their appearance
at fixed observation points in space (stations) is discontinuous, the duration of their
presence being determined by the length of the breeding season of the parent stock
located further upstream (LEGAND, 1967; LEGAND and RIVATON, 1969). Populations
of the myctophid Scopelapsis multipunctatus, for example, appear in sequence along the
study section, at consecutive stages of developmental maturity (LEGAND, 1967; LEGAND
et aI., 1971).
These micronekton populations are the main food of oceanic squid (NESIS, 1970)
which, in turn, are the principal diet of such predatory nekton as sperm whales and
southern bluefin tuna. These large predators are the nomads of the pelagic ecosystem,
maintaining their presence at fixed points in space not by their rapid reproduction as at
low trophic levels, but by their powers of independent movement.
Other components of the pelagic ecosystem whose history is dictated by the forces of
translation rather than succession are the long-lived larvae of eels (leptocephala), rock
lobsters (phyllosoma), and mantis shrimps whose distribution in the eastern Indian Ocean
have been the subject of special studies by CASTLE (1969), CHITTLEBOROUGH and
THOMAS (1969), and MICHEL (1968). Whereas eels spawn in the open ocean, rock
lobsters spawn only near the coast; their larvae are taken first out to sea and then brought
back again some 12 months later. The latter are unique in 3 respects: reproduction of the
species is located at a site peripheral to the pelagic ecosystem of which they form a part;
a proportion of the pelagic larvae must eventually return for the species to survive; and
the duration of larval life is of the order of 12 months. The larvae may be regarded as
upstream "additives" to the ecosystem whose presence or absence at any observation point
downstream is a function of the continuity or discontinuity of the breeding season of the
coastal stocks from which they arise.
The physical requirements for viable recruitment to rock lobster populations are so
unique that spawning of the adults and the subsequent larval drift must surely be
synchronized with seasonal changes in the water movements. Some form of closed
physical system must exist-either a two-way translation with a seasonal frequency, or
a closed gyre with an orbital period of approximately 12 months. It is of considerable
practical and theoretical importance to determine the nature of this physical mechanism.
This is relevant not only to rock lobster recruitment but also to the presence of subtropical concentrations of micronekton and nekton, and to the stability anomalies already
noted.
ROCHFORD (1969) has shown that there are seasonal intrusions of low-salinity water
within the subtropical region in autumn and winter, and seasonal intrusions of highsalinity water within the tropical zone in spring and summer. These results, and those of
NEYMAN, FILJUSHKIN and SCHERBININ(1966) on the prevailing currents, suggest a seasonal
interaction between the waters of the tropical and subtropical zones. MICHEL (1968),
GUEREDRAT (1971), and LEGAND et al. (1971) accept this as the physical mechanism
responsible for the characteristic distribution of the midwater trawl components which
they studied.
Further information is available in the Oceanographic Atlas of the Indian Ocean (WYRTKI,