UC San Diego Library – Scripps Digital Collection

The Gulf of Thailand and the South China Sea were each sampled during five cruises of the Naga Expedition, 1959-1961. The following twenty species were present: Cavolina gibbosa, C. globulosa, C. inflexa, C. longirostris, C. uncinata, Clio recurva, C. cuspidata, C. pyramidata, Creseis acicula, C. chierchiae, C. virgula, C. bulgia, Cuvierina columnella, Diacria quadridentata, D. trispinosa, Hyalocylis striata, Limacina bulimoides, L. inflata, L. trochiformis, and Styliola subula. Distribution maps are presented for each species and variations in distribution are related to climatic and hydrographic parameters. Statistical treatment of the data helped to establish recurrent species groups among ten selected species and to relate the distribution of these groups to environmental configurations. Broadly speaking, there are two recurrent species groups. The first is restricted to the deeper-water areas of the South China Sea, while the second occurs in both the Gulf of Thailand (a shallow, estuarine environment) and all of the South China Sea. The members of the first group, Group A, are Clio pyramidata, Creseis virgula virgula +; C. virgula conica, Limacina bulimoides, L. inflata, and Styliola subula. This group has the narrowest environmental tolerances. It does not occur in the low salinity, high temperature, low oxygen environment of the Gulf of Thailand and the extent of its penetration of the Sunda Shelf area of the southern South China Sea varies with season. Because of their relative rarity in the samples Cavolina gibbosa, C. globulosa, C. inflexa, C. uncinata, Clio recurva, C. cuspidata, Cuvierina columnella, and Diacria trispinosa were not included in the statistical treatment, but their distribution patterns place them with Group A. The second broad group has been divided into two, based on the extent of tolerance for the most extreme conditions in the Gulf of Thailand. Group B, consisting of Diacria quadridentata, Hyalocylis striata, and Limacina trochiformis, is less tolerant of extreme Gulf conditions than the following Group, C, and tends to be concentrated in the central, deeper area of the Gulf of Thailand. H. striata was absent from the Gulf during January-May, 1960 (Cruises S-3 and S-5), and D. quadridentata was absent during April-May (S-5). The members of Group C, Creseis acicula and Cavolina longirostris, have the broadest environmental tolerances and were present and abundant even at nearshore stations in the Gulf of Thailand where salinities were below 29 o/oo;. Group C tended to be most abundant around the margins of the Gulf. The members of both Groups B and C were widely distributed in the South China Sea as well.

Euphausiids collected by R/V Stranger during 1959-61 were examined with respect to 1) seasonal change in distribution, abundance and recruitment in the Gulf of Thailand and South China Sea and 2) range through the Indo-Australian seas. Discussion of the species is preceded by a description of the distribution of zooplankton biomass. In the Gulf of Thailand, biomass was found to be three to five times greater than in the open South China Sea. In eastern waters of the Gulf, the peak appeared during the intermonsoon period following northeast winds. The western and northern parts of the Gulf were richest under the influence of the southwest monsoon. Northern coastal waters of South Viet Nam were richest toward the end of each season: 1) the northeast monsoon when coastal upwelling and southerly flow from the Gulf of Tonkin took place and 2) the southwest monsoon season of northerly flow. The southern shelf and the basin region of the South China Sea yielded the greatest biomass during the southwest monsoon season when flow was from the direction of adjacent neritic waters to the southwest. The Sunda Shelf waters, including the Gulf of Thailand, constitute the extensive neritic province in which Pseudeuphausia lives. The South China Sea basin, though the largest basin in the inter-ocean region, is essentially a cul-de-sac with respect to its oceanic euphausiid assemblage, the species of which are derived from both the tropical and subtropical Pacific to the northeast. They are transported southwestward as far as the Sunda Shelf. During the northeast monsoon season many of the oceanic species tend to extend into shelf waters. Evidently the presence over the shelf of a dense neritic plankton, together with the shallowness of the water which prevents the normal range of euphausiid vertical migration, precludes presence of the oceanic species. Only Stylocheiron species, which do not migrate vertically, extend much beyond the edge of the shelf, rarely entering the Gulf of Thailand. The taxonomic diversity of the predominant species of the South China Sea (each belongs to a different species group) indicates that this assemblage is part of a regional plankton community. The annual reproductive cycles of the oceanic species were like that of neritic Pseudeuphausia, with larvae produced year-round but with maximum spawning taking place during, and at the end of, the northeast monsoon season. Production of Pseudeuphausia in the Gulf of Thailand shifted seasonally from southwest to northeast, evidently in response to coastal enrichment processes initiated by southwest and northeast monsoon winds respectively. Of the parameters measured, only low salinity (‰) correlates with the paucity of larvae. Local occurrences of the species are considered in relation to the global distributions in a study of the effectiveness of the inter-ocean seas in maintaining or interrupting flow between Pacific and Indian Ocean populations and in providing local habitats. The pelagic connection between the two oceans is limited to a succession of deeper seas: Celebes-Molucca-Banda and Flores-Timor. The large number (46) of euphausiid species in this inter-ocean region apparently reflects complexity in the oceanography rather than in the plankton community. Regularly occurring species are either endemic (e.g. Stylocheiron insulare), Indo- Pacific equatorial (e.g. Euphausia diomedeae) or broadly ranging warm-water cosmopolites (e.g. epipelagic Stylocheiron carinatum and mesopelagic Nematobrachion boöpis). Species associated primarily with central water masses of the Indian and Pacific Oceans are sparse though seasonally consistent in occurrence. The most widely-ranging species in the global sense (e.g. Nematobrachion boöpis, Stylocheiron carinatum) exhibit inter-oceanic and inter-hemispheric continuity that appears to have long been firmly established. These species do not have close relatives, with the exceptions Stylocheiron longicorne and S. maximum, each a member of a sub-generic species group. This suggests that the speciation process (population isolations + re-invasions) in Stylocheiron, a genus consisting of members which do not migrate vertically, each occupying a discrete depth interval, may differ from that process in strongly migrating genera such as Euphausia and Thysanopoda. The importance of the Indo-Australian region in the evolution of sub-tropical and tropical species is indicated by 1) the large number of species found there, 2) the present partitioning of central water mass species distributions (e.g. Euphausia brevis exists globally in five sub-populations), and of Indo-Pacific equatorial distributions (E. paragibba exists in separated Pacific and Indian Ocean populations), 3) the patchy confluence of some central and subtropical species through this waterway (e.g. E. mutica), and 4) the existence of Indo- Australian endemics (e.g. E. fallax, E. sanzoi, Nematoscelis lobata). The species of the region, including larval stages of most South China Sea species, are illustrated and briefly described. Thysanopoda subaequalis Boden is synonymized with T. aequalis Hansen, the type material of T. aequalis having been found to possess the characteristics defining T. subaequalis. The species presently called T. aequalis is redescribed as T. astylata. A third geographical form of Stylocheiron longicorne is described as the "North Indian Ocean Form."

The Research Vessel Stranger of the Scripps Institution of Oceanography, University of California, San Diego, was engaged in the Naga Expedition in the Gulf of Thailand and the South China Sea during the period of October, 1959, to December, 1960. The expedition was jointly sponsored by the Governments of South Viet Nam, Thailand and the United States of America. It had a two-fold purpose; to collect oceanographic, biological and fisheries data and material and to train scientists and technicians from Thailand and South Viet Nam in oceanography and marine biology. This report is a description of the oceanographic environment in the Gulf of Thailand derived from oceanographic and meteorological data collected for the most part on six cruises in the Gulf between October, 1959, and December, 1960. The cruise plans for the Gulf of Thailand were designed to investigate systematically the distribution and variability of the physical properties of the Gulf waters. The station plan consisted of stations located 30 to 40 miles apart on five parallel lines running perpendicular to the east and west coasts of the Gulf. The lines were 60 to 90 miles apart. Figure 1 is a composite plan for the five Gulf cruises which made complete hydrographic measurements. The stations were numbered chronologically on each cruise. Thus, stations at approximately the same location have different numbers on each of the cruises. Within the limits of navigation the primary stations on each line were at the same location on each cruise. The following physical oceanographic data were collected at each station; reversing thermometer temperatures, salinity and oxygen determinations at standard levels—0, 10, 20, 30 and 50 m—as depth allowed and bathythermograph (BT) temperature observations. The latter were also taken midway between regular stations and at intervals parallel to shore between station lines. Meteorological observations, including wind, air temperature and sea condition, were taken at the time of each BT. Station data and a description of the physical and chemical methods may be found in Faughn, NAGA report, volume 1. The R. V. Stranger of the Scripps Institution of Oceanography carried out an extensive geophysical survey of the Timor Sea between March 29 and April 24,1961, (van Andel and Veevers, 1967) following the completion of the major oceanographic observational programs in the Gulf of Thailand and the South China Sea. During the survey, closely spaced bathythermograph (BT) temperature and wind velocity observations were made. Observed wind velocities, vertical temperature sections along the cruise tracks and horizontal distributions of temperature at standard depth levels based on BT data are the subject of this report. These observations provide a detailed description of the temperature structure of the Timor Sea and complement the more general one given by Wytrki (1961).

The NAGA Expedition conducted a program of marine investigations in the Gulf of Thailand and the South China Sea during the years 1960-1961. In connection with this program, a bottom fauna survey was made in the Bay of Nhatrang, South Viet Nam. Identifiable amphipods, representing 14 established species and 21 new species, were recovered from 97 of more than 360 stations sampled in the bay and form the basis of this paper.

Although the collections of NAGA Expedition are small and contain many well-known and widely distributed species of the Indo-West Pacific area, they also contain several little-known forms (e.g. Charybdis edwardsi Leene and Buitendijk, Portunus minutus [Shen] and P. tweediei [Shen]) and two new species. A few additional Indo-West Pacific Specimens from the collections of the Scripps Institution, other than NAGA material, are included in the present report. With the better-known species the synonymy has been abbreviated to one or two recent references which permit ready identification; in other cases a full synonymy is given. NAGA collections are reported in the text as Station Numbers (followed by NAGA Catalogue Numbers) with detailed localities and their lists of species in the Appendix. These localities are also plotted on the map. Measurements are of total breadths, including last anterolateral teeth; those over 100 mm to the nearest 0.5 mm, and the remainder to the nearest 0.1 mm.

This book is the outcome of my analysis of all available knowledge of the Southeast Asian Waters. It is hoped that workers in the region, whether in oceanography or other branches of science may find it a source of information and a stimulus to undertake further research in these waters. Some chapters in this book are summaries and condensations of already known facts, but others offer new ideas and interpretations, particularly those chapters on monsoon circulations and their dynamics, on deep circulation and its relation to surface circulation, on the energy exchange between sea and atmosphere, and on the quantitative description of the exchange of water in the deep sea basins.