57 Physical properties of southern Fiji waters Savin Chand, Than Aung, Shivanesh Rao The University of the South Pacific, Suva, Fiji ABSTRACT CTD-O 2 (conductivity, temperature, depth and oxygen) data collected in the southern part of Viti Levu during the cruises (`96, `97, `98, `99 & `03) of the Japanese fisheries research vessel, Koyo-maru were analyzed to identify the isothermal layers, thermocline depths, and their annual variations. Despite the difference in years and locations during the survey, seasonal thermoclines at 50-100 m depth, permanent thermoclines at 100-600 m depth and haloclines at 250-600 m depths were found. In conjunction with the temperature, salinity and depth analysis, water mass properties were also investigated, and T-S diagrams were used to identify the water mass movement and formation. The results indicate that the water mass extending from 200-600 m originated from West South Pacific Central Water and the water mass between 600-1100 m originated from Antarctic Intermediate Water. 1 INTRODUCTION The Japanese research vessel, Koyo-maru makes an annual voyage as part of the training of engineering and navigation student cadets. The Koyo-maru visits Southern Fiji waters in December each year to do CTD-O 2 measurements. The Koyo-maru data, in December each year from 1996 to 1999, measured at different locations of Southern Fiji waters are used to construct temperature and salinity depth profile to deduce the physical property of seawater around Fiji. The time period of these data spans El Niño, La Niña and normal seasons. December 1996 marks the onset of 97/98 El Niño, December 1997 shows the mature phase of 97/98 El Niño, December 1998 marks the La Niña and December 1999, the normal condition. Due to political situation in Fiji, there was no cruise for 3 years and the last Koyo-maru cruise took place in December 2003. The physical properties of the latest data sets were analyzed and compared with earlier data from 1996 (Ferland and Suda, 1998). This provides a good signature of seawater temperature and salinity variations in an interannual time domain. The temperature and salinity fields also form an essential tool for identifying the water mass property with common formation history through T-S diagrams. Tomczak and Godfrey (1994) have shown the existence of Western South Pacific Central Water (WSPCW), Antarctic Intermediate Water (AAIW) and the Antarctic Bottom Water (AABW) in the tropical Pacific Ocean. 2 OBJECTIVES For Pacific island countries like Fiji, the marine resources are an extremely important basis for economic development. However, there is only scant information available about the distribution of water properties within Fiji’s EEZ. It is necessary to understand the nature of existing resources to develop and maintain a sustainable marine environment in the future. For example, temperature and salinity, together with pressure, are the most important physical properties affecting seawater density and control the dynamics and thermodynamic behavior of the ocean. No systematic studies of CTD-O 2 (conductivity, temperature, depth and oxygen) in the Fijian archipelago have been published previously except the reports from Ferland and Suda (1998) and Pickering and Suda (2003). Therefore, it is expected that data analyses and interpretations are important for future studies in oceanography in Fiji. In the absence of user-friendly papers on oceanographic data analysis and basic interpretation of analyzed data, for basic oceanographic research, our paper will pave a way for the beginners who need a guideline to deal with the marine data sets collected in the USP region. 3 DATA AND METHODS In this paper we briefly looked at the CTD data collected during 1996-1999 Koyo-maru cruises and interpreted for the physical properties of Southern Fiji waters. Study areas varied every year based upon the different purposes of the cruises and the requirements of the scientists involved, however, the Koyo-maru cruises focus on the southern part of Viti Levu every year due to the time constraints and the insufficiently charted waters north of Fiji. In addition to the earlier data sets, temperature and salinity depth profiles focussed on data collected during the two transects of the December 2003 cruise shown in Figure 1. Using a CTD meter provided by the National Fisheries University of Shimonoseki under its joint research programme with the University of the South Pacific the measurements were taken during the cruise of 2003. Transect 1 consisting of 6 stations is located between the 18.20º and 18.45º south latitude and along the north-south line of 178.50º longitude whereas transect 2 is located between 18.25 and 18.33º south latitude and along the north-south line of 179.31º longitude. Table 1 summarizes the location of each station for the two transects. Furthermore, the mean temperature and salinity depth profiles for each transect were determined, together with their standard deviations, to identify their variability along the vertical stratification. The T-S relationship has been used to identify the different water mass properties and their common formation history.