Damage to coral reef ecosystems is a major problem on the islands of Gili Air and Gili Trawangan. This will have an impact on the presence of reef fish in the area. This study aims to look how much relation caused by associated between live coral cover and reef fish in Gili Air and Gili Trawangan island.. This research method using Line Intersept Transect (LIT), Underwater Visual Census and simple linear regression analysis to know the relation. The percentage of coral cover on Gili Air Island and Gili Trawangan Island on reef flats (1-5 m) is 11.75% and 11.67% respectively, on the reef slopes (6-10 m) the percentage is 50.4% and 48.9%. In addition we observed the existence of 11 families reef fish. The abundance of reef fish on Gili Air and Gili Trawangan islands on reefs flat with an average of 0,406 ind / m2 and 0,137 ind / m2, on the reef slope on average - respectively 0,434 ind / m2 and 0,274 ind / m2. The determinant value in Southern part of both island indicates a value close to +1 and in Northern part of both island indicates a value close to -1.

Sunda Strait is an important passage for the Java Sea water to flow into the Indian Ocean. There is Java sea in North close to Karimata Strait and Eastern Indian Ocean in South part. Headed from north to south of Sunda Strait, has a high primary productivity that signifies the fertility of water. The strong El Nino (SOI) in 2015, hypothesized to affect variability values of a-chlorophyll content and sea surface temperature in this area. The research aims to know the distribution pattern of chlorophyll-a, and sea surface temperature due to El Nino effect on transition season I (March, April and May 2015). The relationship of both with SOI was analyzed by simple linear correlation analysis. The results showed that the distribution of chlorophyll-a in Northen Sunda Strait is not affected by El Nino but comes from the mouth of the Musi River. The pattern of sea surface temperature distribution from Northern to Southern of Sunda Strait showed in this area affected by El Nino so that the temperature is cooler. The relationship of chlorophyll-a with SOI has a negative moderate correlation (-0.532), indicating that chlorophyll-a in this waters have the direction opposite to SOI and sea surface temperature with SOI has a strong positive correlation (of 0.959).

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The Indonesia Continental Shelf (LKI) expedition was held during September - October 2020. During the survey, there were ten Conductivity Temperature Depth (CTD) measurement stations that located extending from the west of Mentawai Island to the Indian Ocean. In this study, two-line of subsurface temperature, salinity, and density data were plotted longitudinally. The results show the unique feature between the open ocean and coastal area, the characteristic from open ocean did not affect the characteristic in coastal zone, it is shown from the salinity data. The maximum salinity found in the thermocline layer, between 100-150 m in both of line. The salinity increases from the surface until the thermocline, then slightly decreases to the deep layer. The surface salinity in the coastal area significantly different from the open ocean, it is less than 34 PSU. That is the fact that Wyrtki Jet current did not induce the open ocean water to the coastal water in the subsurface. Otherwise, the temperature and density have a similar pattern with range values around 9-31°C.