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Yellowfin tuna is one of the main catches in West Sumatera waters and has a high economic value. Fishing optimization can be improved through spatial and temporal predictions of fishing areas for effective fisheries management. This research aims to predict fishing areas using an oceanographic data approach through maximum entropy (MaxEnt) modelling. Data collection incorporating sea surface temperature and chlorophyll-a were obtained from the Aqua-Modis satellite in 2022 as well as fishing point data from Ocean Fishing Port (OFP) Bungus. The MaxEnt model shows a good level of accuracy in the West season (AUC 0,860), 1st Transitional season (AUC 0,918), East season (AUC 0,918), and 2nd Transitional season (0,920) The contribution of chlorophyll for one year is more significant, accounted for 75,3%, compared to sea surface temperature, constituted 24,7%. The Sea Surface Temperature (SST) values in West Sumatera waters vary on each season, ranging from 26,4 to 34.5 ℃ and the chlorophyll-a fluctuate between 0,03 and 5,45 mg/m3. The distribution of yellowfin tuna habitat on the map shows that most fishing vessel coordinates are in the Habitat Suitability Index (HSI) HSI value of 0,6 – 0,9. Potential areas for catching yellowfin tuna are around the Mentawai Islands. Keywords: Chlorophyll-a, HSI, Maximum Entropy Model, Sea Surface Temperature, Yellowfin Tuna

Abstract Mixed Layer Depth (MLD) plays an important role in various aspects of oceanography. MLD has a characteristic parameter value that is uniform with depth. MLD has an important role for local, regional, and global phenomena. Indonesia, which is surrounded by the East Indian Ocean, will be directly influenced by the dynamics of MLD. This study aimed to analyze seasonal variability and MLD between years. Mixed layer depth data from the ARMOR3D Dataset Copernicus-Marine Environment Monitoring Service was used for MLD analysis with a threshold of 0.2oC for temperature. Wavelet analysis showed that MLD variability in the eastern Indian Ocean spans from intra-seasonal to interannual scales. Time series analysis showed a complex relationship between MLD and SST in the annual and interannual periods which indicates a different process. The MLD monthly climatology at point 90E, 0 showed the depth of mixed layers is deeper during the east monsoon (JJA-SON) ranging from 50-65 m compared to the west monsoon (DJF-MAM) which has a range of 20-40 m. Spatially the MLD in the south of the equator is deeper than in the north. Interannually, MLD is heavily influenced by the Indian Ocean Dipole. MLD depth is deeper in nIOD with a maximum depth in the range of 100 m compared to pIOD. MLD with maximum depth in the strong nIOD phase is around the equator and the pIOD phase is south of the equator. The study also showed that inter-annual variability in regions around the mainland showed a stronger response. Highlight Researh • ARMOR3D data has fairly good accuracy as evidenced by data validation • In general, EIO is influenced by intra-seasonal, seasonal and inter-annual variability • The shallowing and deepening of the MLD are strongly correlated with the wind speed associated with the annual cycle • IOD has a strong role in MLD which looks significantly different, especially when nIOD and pIOD

The waters of the Bangka Belitung islands are shallow waters that are affected by tides. The tides in these waters originate from the propagation of the Natuna and Java Seas, so they have unique characteristics. This study aims to analyze the amplitude and phase difference at 2 stations representing Bangka and Belitung Islands with the final output of LAT and HAT information as marine cadastral recommendations at Tanjung Pesona and Tanjung Tinggi stations and safe depths for ports in PPN Sungailiat and PPN Tanjung Pandan. . Tidal data for 1 year with time intervals of 1 hour in 2015 were used to extract the harmonic components to determine the type of tide and calculate the tidal level at each station. The results obtained are 19 harmonic components with a diurnal tide type. The water level at Tanjung Pesona is higher than the high Tanjung, which is described by tidal levels with HAT 2.64 and 1.72 m. Safe depth of harbor pond. In the LAT condition (lowest recess), the water level at Tanjung Pesona station is above the Sungailiat PPN pool's maximum depth; on the other hand, the Tanjung Pandan PPN LAT is 0.44 m below the maximum depth of the port pool. Key Word: Bangka Belitung, chart datum, LAT, HAT, tidal level

The Lombok Strait, as one of the outlet straits, is part of the ITF route, which is directly adjacent to the Indian Ocean. There is a sill in the Lombok Strait, which is a place for internal wave generation. Leg-1 data from the Japan Agency for Marine-Earth Science and Technology in collaboration with the Agency for the Assessment and Application of Technology which is part of the Tropical Ocean Climate Study Expedition including CTD Yoyo and ADCP taken using ship vehicles R/V Kaiyo. CTD Snapshot from PUSHIDROSAL using the KRI Spica 934 vehicle part of the Opssurta Baruna Jaya 2 Expedition. Determination of seawater mass stratification with the criteria for the thermocline layer is ≥ 0.05 °C.m-1. Four types of water masses were identified, Java Sea, mixed seawater mass (Java Sea - ITF) which occurred diapycnal mixing, North Pacific Subtropical Water (NPSW) and North Pacific Intermediate Water (NPIW). The seawater mass stratification in the Lombok Strait based on temperature, salinity and density which are seen to follow the internal tidal pattern. The average values for energy dissipation and vertical diffusivity for each layer and replication were 5.73 x 10-7 W.Kg-1 and 3.67 x 10-2 m2.s-1 for CTD Yoyo and 2.25 x 10-6 W.Kg-1 and 7.38 x 10-2 m2.s-1 for CTD Snapshot. The value obtained is greater than the open ocean and straits in other studies. The high shear value confirms this in the thermocline layer. The Richardson gradient value> 0.25 is relatively constant in the thermocline layer.