Spatial and temporal dynamics of sea surface temperature and chlorophyll-a in Makassar Strait observed from satellite images: oceanographic characteristics identifying fish distribution and

abundance

Mukti Zainuddin

Faculty of Marine Science and FisheriesHasanuddin University

AbstractTo describe spatial and temporal dynamics of sea surface temperature (SST) and chlorophyll-a concentration (Chl-a) and to identify the predicted area of fish distribution and migration in Makassar Strait, monthly mean remotely sensed satellite data in 2009 were used. Results showed that SST and Chl-a have different spatial and temporal patterns along Makassar Strait. SST tended to be high during the first transition season (March-May) and reached a peak in April at the level of approximately 30°C especially in southern area. Whilst, Chl-a appeared to be high during the west season (January-March) in the central and south of Makassar Strait. During that period, Chl-a ranged from 0.5 to1.0 mg m -3. The Chl-a started to significantly decrease in April and then tended to be constant through December. Most of potential predicted areas for pelagic fish such as for skipjack and mackerel in Makassar Strait developed in the southern area along South Sulawesi waters when progression of warm waters SST moved from southern area to the north from April to July. At the same time and location, Chl-a concentrations tended to gradually well enhance. As a result, skipjack tuna and mackerel were clearly identified to have the high probability of finding the fish in the southern area both December and in March. It suggests that satellite images provide a powerful tool to identify spatial and temporal fish distribution and abundance throughout a year in Makassar Strait.

Keywords: SST, Chl-a, satellite images, predicted area, fish distribution and abundance

INTRODUCTION

Marine fishery resources rely on specific oceanographic structures for feeding, reproduction, and migration and the link between the availability of potential fishing location and fish population dynamics is widely documented in the coastal environment (Laurs, 1984; Inagake et al., 2001). In the tropical coastal environment, spatial and temporal dynamics of oceanographic conditions such sea surface temperature (SST) and Chl-a concentrations play an essential role for identifying pelagic fish distribution and abundance (Zainuddin, 2009; Zainudin and Jamal, 2009). A fruitful approach to understanding the impact of the two oceanographic signatures on pelagic ecosystem dynamics may be to understand the impact of environmental variability on key habitats.

From historical point of view, satellite remote sensing has proved an important source of information to marine science and fisheries oceanography research. SST and Chl-a concentrations are important oceanic parameters in explaining the dynamics of pelagic fishing ground. Thermal features of sea surface provide reliable proxy indicators of the spatial and temporal distribution of prey abundance, which subsequently affects tuna distribution (Humston et al., 2000). Ocean color feature is a good indicator of seasonal variations of chlorophyll fronts (TZCF), which are linked to tuna forage habitat (Polovina et al., 2001).

Makassar Strait is one of the most productive fishing grounds in the eastern part of Indonesia. This area is one of the primary pathways of Indonesian throughflow (ITF) from North Pacific which has an estimates of total volume transport of water-mass distribution at approximately 8-9 Sv (one Sv / Sverdrup, is an ocean transport unit, equal to a current of one million cubic meter per second) (Gordon, 2005). The ITF influences oceanographic structures in the strait. Dynamics of oceanographic parameters such SST and Chl-a within Makassar Strait is very important for pelagic fish distribution, migration pattern and abundance.

Since 2002, Aqua satellite sensor, Moderate-resolution imaging spectrometer (MODIS) has provided high spatial and temporal resolution of SST and Chl-a that has proved the key to recent studies on physical-biological links (Druon, 2010). This paper will use the SST and Chl-a data derived by remote-sensing to focus on the spatial and temporal dynamics of the oceanographic conditions in Makassar Strait. The objectives are to describe spatial and temporal dynamics of SST and Chl-a and to identify the predicted area of fish distribution and migration in Makassar Strait.

DATA AND METHODSThe physical and biological environmental data used to describe the dynamics of oceanographic conditions in Makassar Strait during January-December 2009 were SST and Chl-a data derived from Aqua/MODIS. The NASA distributes Standad Mapped Image (SMI) level 3 binary data with HDF (Hierarchical Data Format) format (http: oceancolor.gsfc.nasa.gov). This study used the satellite data with monthly temporal resolution and 0.044° of longitude and latitude spatial resolution. .This study divided the study area into three groups: (1) north of Makassar Strait: 1 N- 1 S and 117.5-120 E; (2) central area: 1-3.5 S and 117-119.5 E; and (3) south of Makassar Strait: 3.5-5.5 S and 117-119.5 E. All SST and Chl-a values for all pixels on each group were extracted to calculate the trend of monthly mean variables from January to December 2009.

To identify the predicted area of fish distribution and abundance in Makassar Strait, potential fishing location map was constructed from the significant and favorable ranges of the two environmental variables (SST and Chl-a). Preferred SST and Chl-a for skipjack and mackerel were based on previous study (Zainuddin et al., 2008; Zainuddin and Jamal; 2009). The map consisted of binary output in which the black color represents non preferred area, and the white one denotes the high probability area of finding pelagic fish (skipjack and mackerel). This study used these species as examples since they are the main target for commercial fisheries in Makassar Strait. The potential fishing ground map was computed by combining the three favorable oceanographic conditions of satellite images (significant variables) into a single map with the same spatial and temporal scale for each grid satellite data using the Interactive Data Language (IDL 6.3) software package. All data processing and mapping in this paper were made using the software package.

RESULTS

1. Spatial Distribution of Sea Surface Temperature (SST)

Spatial distributions of monthly mean SST estimated from satellite data showed that cold surface water in Makassar Strait occurred during January-February 2009 (Fig.1). The relatively low SSTs were mainly found in the central and southern Makassar Strait during the period. Warm water SST started from the southern area in April with the SST level of more than 30°C and move to the north through July and then the SST tended to decrease from August to November. In the southern area, the lowest SSTs appeared in August around Takalar coastal waters and the SST gradually increased till December.

Fig. 1. Remotely sensed sea surface temperature estimated from Aqua/MODIS for the Makassar Strait from January to December 2009.

Figure 2 showed that the lowest temperature in Makassar Strait occurred during the west season (December-February) in the central and south of the strait. The SST level in the period was less than 28°C on average. The highest SST occurred in the first transition season particularly in the southern area of the strait with the SST of more than 30°C. The average SST in that period was about 29°C. During the east season, SST was highest (>30°C) in the central and northern side of Makassar Strait especially near Celebes (Sulawesi) island.

However, the lowest SSTs were encountered in the south of South Sulawesi Province with the level of less than 28°C SST . The surface temperature tended to be stable (less variability) during the second transition season.

Fig. 2. Remotely sensed sea surface temperature estimated from Aqua/MODIS for the Makassar Strait for all seasons (west season, first transition season, east season and second transition season) in 2009.

2. Spatial Distribution of Chl-a

The Chl-a concentration tended to high from January to March in eastern and southern part of Makassar Strait (Fig.3). In these areas, Chl-a density were mostly greater than 0.5 mg m -3. The highest Chl-a was found in eastern area closed to Borneo Island with the density of more than 0.84 mg m-3. From April to December, the Chl-a appeared to be constant. It is interesting to note that in August 2009, the high Chl-a concentrations were observed along Takalar, Jeneponto and Bantaeng coastal waters.

Fig. 3. Remotely sensed surface chlorophyll density estimated from Aqua/MODIS ocean color for the Makassar Strait from January to December 2009.

Figure 4 showed that Chl-a concentrations were consistently high in eastern side of Makassar Strait for all seasons in 2009. The Chl-a densities in that area were observed to be greater than 0.84 mg m-3. The greatest Chl-a were found during the west season. Chl-a concentrations were lowest in the second transition season along the center of Makassar Strait. The relatively low Chl-a densities in that area were about less than 0.18 mg m -3. The

relatively high Chl-a concentrations along the coastal area of South Sulawesi Province can be found in the east season, particularly in Takalar coastal waters.

Fig. 4. Remotely sensed Chl-a estimated from Aqua/MODIS for the Makassar Strait for all seasons (west, first transition, east and second transition season) in 2009.

3. Temporal Variability of SST and Chl-a

It is important to point out that in Makassar Strait, the temporal variability of SST appeared to be more fluctuated than that of Chl-a (Fig.5: left). A high variability of SST occurred during January-February. SSTs tended to be high in the first transition season and reached a peak in April in which the SST level was approximately 30°C especially in southern Makassar Strait. From May to November, the trend of SST appeared to decrease. Although the increased SST can be seen in December in the south of Makassar Strait, but the lowest SST can be clearly found in January where the SST level was about 27°C. Figure 5 showed that the southern area of Makassar Strait had a highly SST variation compared with the other areas. The northern area was less variable SST spatial pattern.

The Chl-a concentrations have different temporal variability with the SST (Fig.5: right). All three areas in Makassar Strait have similar Chl-a trend along the year of 2009. Chl-a tended to increase in the west season especially in January where the density levels ranged from 0.5 to 1.0 mg m-3. In general, the level of Chl-a concentrations seem to be stable from April to December. For all areas along the strait, the Chl-a densities during that period were about 0.5 mg m-3 on average. The Chl-a appeared to slightly increase only in July. In the northern Makassar Strait, the Chl-a concentrations were much more variable than both in central and southern area.

Fig. 5. Temporal variability of monthly mean SST (left panel) and Chl-a (right panel) with ± 1 standard deviation estimated from Aqua/MODIS in the Makassar Strait for January-December in 2009.

4. Predicted Area of Fish Distribution and Abundance

Figure 6 indicated that spatial pattern of preferred oceanographic conditions for skipjack and mackerel was considerably different. Skipjack tended to inhabit the specific area of Makassar Strait both in March and in December. Whilst, mackerel appeared to occupy a wide

area of the Strait in same period. In March, skipjack tuna were predicted to occur along the north and center of Makassar Strait closed to the Sulawesi coast. It seems different in December where skipjack tended to concentrate in the southern area. Predicted area for mackerel were more developed in northern and southernmost Makassar Strait in March. Meanwhile, in December the high probability areas of finding mackerel were mostly in central and southern Makassar Strait particularly along the coastal of South Sulawesi Province.

Fig. 6. Spatial distribution of predicted area for skipjack (left) and mackerel (right) along Makassar Strait in March (top) and December (bottom) 2009.

DISCUSSION

With just a year of high resolution remote sensing datasets, it is already apparent that spatial and temporal dynamics of the SST and Chl-a could be observed. A description of these

oceanographic parameters is very important to understand spatial and temporal patterns of pelagic fish distribution and abundance. It is important to note, however, that the predicted areas are just proxies indicating the high probability locations of finding fish. The prediction map is based on the joint preferred environmental ranges for a certain species, so that the accuracy of the prediction is strongly dependent on the quality and quantity satellite based-oceanographic datasets and knowledge about fisheries oceanography and ecology for each species. Therefore, this study could be as preliminary nature of results in providing insight into identification of potential pelagic fishing ground.

During 2009, the spatial and temporal patterns of SST are different with those of Chl-a. It is probably that both these oceanographic parameters influence concurrently to the spatial pelagic fish distribution and abundance. Most of potential predicted areas in Makassar Strait such as for skipjack and mackerel develop in the southern area (South Sulawesi waters) when progression of warm waters SST moves from south to the north from April to July (Figs. 1 and 2). At the same time and location, Chl-a concentrations tended to gradually well enhance (Figs 3 and 4). As a result, during this period (first transition-east seasons), many pelagic species distribute and associate with oceanographic structures existing in this area including skipjack and mackerel (Zainuddin et al., 2008; Zainuddin, 2009). This phase was well identified where the SST and Chl-a were more than 28.5°C and about 0.25 mg m -3, respectively.

The prediction map also indicates that the most potential fishing grounds in Makassar Strait occur in the southern area (Fig. 6). Both in March and December, the predicted areas are well formed for skipjack and mackerel within the area of south of Makassar Strait. It is most likely that the availability of upwelling and frontal zones strongly supports a highly productive habitat in that area (Ilahude, 1971; Zainuddin, 2009). The understanding of spatial and temporal SST and Chl-a is very useful to identify high productive fishing locations including upwelling and frontal zones particularly along Makassar Strait.

CONCLUSIONSThe spatial and temporal dynamics of SST and Chl-a are very important to understand the preferred area of fish concentration and distribution in Makassar Strait. The space-time patterns of the oceanographic conditions during 2009 are different, but complement one another to create potential fishing grounds. The most potential predicted area for fish distribution and abundance occur in the southern area of Makassar Strait (South Sulawesi coastal waters). This area mainly develops from April to July in which most pelagic species well associate. REFERENCES

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