Mapping the Suitability of Milkfish Ponds in Sidoarjo District Using Geographic Information Systems (GIS)

Milkfish is one of the potential fishery products in Sidoarjo Regency. Sidoarjo District, one of the subdistricts in Sidoarjo Regency, has significant potential for milkfish development due to its 3271.96 ha of land area. Pond suitability analysis is a crucial aspect of supporting the success of milkfish cultivation. Land suitability can be analyzed using Geographic Information Systems (GIS). This research involved two phases: data collection and analysis. The data parameters collected are temperature, pH, DO, phosphate, and salinity. The data analysis employed overlay and scoring methods. The final scoring results were categorized into four classes, including classes S1 (Very Suitable), S2 (Quite Suitable), S3 (Conditionally Suitable), and N (Not Appropriate). Based on analysis of Landsat image data, the pond area in Sidoarjo District reached 2991.03 Ha. The final scoring results showed that the pond area in the S1 category was 1,816.53 Ha (60.73%), then the S2 category was 1174.50 ha (39.29%). Referring to the calculation of the percentage of pond suitability, most of the ponds in Sidoarjo District are very suitable for cultivating milkfish.


Introduction
As a maritime nation, Indonesia is intrinsically related to fishery cultivation.Since primordial times, the fisheries industry has consistently generated a substantial and lucrative of the nation's foreign exchange.The number of cultivated fish produced was recorded as 11.53 million tons in 2021 [1].The fishing industry, particularly freshwater fishing, is one of Sidoarjo's primary economic sectors.Ponds occupy 15,539 hectares of land in the Sidoarjo Regency, or 21.9% of the total land area [2].Sidoarjo Regency is well-known for its exceptional fishing goods, such as milkfish and prawns.Milkfish is a prominent agricultural product in Sidoarjo Regency, where its overall production amounted to 34,548,900 kg in 2021 [3].With an area of 3271.96ha, Sidoarjo District is one of the sub-districts in Sidoarjo Regency with significant potential for milkfish development [4].
Population growth is the primary concern in expanding the milkfish pond fisheries industry.As the population grows, so does the demand for land, resulting in a decrease in possible land for milkfish ponds.According to Sababalat et al., actual land suitability is land suitability for land use assessed in its proper condition without any changes.On the contrary, potential land suitability refers to the land's appropriateness assessment for future land adjustments [5].It is necessary to evaluate the suitability of land used for aquaculture to increase fisheries productivity and encourage sustainable aquaculture.Thesuitability of land can be assessed by knowing the factors that influence the quality of the land, such as the nature of the soil, water, hydrology and so on.[6].The land utilized for a pond must satisfy specific criteria to be suitable for use and provide satisfactory results.Therefore, it is necessary to conduct a land suitability study based on the conditions that affect the life, safety, and continuity of milkfish cultivation to ensure that the environmental conditions of the land are suitable for milkfish cultivation.Pond suitability analysis plays a critical role in ensuring the prosperity of milkfish cultivation [7].The Geographic Information System (GIS) is a method that can be employed for the analysis of land suitability.Geographic Information Systems (GIS) are utilized as analytical instruments for the purpose of spatial determination.Geographic Information Systems (GIS) is a spatial analysis that can integrate multiple datasets and pieces of information on the cultivation of fisheries to generate a thematic map with a relatively high degree of precision and efficiency [8].
The metrics utilized for the analysis of land suitability encompass physical factors, such as temperature and salinity, as well as chemical factors, including pH, dissolved oxygen (DO), nitrate, and phosphate levels.This finding aligns with the research conducted, which suggests that water quality assessments can serve as a reliable indicator for evaluating the quality of milkfish aquaculture [9].Several parameters that are used must be taken into consideration.Given that not all factors influencing the suitability of pond land are of equal importance, determining the weights of criteria and sub-criteria requires the application of multiple methods [10].By understanding the water quality criteria, it is possible to enhance the technology of milkfish cultivation by utilizing an alternative method for water quality management.
Milkfish cultivation is comparatively more manageable and exhibits significant economic potential compared to other fish.Nevertheless, there is a deficiency in the development of the milkfish pond fisheries sector in Sidoarjo Regency.The potential for milkfish pond development in Sidoarjo Regency remains substantial because some land that has the potential to be suitable for ponds is still not optimally utilized.Mapping the suitability of ponds represents a potential approach to enhancing the productivity of milkfish ponds.Nevertheless, there is currently a lack of a comprehensive map in Sidoarjo District detailing the locations of milkfish ponds suitable for cultivation.Therefore, to determine the suitability of milkfish ponds in Sidoarjo Regency, research was conducted that integrated water quality parameters with Geographic Information Systems (GIS).

Materials and Method
The primary data included in this study encompassed measurements of pH, salinity, dissolved oxygen (DO), temperature, and phosphate levels.The primary data samples were collected using a random sampling method.Specifically, six distinct pond points were selected, and from each of these points, three samples were gathered.Consequently, a total of eighteen samples were utilized for the study.The secondary data included in this study encompasses the administrative base map of Sidoarjo Regency, as well as Landsat satellite imagery.The phases of research can be seen in figure 1.
The initial phase of the research was conducting an examination of the challenges encountered in the cultivation of milkfish ponds in Sidoarjo Regency, with particular emphasis on the Sidoarjo District.The subsequent phase involves the gathering of primary and secondary data.The primary data obtained contains measurements of pH, salinity, dissolved oxygen (DO), temperature, and phosphate levels.The secondary data included in this study encompasses the administrative base map of Sidoarjo Regency and Landsat satellite imagery.
The acquired spatial data is subsequently superimposed onto a base map generated using administrative maps, satellite imagery, and land use maps.Based on the suitability parameters of the pond land, the overlay results are subsequently assessed.The map was then displayed in accordance with the outcomes of the scoring.The parameters for pond suitability are detailed in table 1.Source: [6], [11] The evaluation of pond land's suitability is conducted by employing a suitability matrix to assess physical and chemical parameters.The formulation of suitability criteria or matrices was founded on the influence of each parameter on potential milkfish pond cultivation sites.The level of suitability exceeded four classes, namely S1 (Very Suitable), S2 (Quite Suitable), S3 (Conditionally Suitable), and N (Not Appropriate).Milkfish pond suitability scoring results can be seen in the following table:

Result and Discussion
The location of the sample points used is in three villages, namely: Kemiri Village, Sekardangan Village, and Gebang Village.These three villages have quite large potential for developing milkfish cultivation so that the mapping of the suitability of milkfish pond land in these three villages can be used as a reference.The figure 2 presented below illustrate the sites where samples were collected for data collection purposes.

Figure 2. Research sampling point
The water quality was measured at seven distinct locations, where a total of 21 samples were collected, with three samples taken at each location.The temperature, dissolved oxygen (DO), and pH characteristics of each sample were directly measured.Meanwhile, phosphate and salinity parameters were tested in the laboratory.The seven points located in the three villages are a form of strengthening parameters related to milkfish cultivation activities, ensuring that temperature, salinity, DO, pH and phosphate comply with predetermined standards so the potential for carrying out milkfish cultivation activities increasesas.The outcomes obtained from monitoring various water quality metrics can be seen on table 3. The criteria parameters derived from table 1 are subsequently interpolated in order to ascertain the spatial distribution.The employed interpolation technique is Inverse Distance Weighted (IDW).The Inverse Distance Weighting (IDW) approach was developed on the fundamental premise that the values observed at sample sites exhibit a direct correlation with the spatial proximity between these places.The determination of the target's position was carried out by utilizing measurement values in the proximity to the location under consideration [12].This method provides measurements that are quick and accurate.This interpolation methods provides a reasonable approximation of the distributed point data [13].The outcomes of the interpolation of measurement parameters are shown as follows:

Temperature
The results of measuring the temperature of ponds in Sidoarjo District using several sample points obtained measurement results ranging from 27.9 to 31.9 °C.The acquired temperature readings indicated that the pond water temperature was not excessively warm.Based on several studies, fish that live in tropical waters could grow well at temperatures of 30-35 °C in conditions of excessive levels of dissolved oxygen [8].Then, referring to the suitability indicator for pond cultivation, this value was within the safe limit.Milkfish seeds have a tolerance limit for high temperatures up to 38-41 ˚C in ponds, but are sensitive to low temperatures [14].

pH
The resulted of pH measurements in pond waters in Sidoarjo District showed measurement values ranging from 7.41 -8.53.This pH value showed that pond water conditions tend to be alkaline because they have a pH above 6.The optimum pH conditions used for the feasibility of cultivating milkfish were in the range of 7.5 -8.5 [15].

Dissolved oxygen (DO)
Dissolved oxygen (DO) was measured in situ with a DO-meter.The range of measured DO concentrations in pond waters was 5.63 to 8.97 mg/l.This range value indicated that the conditions of the pond water were conducive to milkfish cultivation.In order to successfully cultivate milkfish in pond water, oxygen levels of >3 mg/l were required [9].The dissolved oxygen used for milkfish cultivation ranges from 4 to 8 mg/l; optimizing the DO value can be accomplished with a filter machine or through basic methods such as regular water changes [16].

Salinity
Salinity levels ranging from 3 to 23 ppt were determined in pond waters using a refractometer.Pond areas that were closer to the sea had higher salinity values, while pond areas that were further away from the sea had lower salinity values.The highest salinity value in the measurement data was at point 6, which is very close to the ocean.Milkfish could adapt to fresh water with a salinity of 0 -5 ppt or seawater with a salinity of 30 -50 ppt.However, the best salinity for cultivating milkfish was 12-20 ppt [6].

Phosphate
Phosphate in water was a nutrient source for phytoplankton, klapa, and vegetation.Phosphate availability significantly affects the proliferation of aquatic microorganisms [17].The phosphate content in pond waters supports the availability of natural food for farmed fish.In the pond region of Sidoarjo District, phosphorus concentrations range from 0.19 to 2.33 mg/l.A phosphate level of 0.21 or higher indicated that the pond water area was quite suitable for milkfish pond development [14].The data parameters collected are temperature, pH, DO, phosphate, and salinity.The data analysis employed overlay and scoring methods.The final scoring results were categorized into four classes, including classes S1 (Very Suitable), S2 (Quite Suitable), S3 (Conditionally Suitable), and N (Not Appropriate).Based on analysis of Landsat image data, the pond area in Sidoarjo District reached 2991.03Ha.The final scoring results showed that the pond area in the S1 category was 1,816.53Ha (60.73%), then the S2 category was 1174.50 ha (39.29%).Referring to the calculation of the percentage of pond suitability, most of the ponds in Sidoarjo District are very suitable for cultivating milkfish.The suitability map of milkfish ponds based on scoring calculations can be seen in figure 8 below.

Conclusion
Sidoarjo District has great potential in developing milkfish ponds.Referring to the five suitability parameters, temperature, pH, DO, phosphate, and salinity, the final value was obtained in the S1 category (Quite Suitable) of 1,816.53Ha (60.73%), the S2 category of 1174.50 ha (39.29%) (Suitable).Optimizing ponds can be done by maintaining environmental parameters so that they can continue to support the growth of milkfish.

Figure 3 .
Figure 3. Temperature distribution in the pond area of Sidoarjo District

Figure 4 .
Figure 4. pH distribution in the pond area of Sidoarjo District

Figure 5 .
Figure 5. Distribution of DO in the pond area of Sidoarjo District

Figure 6 .
Figure 6.Salinity Distribution in the pond area of Sidoarjo District

Figure 7 .
Figure 7. Phosphate Distribution in the pond area of Sidoarjo District

Figure 8 .
Figure 8. Map of the suitability of milkfish ponds in Sidoarjo District

Table 2 .
Milkfish pond suitability scoring results

Table 3 .
Pond water quality measurement results