Bivalve Spat (Anadara granosa) recruitment in rehabilitation mangrove ecosystem of Rangsang Island, Riau province Author Oktavian, Rizki Marine Science Postgraduate, Faculty of Fisheries and Marine Science, IPB University, Dramaga Bogor, West Java, Indonesia Author Bengen, Dietriech G IPB University, Dramaga Bogor, West Java, Indonesia Author Natih, Nyoman Mn Department of Marine Science and Technology, Faculty of Fisheries and Marine Science, IPB University, Dramaga Bogor, West Java, Indonesia text International Journal of Fisheries and Aquatic Studies 2023 2023-01-01 11 1 31 35 http://dx.doi.org/10.22271/fish.2023.v11.i1a.2766 journal article 10.22271/fish.2023.v11.i1a.2766 5801a302-419c-42be-a82f-3bc68d191021 2347-5129 11659175 Bivalve spat sampling of A. granosa Bivalve spat sampling was carried out at low tide by lifting the collector (net) and washing it in a tub to get the attached clam seeds. Collector removal is carried out once a month in November 2020 , December 2020 , and January 2021 . First, a sampling of bivalves entangled in the collector was done by hand, then put into a filter bucket. Next, the substrate was filtered using a 1 mm mesh sieve. Bivalves seeds found were put into plastic bags and then given 10% formalin. Finally, samples are taken to the Marine Biology Laboratory to be washed, counted, and classified. Sediment sampling The sediment contained in each subzone was taken using a shovel at a depth of 10 cm , put into plastic as much as 500 grams, and then into an ice box to cool (Juandi, 2013) [ 14 ] . Finally, an analysis of sediment samples was carried out in the laboratory to determine the type and fraction of sediment and analysis of the organic matter. Environmental characteristics measurement Water quality parameters are measured at high tide around the subzone area. Water quality parameters include pH, salinity, and temperature. First, measure the degree of acidity by dipping the pH indicator into the water and seeing the color changes, then compare it with the standard pH color on the pH indicator box. Next, salinity is measured using a hand refractometer calibrated with distilled water. A sufficient amount of seawater is dripped with a dropper, and look at the number shown, then recorded the number in ppt units. Finally, measure temperature using a thermometer inserted into the water, then wait a while and record the numbers shown on the tool in units of °C. Mangrove density and bivalve spat abundance of A. granosa Mangrove density and bivalve spat abundance of A. granosa was calculated using Odum (1993) [ 15 ] and Kurniawan (2016) [ 13 ] . Result and Discussion Mangrove Density Table 1: Environmental Characteristics of the Rehabilitation Mangrove Ecosystem on Rangsang Island, Riau The density condition of the rehabilitation mangrove ecosystem in Anak Setatah Village , Rangsang Island of Riau , is classified as dense, with an average density value of 2283.33 individuals/ha. The total distribution of mangrove density at each station can be seen in Figure 2 .

Time	Temperature ( ◦ C)	pH	Salinity (PPT)
November 2020	27.80	7.67	28.67
December 2020	27.83	7.62	28.17
January 2021	27.33	7.77	28.33

Fig 2: Distribution of Mangrove Density in the Mangrove Ecosystem Rehabilitation of Rangsang Island, Riau The highest mangrove density was found at Station 1.2 with a density value of 3000 ind/ha, and the lowest was found at Station 2.2 at 1900 ind/ha. Based on the Minister of Environment Decree 201 (2004), all stations fit the good criteria where the mangrove density value of 1500 ind/ha falls into the high-density criteria (density). However, the density values at each station are influenced by the number of individual mangroves. The same study conducted by Nadeak et al . (2020) [ 12 ] found that the density of mangroves in several Mangrove Ecosystem areas on Rangsang Island of Riau is in the dense category with the highest density value of 2266.7 ind/ha. Table 2: Abundance Distribution and Average Length of A. granosa

Time Abundance (Ind/100 m 2 ) Average length (mm)
November 2020	51600	7,19 (±2,41)
Desember 2020	79100	7,67 (±3,94)
January 2021	28650	6,11 (±2,52)

Species in the rehabilitated mangrove ecosystem on Rangsang Island of Riau consisted of Avicennia alba and Avicennia marina . Both of these species have characteristics that are resistant to high salinity; according to Harnani (2017) [ 19 ] , the distribution of Avicennia , especially A. alba , is spread throughout Indonesia . Furthermore, according to Khairijon et al . (2015) [ 11 ] , these two species also prefer mud substrate conditions, where environmental conditions in the Mangrove Rehabilitation Ecosystem on Rangsang Island of Riau have suitable environmental conditions. Variation of environmental characteristics Environmental characteristics in the mangrove ecosystem rehabilitation of Rangsang Island are shown in Table 1 . The distribution of temperature, pH, and salinity values at three months of observation in the mangrove ecosystem of Rangsang Island of Riau did not significantly differ. The water temperature range is 27.33-27-80 C, the pH range is 7.62-7.77, and the salinity range is 28.17-28.67 ppt. Environmental characteristics such as temperature, pH, and salinity are in the excellent category for the life of A. granosa . According to Winanto (2004) [ 20 ] and Oktaviani et al . (2018) [ 17 ] , the temperature range for living bivalves is 27-31 C, then according to Lukmana (2021) [ 18 ] , a good salinity range is no more than 35 ppt, and a pH range is 7-8.5. Abundance and length of bivalves Spat ( A. granosa ) The abundance and average length of A. granosa found at the study site are shown in Table 2 . The results found at the study site were different for three consecutive months. A. granosa found in November 2020 was 3096 ind with an abundance of 51600 ind/ 100m 2 and an average length of 7.19 (±2.41) mm. Furthermore, in December 2020 , 4746 ind was found with an abundance of 79100 ind/ 100m 2 and an average length of 7.67 (±3.94) mm. In January 2021 , it was found about 1719 individuals with an abundance of 28650 ind/ 100m 2 and an average length of 6.11 (±2.52). Fig 3: Distribution Range of A. granosa Length in Rangsang Island, Riau (A) November 2020 (B) December 2020 (C) January 2021 The highest abundance and average length of A. granosa were found in December 2020 , then the lowest value was found in January 2021 . Therefore, the distribution of A. granosa length in different months was caused by differences in the values of environmental characteristics in those three months, which can be seen in Figure 3 . According to Amalia (2010) [ 7 ] , several things cause the difference in length, one of which is temperature. Relationship between Spat bivalve ( A. granosa ) and mangrove Figure 4 shows the relationship between the abundance of A. granosa and mangrove density at three months ( November 2020 , December 2020 , and January 2021 ). The results show different regression values (R2) at three months of measurement. It means that the recruitment of A. granosa bivalve spats may vary from month to month. Figure 4 (A) shows the relationship of mangrove density to have an influence of 64.15% on the abundance of A. granosa , then Figure 4 (B) explains that the density of mangroves has a reasonably significant influence on the abundance of A. granosa , which is 82.57%, and Figure 4 (C) shows that mangrove density has an effect of 92.36% on the abundance of A. granosa . The relationship between mangrove density and abundance of A. granosa is in the strong category in December 2020 and January 2021 , while the relationship between mangrove density and abundance of A. granosa in November 2020 is in the strong category. This categorization is based on Tanjung (2010) [8] and Siahaan (2014) [ 9 ] writings which state that weak strength with R 2 = 0.00-0.25, moderate strength with R 2 = 0.26-0.50, strong strength with R 2 = 0, 51-0.75, and the strength is very strong with R 2 =0.77-1.00. Fig 4: Relationship between Spat Abundance of Bivalve A. granosa and Mangrove Density in Rangsang Island, Riau (A) November 2020 (B) December 2020 (C) January 2021 Correlation of average length of A. granosa with environmental characteristics Table 3 shows the correlation value between the average length A. granosa with environmental characteristics (temperature, pH, and salinity) from 3 months sampling. Table 3: Correlation value between average length A. granosa with environment characteristic

Parameter	Correlation Value	Interception
Temperature	0,97	Very Strong
pH	-0,99	Very Strong
Salinity	-0,11	Not Strong

Based on the correlation values shown in Table 3 , temperature and pH strongly influence the average length of A. granosa , while salinity has no strong effect.