Production, biomass, and turnover of exploited mangrove clams ( Geloina expansa , Mousson 1849) in Kendari Bay mangrove forest, Southeast Sulawesi Indonesia

. Geloina expansa is a front-runner commodity of the mangrove ecosystem. This species is notably experiencing ecological pressures in Kendari Bay. Accordingly, this study aims to determine their production, biomass, and turnover in the mangrove ecosystem. This research is hoped to provide empirical information that will aid in the formulation of the management strategy of mangrove clam resources in Southeast Sulawesi. Clam samples were collected at random in three selected sampling areas using a 1x1 m 2 quadrat-transect sampling approach. The clams were measured for their shell length, total weight and weight of fresh meat. The clam meat was dried to obtain a shell-free dry mass. The production, biomass, and turnover of the clams were calculated using standard formulas. The population density of the clams ranged from 23.78 ind/m 2 (October) to 77.44 ind/m 2 (February), where the remaining months of observations showed similar values throughout. The clams biomass population in each size class ranged from 0.04 to 4.95 g/m 2 . The somatic production, as per the dry weight showed the highest value at 6.9 cm shell length (2.01g/m 2 /year). The lowest individual somatic production was found in the shell width of 9.7 cm (0.55 g/m 2 /year). The turnover rate (P/B) of the mangrove clam was 1.73/year. The density of the mangrove clams in the mangrove forest in Kendari Bay was found to be high. This was accompanied by high productions in the young or small-sized groups, peaking at a size smaller than the size where peak biomass was found.


Introduction
Kendari Bay mangrove ecosystem to the extent that they are massively exploited by fishermen and the like [1] [2].Locally, this species of clam is known as "kalandue" and is a very popular seafood delight.The large market demand for mangrove clams in Kendari City brings about the high fishing pressure for mangrove clams [1].Addedly, Kendari Bay is also experiencing prolonged environmental degradation due to the rapid city developments in the coastal areas [3].The conversion of mangrove forests into aquaculture areas and buildings (real estate & settlements) as well as environmental pollution (heavy metals & plastics: macroplastics/microplastics) [4] [5] as a result of household and factory-scale anthropogenic activities [3], cause Kendari Bay coast to be loaded with matters that physiologically threaten the sustainability of the mangrove ecosystem and its associated organisms, including that clams [6].Lower density and smaller existing size of clams as caused by the previous can affect population dynamics (production, biomass & recoverability) of pelecypods in nature [7] [8].
Research related to the population dynamics of clams, specifically in terms of production, biomass, and turnover, in Indonesia, especially in Southeast Sulawesi, is rarely carried out, contrary to other countries with recognizable research development of the same concern.Reports on production, biomass, and turnover have been previously been made on Glauconome virens [9], Batissa violacea [10], Eurhomalea exalbida [11], Corbicula fluminea [12], Keletistes rhizoecus [13], Batissa violacea var.celebensis [8], and Anodonta sp [14].However, the same research on Geloina expansa has yet to be carried out.Such research will hold significant importance in providing information to devise an optimal and sustainable management strategy of mangrove clam resources.Given that mangrove clams have a high market price, i.e., economically valuable, and that they are under threat of anthropogenic activities and rapid urban development, the lack of scientific information on biomass, production, and turnover of these clams needs to be immediately resolved.Thus, this study aims to determine the production, biomass, and turnover of mangrove clams in the mangrove forest of Kendari Bay, Southeast Sulawesi.It is hoped that the results obtained will provide empirical information crucial to the formulation of management strategies for mangrove clams in Southeast Sulawesi.

Sample and data collection
The material sampled in this study was mangrove clams.Moreover, the data gathered were the population density of mangrove clams, shell width, the total weight of mussels, fresh meat weight, and dry meat weight.

Sample collection and examination
Clam samples were collected randomly (purposive random sampling) at three points of collection using a 1x1 m 2 quadratic-transect approach on 20 different sampling occasions per month.Subsequently, the total number of mangrove clam samples was count and population density (ind/m 2 ) was calculated.The clam samples were then measured for shell length (SL), total shell and meat mass (TW), and shell-free fresh mass (FM).The meat was then dried in an oven for 48 hours at 70 o C to obtain shell-free dry mass (SFDM) (Abraho et al., 2010).The sampling of water quality and substrate was carried out simultaneously with the sampling of clams.

Data analysis Population density
The average density of the mangrove clams is a function of the sample area (ind/m 2 ) analyzed by the Mann-Whitney test (U-test) [15] as follows: Where: K = population density (ind/m 2 ) ni = number of individuals (ind) A = area (m 2 )

Biomass
The average annual biomass (gDM/m 2 ) was calculated using the equation [16] :

Production
Total annual production was calculated using the mass-specific growth rate method [17][18] as follows: Where: P = annual production (g DM.m 2 /year) Ni = average population density (ind/m 2 ) in the i-th length class Mi = average dry mass (DM) of individuals in i-th length class Gi = mass-specific growth rate The specific mass growth rate (Gi) was calculated using the following equation [16] : The value of the coefficient b was obtained from the relationship between shell length (L) and dry mass (DM) which was determined using the following simple regression analysis: Where: a and b = coefficients L = shell length DM = dry mass (g) The growth coefficient (K) and asymptotic length (L∞) were computed using ELEFAN I (Electronic Length Frequency Analysis) integrated with the FiSAT II version 3.0 program package.K and L∞ were adapted from another research [18] .

Turnover rate
The turnover rate was calculated from the total annual production (P) divided by the average annual biomass (B).
The shell-free dry mass (SFDM) was converted into ash-free dry mass (AFDM) with a conversion factor of 82.7% [19] .

Population density
The population density of G. expansa clams in Kendari Bay waters varied during the study period.The highest density occurred in February at 77.44 ind/m 2 and the lowest was in October at 23.78 ind/m 2 (Figure 2).The Mann Whitney test for inter-month density (P-value 0.1) showed that the densities of the mangrove clams in the remaining months did not differ significantly.

Biomass
The population biomass analysis results of G. expansa demonstrated varied values ranging from 0.04 to 4.95 g/m 2 .The pattern surrounding the peak population density value of G. expansa clams was not concurrent with that of the peak biomass value.Although, the increase and decrease in the population density value tended to have the same relative pattern.The highest density was found at 4.1 cm (8.51 ind/m 2 ), while the highest biomass was found at a larger size, namely 5.2 cm (4.95 g/m 2 ).At the size of 5.2 cm, the density of G. expansa clams was smaller than the peak density of 6.43 (ind/m 2 ) (Figure 3).

Production
The (dry mass) somatic production analysis results of G. expansa indicated the highest value at 6.9 cm shell length (2.01 g/m 2 /year).The lowest individual somatic production was found in the shell length of 9.7 cm (0.55 g/m 2 /year) (Figure 4).Annual population production of G. expansa ranged from 0.009-9.57g/m 2 /year with a total production of 54.44 g/m 2 /year.Population density demonstrated a relatively similar size class pattern to the production of G. expansa clams.The highest production and density occurred from the size class of 4.1 cm to 5.8 cm (Figure 5).

Relationship between biomass and production
The increasing and decreasing production rate pattern in each size class of G. expansa clams in Kendari Bay waters generally coincided with the increasing and decreasing pattern of biomass values.The highest production and biomass of G. expansa clams occurred from the size class of 4.1 cm to 5.8 with a P/B ratio of 1.73/year (Figure 6).

Discussion
The population density of mangrove clams (G.expansa) in the mangrove forest of Kendari Bay, despite fluctuations, was quite steady over the observation time.That is, temporal variations in density values were not significantly different.This implies that mangrove clams were evenly distributed in the mangrove ecosystem, which is a hotspot area for harvesting mangrove clams.This also indicated the success of the reproductive process, i.e., the population recruitment in the mangrove forest of Kendari Bay.Mangrove clams can spawn all year round on Chorao Island, particularly when there is an increase in temperature and salinity concentrations [20] .Likewise, such conditions might have factored the spawning of the mangrove clams in Kendari Bay.This condition afforded fishing activities in the coastal waters of Kendari Bay to not significantly affect the population density of the mangrove clams.Moreover, limited fishing gear and difficulties in accessing harvesting locations also posed a challenge for the fishermen to fully exploit the mangrove population.The mangrove clam population in Kendari Bay could thus be maintained within the balanced or under-fished category [1] .
The density of mangrove clams in Kendari Bay was relatively higher than other similar clams in other places.The population density of mangrove clams (Polymesoda bengalensis) in Muaro Pasia Kapa Padang Tae was 2.37 ind/m 2 [21] .For this case, the highest density was found on sandy mud substrates with a dense nipa palm population, which was 1.0 ind/m 2 .The lowest density was found in mud substrates with a sparse presence of nipa palm at 0.63 ind/m 2 .Furthermore, from another research reported that the population density of mangrove clams (P.bengalensis Lamarck) in the Kenagarian Mangguang mangrove area, Pariaman City was 1.8 ind/m 2 and fell under the very low category [22] .Accordingly, the population density of mangrove clams depends on the presence of mangrove trees that form dense canopies, supply large quantities of nutrients from plant residues, and stimulate water fertility as they attract phytoplankton [23][24] [21]22].
The density of the mangrove clams in Kendari Bay was relatively lower when compared to the density of Cerastoderma edule of 32-704 ind/m 2 [25] , Mesodesma mactroides of 110-543 ind/m 2 [26] , Donax trunculus of 36-272 ind/m 2 [27] , Anomalocardia brasiliana of 22-1592 ind/m 2 [28] , and Darina solenoides of 56.4-779.4ind/m 2 [29] in other beaches and estuaries.Differences in the density of clams both of differing and the same species are closely related to geographic location and climate [30][31] , habitat type and conditions [32][33] , fluctuations in environmental parameters including food and nutrient availability [23][34] as well as predator and prey density [35] .Certain ecological characteristics of habitat and niches that provide safe and optimal living space are also preferred by mangrove clams.This is in line with the statement of several researches in which substrate and morphological structure of mangrove plants such as Rhizophora, Sonneratia, Nypa fruticans, and others, provide collection sites for plant residues that are utilized by macrozoobenthos or bottom feeders as food, particularly clams associate with mangroves including G. expansa [24][36][37] [38] .
The annual production of the mangrove clam showed an increasing trend in the smaller or younger clam and reached a peak at a shell length of 5.2 cm shell.Beyond this size, the mangrove clams experienced a very significant decrease and then a stagnation at sizes close to the maximum (L∞), i.e., when the clams were mature/old.The decrease in population production when approaching the maximum size (L∞) is related to productive age, decreased reproductive potential, low density, as well as the preferred exploitation of clams of larger size [39] .The results indicated that, empirically, clams with a shell length of 5.2 cm contributed to the high production value of the mangrove clam population of 54.44g/m 2 /year.This also implies that variation in densities of the size groups determined the annual production value of mangrove clams.This is despite the fact that peak density and production occurred at different shell length sizes, unlike other clams which typically show a corresponding relationship between production and density [15][40][19] .This occurrence is thought to be caused by the harvesting pressures of mangrove clams in Kendari Bay, in particular, due to fishermen's preference towards certain clam sizes (large/old).Consequently, the peak density and production of mangrove clams in Kendari Bay occurred at smaller sizes of 4.1 cm and 5.2 cm, respectively.The high population density of the young or smaller-sized clams contributed greatly to the annual production.
The availability of food was also a key factor that generated the high annual production of mangrove clams in the waters of Kendari Bay.Moreover, the mangrove forest provides habitats and living niches that are very rich in nutrients.That is, biota associated with mangrove forests will have access to abundant food sources [23][24] .One of the factors that can produce a high total annual production of a clam population is the high availability of food as well as its effective or optimal use [40] .
The living tissue generation, in mass per m 2 area, of mangrove clams was very high and peaked at 5.2 cm in shell length.Typically, a pelecypod will have a distinct population group (based on size/age) in which the annual production peaks.Pokea clams (Batissa violacea), for instance, will have increased annual production along with the increase in size until it reaches a peak of 94.76 AFDM/m 2 /year, whereby from then on will decline until it reaches its maximum size [10] .Additionally, M. mactroides reaches an annual production peak at 4.7 cm at 0.12 AFDM/m 2 /year, which will then decrease along with growth to its maximum length (L∞ = 8.5 cm) [26] , E. exalbida clams with L∞ = 7.4 cm reaches a peak at 5 cm at 22.2 AFDM/m 2 /year [11] , and Donax striatus clams with L∞ = 36.1 mm reaches maximum production at 24 mm with a value of 6.11 AFDM/m 2 /year [15] .The density of the mangrove clams in the mangrove forest in Kendari Bay was found to be high.This was accompanied by high productions in the young or small-sized groups, peaking at a size smaller than the size where peak biomass was found.The decrease in the production and biomass in larger size or old/mature groups as a result of pressures from harvesting activities of the Geloina expansa clams prompted faster turnover of the mangrove clams.

Fig. 1 .
Fig. 1.Map of mangrove forest research location in Kendari Bay

Fig. 5 . 6 BIO
Fig. 5. Graphs of average density and estimated secondary production of G. expansa clams in each size class

Fig. 6 .
Fig. 6.Graphs of secondary production and biomass of G. expansa clams in each size class