The biomass and distribution of black clam (Villorita cyprinoides) in Vembanad, a tropical estuary located along the southwest coast of India varied significantly. Sampling was done in freshwater-dominated zone in the south (distal) and brackish water zone in the north (proximal), during pre and post monsoon seasons. Clam biomass was estimated from samples, collected from different stations during the study period. Water transparency and temperature were measured at the sample sites. Water samples were collected and analysed for salinity, dissolved oxygen (DO), pH and hardness. There was a significant difference in the clam biomass during the two seasons in the distal zone, and those collected from the distal and proximal zones during pre-monsoon season. The data were further analysed to determine the factors affecting the clam biomass distribution in the two zones and seasons. Factor analyses, comparing the distal zone during two seasons and zonal variations were similar to earlier observations. Step wise regression analyses found that dissolved oxygen (adjusted R2 = 0.3) is the only variable affecting clam survival during pre-monsoon period in the distal and proximal zones. A geographic map of the region obtained from the Indian satellite sensor LISS (Linear Image Self Scanner) was used along with in situ data to map the results using inverse distance weightage model.
Tropical estuaries are highly productive and critical to the maintenance of coastal fisheries [
Villorita cyprinoides which is a major molluscan resource of Vembanad estuary contribute 97% of the molluscan catches from India [
The distribution of clams in the Vembanad estuary is patchy. But being sedentary organisms, their survival and growth are dictated by a combination of favourable hydrographical conditions. The observations on environmental variables and their relation on clam biomass distribution in this study would help the fishing community to identify easily the zones of high abundance of the clams, in various geographic areas across the estuary and during the two seasons studied in the estuary.
Vembanad estuary, situated in Kerala, is the largest tropical wetland ecosystem along the south-west coast of India. This tropical estuary, fed by ten rivers draining into the Arabian Sea, covers an area of 21,050 ha spread between latitudes 9.59˚N and 10.61˚N and longitudes 76.19˚E and 76.43˚E. The rich biodiversity of shell fish and other aquatic resources and their socio-economic importance, made Vembanad estuary a wetland of international importance, and is designated a “Ramsar site”. A unique characteristic of the estuary is the Thaneermukkom salt water barrier which divides the water body into two zones―one with freshwater fed by the rivers on the southern side, and the other with brackish water fed both by rivers and Arabian Sea on the northern side (
different seasons observed in this tropical monsoon-influenced environment. Usually three seasons are identified in Kerala, namely pre-monsoon (February - May), monsoon (June - August and October - December) and post-monsoon (December - January) [
Ten sampling stations were identified in the distal zone of the lake, and sampled during pre- (March - April, 2008) and post-monsoon (December, 2007 - January, 2008) seasons to study the variations of clam population in relation to physico-chemical variables (
Similarly, ten sampling stations were identified in the proximal zone of the estuary, and sampled during pre-monsoon season to study the impact of dredging on the clam population in the estuary (
Sampling was not done during monsoon and post-monsoon season in the dredged area (proximal zone) as dredging was suspended at this time. The distal sampling stations were named D1-10 and d1-10 for the pre and post-monsoon sampling respectively, whereas the proximal stations were named P1-10 for the pre-
Distal Zone | Proximal Zone | |||||
---|---|---|---|---|---|---|
Pre-monsoon | Post-monsoon | Pre-monsoon | ||||
Stations | Longitude | Latitude | Longitude | Latitude | Longitude | Latitude |
1 | 76.4207 | 9.5924 | 76.4188 | 9.5912 | 76.36715 | 9.72436 |
2 | 76.4202 | 9.5881 | 76.4202 | 9.5888 | 76.37196 | 9.72223 |
3 | 76.4116 | 9.5651 | 76.4109 | 9.5658 | 76.37341 | 9.72039 |
4 | 76.4025 | 9.563 | 76.4006 | 9.5634 | 76.37594 | 9.71786 |
5 | 76.3927 | 9.5517 | 76.3871 | 9.5502 | 76.3773 | 9.71667 |
6 | 76.3799 | 9.5476 | 76.3797 | 9.5455 | 76.37417 | 9.72444 |
7 | 76.3691 | 9.5692 | 76.3629 | 9.561 | 76.37391 | 9.72258 |
8 | 76.3799 | 9.6191 | 76.3883 | 9.6162 | 76.3756 | 9.72131 |
9 | 76.3883 | 9.6519 | 76.3919 | 9.6512 | 76.375 | 9.728 |
10 | 76.4006 | 9.6689 | 76.4018 | 9.662967 | 76.377 | 9.726 |
monsoon sampling. The clams were collected randomly from the sampling stations using a 1 m × 1 m transect during pre and post-monsoon seasons. Live clams found within the transect were counted in situ and weighed to the nearest gram and maximum width measured to the nearest millimetre in the laboratory. A transect is a square-shaped construct built with iron pipes of half inch thickness. The physico-chemical variables such as temperature and transparency of the water samples were measured at the sampling site where as other environmental variables such as dissolved oxygen, hardness, pH and salinity were analysed using standard methods [
The environmental variables were normalised using the equation
where x is the variable and n is the sample size, to transform all the variables in proportion to one another. Significant differences, in the hydrographic variables between the proximal and distal zones were investigated using student’s t statistics. The dominant variable affecting the clam biomass in proximal and distal zones during pre and post-monsoon seasons was identified using factor analysis. The factor analysis generated the principal components, eigen values, percentage of variance explained by each factor, cumulative percentage of the variance in data explained by inclusion of successive factors and bivariate plots allowing inferences on relations between all variables and their variation. Stepwise regression analysis was also carried out on the variables measured to confirm the critical ecological variable affecting clam distribution in various zones across the seasons.
The Vembanad estuary is covered in four topographic sheets produced by the Survey of India numbered 58B/4, 58B/8, 58C/6 and 58C/5 & C/1 of the year 1967 which are all large-scale maps. Satellite image of the estuary was obtained from LISS-III sensor of IRS-P6 satellite in LGSOWG (Landsat Ground Station Operators Working Group) format. The path, row and map ID of the image obtained are 099, 066, 58C/6 and 58C/5 & C/1. The satellite imagery was processed using ERDAS IMAGINE 8.7 and ArcGIS 9.2.
Protocol used for mapping the distribution of Villorita cyprinoides using environmental variables and satellite image was adopted from [
the most correlated variables where subjected to overlapping to identify the deterministic variable in clam distribution (
Inverse distance weightage modelling is a spatial interpretation technique of using points with known values to estimate values at other unknown points. This is a type of deterministic method for multivariate interpolation.
The hydrographic variables of water at sampling stations in the distal fresh-wa- ter zone during pre and post monsoon seasons and that of the stations in the proximal brackish water zone during pre-monsoon season is summarised in
T-test on clam biomass during pre and post monsoon seasons in the distal zone indicated a significant difference in the clam biomass between the two seasons (P < 0.05). T-test on hydrographic variables in the distal zone during the sampling periods indicated a significant difference in the environmental variables between the two seasons (P < 0.05). The post-monsoon season in the distal zone showed a higher mean temperature (31.55˚C ± 1.01˚C) in comparison with the pre-monsoon season (31˚C ± 0.53˚C) similar to the results reported earlier [
T-test on clam biomass sampled from the proximal and distal zones during the pre-monsoon season showed that there was a significant difference between the two zones. Similarly, T-test on the hydrographic variables sampled from the proximal and distal zones during the pre-monsoon season showed a significant difference between the zones (P < 0.05).
The measurements indicated that mean weight of the clam, Villorita cyprinoides, was more in the distal zone during pre-monsoon season compared with
Seasons | Pre-monsoon | Post -monsoon | Pre-monsoon |
---|---|---|---|
Months | (Mar - Apr’08) | (Dec’07 - Jan’08) | (Mar - Apr’08) |
Zone | Proximal zone | Distal Zone | |
Area | Dredged area | Non-dredged area | Non-dredged area |
Mean of the variables/Stations | P1-10 | d1-10 | D1-10 |
Temperature (˚C) | 31.5 ± 0.93 | 31.55 ± 1.01 | 31 ± 0.53 |
pH | 7.9375 ± 0.42 | 8.05 ± 1.21 | 8 ± 0.00 |
Transparency (cm) | 85 ± 39.01 | 305.5 ± 65.76 | 129.5 ± 53.46 |
Salinity (ppt) | 9.0375 ± 0.42 | 0.34 ± 0.35 | 1.75 ± 1.52 |
DO (ppm) | 5.25 ± 1.59 | 5.5 ± 1.54 | 6.22 ± 1.73 |
Biomass (kg/m2) | 408.5 ± 340.93 | 660.66 ± 565.19 | 1203.1 ± 1460.58 |
Hardness (ppm) | 16.2125 ± 0.5 | 9.16 ± 2.61 |
Seasons | Pre-monsoon | Post ?monsoon | Pre-monsoon |
---|---|---|---|
Months | (Mar - Apr’08) | (Dec’07 - Jan’08) | (Mar - Apr’08) |
Zone | Proximal Zone | Distal Zone | |
Area | Dredged area | Non-dredged area | Non-dredged area |
Stations | D1-10 | d1-10 | P1-10 |
Number of samples | 414 | 1140 | 490 |
Length range of samples (mm) | 1.2 - 3.6 | 1.1 - 5.0 | 1.4 - 3.8 |
Mean length (mm) | 2.12 | 2.40 | 2.56 |
Weight range of samples (g) | 0.8 - 22.69 | 1.01 - 31.64 | 1.08 - 20.86 |
Mean weight (g) | 6.02 | 9.41 | 8.85 |
Variables | Factor 1 | Factor 2 | Factor 3 |
---|---|---|---|
Temperature | −0.19 | −0.11 | 0.18 |
pH | 0.05 | 0.10 | 0.49 |
Transparency | 0.01 | 0.62 | 0.35 |
Salinity | 0.21 | −0.12 | −0.36 |
DO | −0.02 | 0.68 | 0.04 |
Hardness | 0.95 | −0.28 | 0.11 |
Biomass | 0.79 | 0.50 | −0.34 |
Eigen Values | 1.76 | 1.51 | 1.11 |
% Cumulative variation | 27.43 | 50.98 | 68.33 |
that in the proximal zone where dredging was carried out (
The factor analysis for the pre and post-monsoon seasons in the distal zone of Vembanad estuary (
The bivariate plot (
oxygen and transparency on the second factor and that of salinity on the first factor could not be ruled out as a determinant of the clam distribution in the distal zone (
The mean dissolved oxygen observed in these stations during pre and post monsoon seasons in the distal zone were similar to earlier observations [
Since dredging was taking place on the distal side (northern side of the bund) and not in the proximal zone, there was a possibility that the variation in clam biomass and the associated hydrographic variables in the distal might be different from the proximal zone.
The factor analysis of variables during the pre-monsoon season in the distal and proximal zone of the estuary indicated that the first three factors explained 72.10% of the cumulative variations in the variables measured, with eigen values of 2.18, 1.32 and 1.15 respectively (
Factor loadings | Factor-1 | Factor-2 | Factor-3 |
---|---|---|---|
Temperature | −0.03 | 0.39 | 0.07 |
pH | −0.05 | 0.06 | 0.99 |
Transparency | 0.39 | 0.68 | 0.12 |
Salinity | 0.28 | −0.29 | 0.14 |
DO | 0.98 | 0.16 | 0.11 |
Hardness | −0.12 | −0.85 | 0.12 |
Biomass | 0.61 | 0.07 | −0.27 |
Eigen values | 2.18 | 1.32 | 1.15 |
% cumulative variation | 33.77 | 54.24 | 72.10 |
in the second factor may influence the clam distribution during pre-monsoon period in proximal and distal zones of the lake. The decreased transparency due to dredging during pre-monsoon period and the consequent reduction in clam biomass could be observed. The stepwise regression revealed that DO and salinity act as crucial variables influencing biomass and distribution of clams as in the Equation (2) (Adjusted R2 = 0.3) as
Water transparency and temperature showed a negative correlation during pre-monsoon analysis. This may be due to the highly turbid water at the proximal zone, which is a dredging site and the high mean water temperature during the season. Similar results were observed previously by [
Environmental variables such as temperature, pH, dissolved oxygen, transparency and hardness are important in deciding the distribution of an aquatic species [
The predominance of dissolved oxygen over salinity and temperature as represented in the bivariate plot (
distal zone in the estuary, IDW of temperature was not prepared. IDW model of clam biomass in distal zone in pre-monsoon (
The spatial variation of the dissolved oxygen in the proximal zone during pre-monsoon season showed that dissolved oxygen in the proximal zone was low, on account of low transparency due to dredging as well as the suspension of dredged materials due to entrenchment. The dissolved oxygen was minimum near the dredger and it gradually increased away from the dredger as observed in
The IDW representation of clam distribution in various seasons and zones indicated a parallel shift in the clam distribution in relation to the environmental variables emphasising the predominant role of the latter in determining availability of the species. The suitable habitat sites for clams during various seasons in the distal zone were identified using the IDW models of salinity and DO in distal zone during pre (
The IDW representation of clam distribution in various seasons and zones indicated a parallel shift in the clam distribution in relation to the environmental variables emphasising the predominant role of the latter in determining availability of the species. The suitable habitat sites for clams during various seasons in the distal zone were identified using the IDW models of salinity and DO in distal zone during pre (
The predictive spatial distribution map of clam abundance in the distal zone during pre-monsoon season (
zone during pre-monsoon season (
Similarly, the predictive spatial distribution map of clam abundance in the distal zone during post monsoon season (
The crucial role of dissolved oxygen in clam distribution during pre-monsoon in proximal zone as represented in bivariate plot (
Most of the previous studies related to mapping of molluscan resources were used for identification of suitable sites for aquaculture, habitat mapping etc. But they failed in representing the seasonal dominant variables for predicting the biomass of clams and its fishery (
The study is first of its kind, which attempts to map the clam distribution based on the responses to environmental variables and dredging. The study
Major activities during the study | Research output | Reference |
---|---|---|
Development of a data driven method for identifying areas most suitable for hard clam aquaculture in Florida USA using GIS and SRS. | Identified areas of the Indian River Lagoon on the Florida east coast and Charlotte Harbor on the Florida west coast that suits hard clam aquaculture | [ |
Development of an optimum location selection technique to transfer mangrove oyster Crassostrea rhizophorae raft culture technology to selected coastal communities in Margarita Island, Venezuela. | Multiple Criteria Evaluation was used to generate a map of the optimum sites for oyster aquaculture | [ |
Building up a spatial database using those criteria which were considered to have any influence in integrating marine fish-cage culture within the tourism industry in Tenerife (Canary islands). | Three model Criteria (distance to beaches, nautical sports, and view shed), which were mapped to show the most suitable areas for cage culture development in coexistence with tourism | [ |
Identification of the most suitable sites for hanging culture of Japanese scallop using geographic information system (GIS)-based multi-criteria evaluation models | A series of GIS models was developed to identify the most suitable areas for scallop culture using weighted linear combination | [ |
Identify the zones of high clam abundance and develop a model to explain the impact of environmental variables and dredging on clam biomass in an estuary | Developed GIS based IDW-model for the temporal clam biomass distribution in the Vembanad estuary and also during dredging identifying the dominant variable controlling the distribution in each season. | Present study is a combination of different strategies adopted by the earlier researchers with a better mapping protocol based on dominant variables. [ |
identified the sites near to dredger and beyond the dredger to a few distances as unsuitable for growth and survival of black clam because the entrenched material gets settled after being carried by the underwater movements at few distances beyond the dredger. This shows that this area is unsuitable for clam growth and even for any clam culture activities in the near future. The area ahead of dredger (P1) is depicted as safe from the impacts of dredging. The study also predicted the clam abundance sites in the pre-monsoon and post monsoon season in the distal zone, where in the colour gradient of green exhibited the abundance of clam.
Monitoring the distribution of clam biomass during different seasons on the basis of environmental variables helps in identifying the availability of the species and monitoring the catches season wise. The study reveals the importance of monsoon-based seasons in the distribution of the species in the estuary. The study indicates a shift in the clam biomass during various seasons. The clam fishery management may be devised based on the spatial abundance and the present study shows a spatial distribution of abundant clam zones for a proper management.
Long-term monitoring programmes are essential for defining protracted trends of environmental degradation or climatic change. Unfortunately, few studies worldwide have considered continuous long-term data. This paucity of information on long-term benthic change prevails in part because of the high expense associated with collecting, sorting, and identifying benthic samples. Such cost constraints lead to compromising over the intensity, distribution, frequency and longevity of a sampling programme. Truly long-term data sets with high spatial and temporal replication are therefore rarely available. The study requires as suggested a data frame over long term in addition to a detailed study on the distribution of the food of the species to comprehend on the exact distribution pattern of the bivalve.
The authors acknowledge the support from Dr. V. R. Suresh, Director, Central Inland Fisheries Research Institute, Dr. A. Gopalakrishnan, Director, Central Marine Fisheries Research Institute, Dr. Dilip Kumar, ex-director of Central Institute of Fisheries education for supporting the work.
Paul, T.T., George, G., Dennis, A., Athira, N.R., Biradar, R.S., Khandagale, R. and Padmakumar, K.G. (2017) Ecosystem Responses in the Distribution of Black Clam (Villorita cyprinoides) Beds in Vembanad Estuary during Environmental Changes Using GIS and RS. Journal of Geographic Information System, 9, 245-266. https://doi.org/10.4236/jgis.2017.93015