CORRELATION BETWEEN THE EDIBLE FRESHWATER BIVALVES AND SOME ECOLOGICAL FACTORS AT SELECTED SITES OF CAGAYAN RIVER.

Nenette T. Columna. Cagayan State University, Lallo Campus, Lallo, Cagayan Valley, Philippines. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 05 December 2018 Final Accepted: 07 January 2019 Published: February 2019


Methodology:-Study Area
Cagayan River, situated in Cagayan Valley is the longest river in the country. It is about 380 kilometers long and has an average width of about 100 meters. The Cagayan River basin covers an area of about 28,110 square kilometers (Cortes, 1980). The river crosses the provinces of Cagayan north, Isabela in the central portion, Nueva Viscaya and a narrow strip of Quezon province in the south. It flows over the entire length of the valley in the northerly direction from its head Nueva Viscaya to its mouth in the Babuyan Channel near Aparri. On the other side of this fertilizing and mighty river lie the open stretches of Cagayan Valley.
The study area is situated at the northernmost part of Cagayan at approximately latitude 10 0 00‖ to 18 0 23‖, longitude 121o 38‖ to 121o 43‖. It is about thirty kilometers from the mouth of the river starting from station I. the area was divided into four stations designated as Station 1 (Gattaran), Station 2 ( Lal-lo), Station 3 ( Camalaniugan) and Station 4 (Aparri).

Biological collection
Two sampling areas from each town were chosen. Collection of bivalves was done once a month for a period of six months. Dredging time was 20 minutes per 10 meters of distance traveled in each sampling site.
The clams collected were counted and sorted according to species and area of collection. Samples for identification were preserved in 10 % formalin solution.

432
Water and substrate samples were collected from different sampling sites in each station by a hired fisherman/diver. The water and substrate samples were transferred in labeled bottles and plastic bags, respectively, for analyses in the laboratory.

Dredging
An indigenous submarine dredge was used. It is composed of one flat-bottom row boat or motor boat about 10 meters long, 1 meter wide and 0.36 meter deep at the center, a -tako‖ or dredge (Fig. 2) and a pair of boho, Bambusa spp. mattings or -sawali‖ (Fig 1) about 3.1 x 1.3 meters.
Upon reaching the fishing ground and after ascertaining the depth of the bed, the dredge was dropped. When the dredge reaches the bottom, the pair of -sawali‖ wings was set into position in the water, about one and one half meter from the bow end, one at each side in a wing-like formation. The -sawali‖ wings intercept the volume of current that maintains the boat in a downward dredging position.
After the dredge has been set, the boat was allowed to drift with the current, the dredging path changed occasionally the wings. The dredge was hauled every 20 minutes in the four stations.

Shell Size and Density
Shell size was determined using a Vernier Calliper through the measurement in centimeters.

433
The density of each bivalve species was expressed as number of bivalves/ square meter area dredged collected after 20 minutes dredging. An average of 10 meters in each sampling site was measured and used to compute for the density of each bivalve species.

Physico-chemical Parameters
The following Physico-chemical characteristics of water from the four stations of the river were monitored once a month. 1. Temperature of water at two levels (that is at the bottom and at the surface) was taken by the use of an ordinary laboratory thermometer, both temperature was measured by lowering a thermometer tied with a graduated string and a weight at the bottom. After few minutes, the thermometer was retrieved rapidly. An average of two readings was considered. 2. River depth was measured by the use of an improvised depth gauge (nylon string with metric calibration and with one end tied with weight). 3. Dissolved oxygen content (DO) was determined by Iodometric Method or Winkler's Method. 4. pH was determined by the use of a portable pH meter, model HM -1K. 5. Total water hardness was determined by titration (EDTA Titrimetric Method). 6. Salinity and conductivity was determined bythe use of a Salinity and Conductivity Meter, USI Model 33. 7. Transparency was measured using Secchi Disc. The depth at which disc disappeared and reappeared was averaged and reported as Secchi Disk Transparency.

Substrate Analysis
Type and characteristics of the substrate from the four stations of the study area were determined. Samples of substrate collected in each sampling site per stations were air dried and analyzed in the laboratory for its organic content.
Total organic content was determined using the Walkley-Black Method (Allison, 1965). The organic carbon percentage was determined and multiplied by a conversion factor 1.724 to obtain organic matter percentage. Another set of substrate samples was oven and sieved to determine the size and type of particles.

Statistical Analysis:-
Monthly values of the average shell size (length and width) and density for the population of the different species of edible bivalves in the four stations were computed and tabulated. Likewise, the mean of the physico-chemical characteristics of the river water and substrate: temperature, transparency, depth, pH, salinity, conductivity, hardness, dissolved oxygen and organic content were computed and tabulated per month (February to July) from the four stations of the study area.
The relative abundance of collected bivalves was computed using the formula: Relative Abundance = n1 / n where n1 -is the number of individuals of each species n -is the total number of individuals collected The monthly relative abundance for the population of each bivalve species was also computed and tabulated.
Analysis of variance was used to determine the significant differences of the mean of the physico-chemical factors, density, relative abundance and shell size from the four stations. The Simple Pearson Correlation (r) was used to find the correlation between shell size (length and width), density, relative abundance of each bivalve species and the physico-chemical parameters of the water and the organic content of the substrate. Correlation was used to establish the degree of association between pair of variables.
-Cabibi‖ scientifically named Anodonta sp. was a freshwater clam known to occur only in that part of Cagayan River along the municipality of Lallo and vicinity, (Hosillos, 1957) and (Escritor,1959) pointed out that the -cabibi‖ grounds were situated from the southern tip of Camalaniugan extending southward to the southern tip of Lallo. However, an assessment was made by Layugan (1988) on the edible mollusks found in Cagayan river. There were three species of bivalves identified, ‗unnoc‖, ‗bennec' or ‗tulya' and ‗cabibi‖ scientifically name Batissa violacea.-Cabibi‖ was scientifically named Cyrena sp. According to Sowerby( 1878), Cyrena sp. was a big clam that inhabit rivers and estuaries, most likely the same as that of Batissa childreni which were smaller in size. Cyrena sp. and Batissa childreni( Gray, 1878) and Batissa violacea (Lamark, 1818) had the same local name as -cabibi‖.In this study, -cabibi‖ was identified as Batissa childrreni (Gray, 1878) contrary to the findings of Layugan, 1988 and Mayo, A.D., et al., 2016 which is Batissa violacea (Lamark, 1818). This could be due to some revisions on the identification and authentication of some species made by taxonomists.
Batissa childreni has a rounded shell, smooth with few growth lines and greenish to yellowish in color. The shell is more or less thick. The shell of Corbicula manilensis is triangular with striated surface, brownish black periostracum. Psammotaea virescens is a small species with thin shell, colored purplish to brownish.

Fig 3:-BatissachildreniCorbiculamanilensisPsammotaeavirescens
The three edible freshwater bivalves that thrived in Cagayan River were distributed in some identified areas. Batissa childreni were found only in Station 1 and 2 (Gattaran and Lallo). There were more bivalves of this species in Station 2 but bigger clams were noted in Station 1. Corbucula manilensis was collected from Station 1, 2 and 3. A large number were dredged in the month of May. Psammotaea virescens, a small species of bivalves were found only in Station II and III.

Physico-Chemical Factors Depth -
The shallowest depth of the river recorded was 8.25m at Station 1 in the month of February. It, however became deeper from March to July.

Transparency-
It was in July when transparency had the lowest value at because of the heavy downpour of rain. When it rains, runoff coming from nearby farms and mountains flow to the river. Run-off carries fine soil particles that make the river water very turbid thereby light can no longer penetrate in the water.

Temperature -
Water temperature (surface and bottom) did not fluctuate significantly. The highest temperature was observed in April and June in all stations because of the strong heat of the sun during these months, while low temperature occurred in February.

Dissolved Oxygen-
The average DO at Station 1shows a decreasing trend, the highest value was observed in the month of February. Station II was shown to have the highest DO and lowest in Station 4.

pH and Total Hardness-
A pH reading slightly lower than neutral was observed in the month of February and relatively alkaline in the month of June. However, the pH values were almost similar from the four stations. Total hardness was very high at Station 1 and 2, which is classified as very hard. These stations were surrounded with rocks, the river bank was full of fossils shells. Rainwater carries Ca ++ , Mg ++ , other polyvalent metals, carbonates and bicarbonates and goes to the river making the water to be hard.
Salinity and Conductivity -Station 1 and 2 had a zero salinity while it was expected that Station 4 ( Aparri) had the highest because of its proximity to the sea. Station 4 having the highest salinity was expected to have the highest conductivity. It was also noted to have high pH and relatively high temperature. The reasons for these relationship may be assumed on the concentration of ionized substances present brought about by the salt ions and both the H + and OH -.
Substrate quality -Station 1 and 2 had a sandy substrate, the texture was observed to be course. Loamy sand was the type of substrate in Station 3 and Station 4 had a silty loam substrate and had the highest organic content. Stations 1, 2 and 3 had a zero organic content in February and March but there was an increased in April.

Population Density
The survey of population density of each bivalve species shows that Batissa childreni was present only in Station 1 and 2 with highest density at Station 2. Lowest density (3.56/m2 area dredged) was noted in the month of April while highest value (413.53/m2 area dredged) was in March (Fig 4). Figure 5 shows that Corbicula manilensis thrived only in Stations 1, 2 and 3. An increase of density from February to July was observed from the three stations.
Psammotaea virescens was absent in Stations 1 and 4. In Stations 2 and 3, an increase of density from February to May was observed (Fig 6). The density of Batissa childreni was greatly affected by the temperature of water. This was shown in the adjusted effects of the variables of density and correlation analysis.As the temperature of water decreased, more bivalves were collected in the study area, thus the density was high. Apparently, the water temperature and density of Batissa childreni were negatively correlated.
Crumb, S.E. (1977) reported that the density of macrobenthos of Delaware River seemed to be the function of temperature. The maximum amount of oxygen that can dissolve in water at 5 0 C is 12.8mg/L. This means that 1L of water at 5 0 C can dissolve no more than 12.8 mg of oxygen. At 15 0 C the maximum concentration of oxygen in water is 10.1 mg/L. At 20 0 C it is only 9.1 mg/L. It is clear that the maximum concentration of DO in water decreases as the temperature increases (UPSEC, 1986). In this study, the mean density of Batissa childreni was highest in the months of February and March when there was low temperature and the DO was high compared to other months. Thus, these bivalves were supplied with a great amount of oxygen in a lowest temperature resulting in a greater number of individuals. There is a moderate positive correlation of the population density of Batissa violacea on water quality of the river like water depth, current and temperature (Mayor, A.D., et al., 2016). Natural population of freshwater mollusk show extreme variation in the pattern of shell formation which may or may not be related to water hardness (Russel-Hunter, 1981). This was particularly true to Corbicula manilensis in which the density and relative abundance was high when the water has low total hardness. The present study shows that as the total hardness was low, the density of this clam was high and they were more abundant. Density of Corbicula manilensis may not be related to the hardness of water but to other factors.Corbicula fluminea is denser in a fine sand substrate, well-oxygen lotic system with optimum environmental conditions (Belanger, S.E.,et al. 1985) Harrison and Rankin (1978) showed that the greatest densities of Pisidium puntiferum occurred where there was the greatest percentage of organic material in the substrate.

Relative Abundance
Relative abundance of Batissa childreniwas highest in February at Station 2. There was a gradual increase from February to July with Station 1 that has the highest relative abundance (Fig. 7). In Station 3 and 4, there was no Batissa childreni that was dredge while Corbicula manilensis did not thrive in Station 4. The relative abundance of Psammotaea virescens decreased from February to July while Corbicula manilensis increased in number (Figs. 8 & 9). Corbicula manilensis is the most abundant during the six-months study. A number of studies indicate that -hardwater‖ is better than -soft‖ for mollusks. Macan(1938), group mollusks according to hardness of water in which freshwater mollusks were found more. This was particularly true to Batissa childreni in which this species was more abundant in stations with high total hardness. Boycott (1936) reported that fifty percent of the species of mollusk are hardly ever found in water with less than 20mg/L Ca. With decreasing amounts of Calcium below this concentration, the number of species becomes progressively fewer. Figure 10 shows that there was an increase of the shell size in the month of April and June. It was observed that the values of the mean length was higher than the width except for some months where the mean width was higher than the length.

Shell Size (Length and Width)
For Corbicula manilensis, the longest mean length was observed in July and April and the widest width was present in May. The values of the mean length were higher than the mean width. There was an increase in shell length from February to April while increase in shell width was from February to May ( Figure 11). Figure 12 shows the mean shell size of Psammotaea virescens. Big sized clams occurred in May. There was increase in length as the width increased. Longest mean length was noted in May and widest mean width was observed in May. Small sized clams were noted in April. The correlation coefficient between the shell size and the physico-chemical parameters shows that temperature was highly correlated with the shell size of Batissa childreni. As the temperature decreased, bigger bivalves were observed indicating a negative correlation.
Total hardness influenced the shell size of Corbicula manilensis. The higher the total hardness, the bigger bivalves collected. The lower the hardness, there were small sized bivalves collected.
The length and width of Psammoatea virescens was accounted to the organic content of substrate. As the organic content of the substrate increased, the size also increased indicating a positive correlation.Psammotaea virescens (Deshayes, 1855)) is one of the nine species under Family Veneroidea that are found and identified throughout Onagawa Bay, Northeastern Japan. Most of these bivalves occurred in the muddy sand bottom with high organic matter ( Sasaki, K., 1996).
In this study, the three species of edible bivalves thrived on their natural habitat feeding mostly with organic materials. In the study of Mayor