Nghiên cứu quy luật sinh trưởng nghêu trắng Bến Tre (Meretrix Lyrata) tại cửa sông Bạch Đằng !

1. INTRODUCTION

Bach Dang estuary, with two sides distributes into tidal flats, is salient terrain in tidal areas. Fraction of tidal flat is fine sand-silt created by the interaction of tidal and rival flow (Tran Duc Thanh et al, 2000). Fraction of tidal flat is appropriate to habitat of hard clam (Meretrix Lyrata) (MCD, 2009).
There are many advantage conditions as nutrition, current, salinity, pH, DO, detritus and plankton brought by the flow the continent to estuary, so hard clam here is developed widely.  Area of hatchery of hard clam at Bach Dang estuary, concentration at Dong Bai commune – Cat Hai district (figure 1) far from Cat Ba Island about 20km, is 23.9 ha (2000) to 155.5 ha (2007) (Tran Dinh Lan and Luc hens, 2009). With specific character at Bach Dang estuary so rule of growth of hard clam (Meretrix Lyrata) is different from others such as Nam Dinh province, southern Viet Nam (Tien Giang province, Ben Tre province). 
Nguyen Huy Yet et al.(2007) studiedthe growth rate of hard clam (Meretrix spp.) at Nam Dinh province which is central hatchery of hard clam in northern Viet Nam. The results show the significantly relationship between size and growth of hard clam (Meretrix lyrata). Truong Quoc Phu (1999) also showed the relationship between weight and length of hard clam (Meretrix lyrata) following Von Bertalanffy formula which was the correlation among length and time (months old) with growth rate, and seasonable growth. With hard clam grown Bach Dang areas, especially estuary, there hadn’t any result for rule of growth rate. 
The result is signification highly in science that age of hard clam (Meretrix Lyrata) is determined by size of shell (length, width or height). With hatchery of hard clam, the result evaluates growth rate to increase productivity.

2. MATERIALS AND METHODS
2.1. Materials
The article is a part of thesis at HaNoi University of Technology: "Studying on capacity of bioaccumulation mercury of Hard clam (Meretrix Lyrata) at Bach Dang estuary, HaiPhong – VietNam”.

                                               


2.2. Methods
Design of experiment
Tidal range in the studying area is around 2.5 to 3.2 m at flood-tide, and from 0.5 to 1m at ebb-tide (Tran Duc Thanh et al, 2000). So the experiment was designed the following:
- Setup two areas (10m²) on surface to be perpendicular dike.
- Area 1 (symbol: OTN) in high tidal flat with 8 hour dry. Rate sand/ mud are 90:10.
- Area 2 (symbol: AD) in low tidal flat with 5 hour dry. Rate sand/ mud are 70:30.


Specific growth rate
30 individuals of hard clam are monthly sampledfor each area. Hard clams are gathered in defined areas so their size are quite unity.

                                                            
                                                           Figure 2: Growth of Hard clam in Bach Dang eastuary

The result shows that rule of growth rate of bivalves is logarithm function (Truong Quoc Phu, 1999). Growth rate of hard clam in Nam Dinh province (Meretrix meretrix, Meretrix lusoria, Meretrix Lyrara) between size and age is shown in the table (Nguyen Huy Yet et al, 2007). Result is shown by Excel 2003, relationship is logarithm.
The relationship between size (length, width or height) and age (months old) of hard clam is X = alog Y + b. Use software Sigmaplot for Window Version 11.0 (Copyright 2008 Systat Software, Inc).
With:    - X is size of hard clam (cm).
       - Y is age of hard clam (months old).
       - a and b are constant, are determined by experiment.
Statistical analysis
To set up two areas of hatchery of hard clam distributed alongsea level so growth rate of hard clam is different between two areas. Size’sdifferences between hard clam breeding in two areas were tested by analysis of variance (ANOVA). If size of hard clam is different from two areas then to established function (size and age of hard clam) differently. If size of hard clam isn’t different in calculation average size then hard clam at Bach Dang estuary was calculated with only a function. Statistical analysis was performed by use of the software Excel 2003 for Windows (Microsoft Office Excel, USA). F and F_crit (to determine by software Excel 2003) are parameter to access relationship two series of figure. If F> F_critthen supposition (H₁, series is different) is accepted or on the contrary supposition (H₀, series is not different) is accepted.


3. RESULTS
- Growth rate of hard clam at Bach Dang estuarydistinguished from other estuaries as unique local properties of seasons, flow, altitude of tidal flat and nutrient regime. Rule of growth rate was divided intoseveral stages with logarithm rule.
- Hard clam living in high tidal flat grows slower than ones in low tidal flat. Relationship between length and ages is formulated:L = -28.6455+35.5797*ln(T) - 13.3596*(lnT) ² +1.7179*(lnT) ³. The rule is suitable with previous research.
- To determine ages of hard clam at Bach Dang estuary based on size of shell (length, width or height) that formula:
T = . With formula 1.3 is the most correct. 

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Nguồn: NCS. Lê Xuân Sinh - đăng tạp chí Environment and Natural Resouces Researcher, Canada

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