Length-weight relationship of a Cyprinid fish Puntius filamentosus from Chalakudy River, Kerala

Manuscript 1568; © ZOO; Date of publication 21 February 2007 Received 24 May 2006; Revised received 08 November 2006; Finally accepted 15 December 2006 2637 March 2007 | ISSN 0973-2535 (Print edition); 0973-2551 (Online edition) Puntius filamentosus (Valenciennes) is a medium-sized cyprinid that forms an important food and ornamental fish and is easily recognized by the elongated, filament-like extensions of the branched dorsal fin-rays in adult males. The lengthweight relationship is an important tool in fish biology, physiology, and ecology and fisheries assessment. This relationship serves three purposes viz. i) to determine the type of the mathematical relationship between two variables so that if one variable is known the other could be computed; ii) the relative condition can be estimated to assess the general well being of the fish and type of growth, i.e. whether isometric or allometric and iii) it helps to estimate the potential yield per recruit in the study of fish population dynamics. In fishes, generally the growth pattern follows the cube law (Brody, 1945; Lagler, 1952). Such relationship for the fishes will be valid when the fish grows isometrically. In such cases the exponential value must be exactly 3. But in reality, the actual relationship between length and weight may depart from the ideal value due to environmental conditions or condition of fish (Le Cren, 1951). This relationship is expressed by the equation W = aLb. This equation was used by several workers for different species from different habitats. Some of the recent works in this aspect are those by Mercy et al. (2002), Oscoz et al. (2005), Serajuddin (2005), and Harish Kumar et al. (2006). Methodology: A total of 60 specimens (24 males, 18 females and 18 indeterminate) of different size groups (5.4 to 12.0cm) were collected from the fish market close to the downstream stretch of Chalakudy river during January to December 2005. Lengths of fishes were measured with a mm scale and weighed with a top loading electronic balance in fresh condition to the nearest 1mm and 0.1g respectively. The length-weight relationship was estimated separately for males, females, indeterminate and pooled data using the linear form of formula W= aLb (Le Cren, 1951) as log W= log a + b log L, where, W = weight of the fish, L = length of the fish and ‘a’ and ‘b’ are constants. The constants 'a' and 'b' values in the equation were estimated using the methods of least squares. The 't' test was employed to test the level of significance of weight on length. The regression equation was fitted separately for each group and the slopes ('b' values) were tested to find out significant variation among them, if any, with ANCOVA. Statistical analysis was carried out with the help of the statistical software SPSS. Results: The estimated coefficients of the length-weight relationship and other details of statistical analysis are given in Table 1 and the results of the ANCOVA analysis are given in Table 2. The r values and t test indicates a high degree of correlation between length and weight. The ANCOVA results showed that there is no significant difference between the ‘b’ values of males and females. The length-weight relationships (logarithmic) for the different groups of Puntius filamentosus from the Chalakudy River are as follows: