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Length-weight Relationships and Relative Condition Factor of the Mullet, Rhinomugil Corsula (hamilton, 1822) in the Sitakunda Coast of the Bay of Bengal, Bangladesh

Syeda Ismat Ara
Department of Zoology, University of Chittagong, Chattagram 4331, Bangladesh

Mohammad Ali Azadi
International Islamic University Chittagong 4318, Bangladesh.

Munira Nasiruddin
Department of Zoology, University of Chittagong, Chattagram 4331, Bangladesh.

Aftab Hossain
Department of Zoology, University of Chittagong, Chattagram 4331, Bangladesh.

Abstract/ Executive Summary:

This study deals with the length-weight relationship and relative condition factor (Kn) of 2480 specimens of Rhinomugil corsula (Hamilton, 1822), ranging from 3.5 cm to 29.5 cm total length and 0.40 g to 235.34 g weight, sampled monthly for two years periods from March 2016 to February 2018 from the Sitakunda Coast of the Bay of Bengal, Bangladesh. Arithmetic form of equations for length (TL) and weight (W) of fish showed curvilinear relationships in male: W=0.0081TL3.016, female: W= 0.0083TL3.058 and combined sex: W=0.0076TL3.052, while logarithmic scale showed linear relationships in male: LogW=- 2.122+3.016LogTL, female: LogW=-2.09+3.058 TL and combined sex: LogW=- 2.121+3.052TL. In all the cases the coefficient of correlations between length and weight were positive and highly significant (P<0.01). The regression coefficient ‘b’ values were found to be 3.01±0.11, 3.06±0.12, and 3.05±0.12 for male, female and combined sexes, respectively for two years periods together. When ‘t’ test was done, the growth was found to be isometric in the case of males and combined sex and positive allometric in the case of females. The relative condition factor (Kn) varied from 1.0003- 1.009, 1.001-1.008 and 1.001-1.008, respectively, for male, female, and combined sex in 2016-17, whereas 1.001-1.007, 1.003-1.006 and 0.992-1.008 respectively in 2017- 18. The Kn values were always around ‘1’ or a little bit more than ‘1’ for length groups as well as monthly, indicating the excellent growth and robustness of fish due to favorable environmental conditions prevailing in the habitat of the fish, in the Sitakunda coast of the Bay of Bengal.

Keywords: Length-weight relationship, Relative condition factor, Rhinomugil corsula, Sitakunda coast, Bangladesh

Location: Department of Zoology, University of Chittagong.

Start Date: 00-03-2016

End Date: 00-02-2018

Program Area: Animal Health and Management

Commodity: Sea fish

Objectives:

To understand the growth pattern and wellbeing of the fish and health condition of the habitat in the Sitakunda coast of the Bay of Bengal which is very much needed in fisheries management.

Methodology:

Experimental fishes were collected from the three fish landing stations namely (1) Sitakunda Bazar fish landing station (22°37?12??N, 91°46?4??E) (2) Kumira fish landing station (22°30'33"N, 91°41'56"E) and (3) Bhatiari fishing village (22°25'39"N, 91°44'17"E), and sometimes directly from fishermen catch in the Sitakunda coast. After collection, the fish samples were brought to the laboratory of the Department of Zoology, University of Chittagong. Lengths were measured to the nearest centimeter (cm) and weight up to the nearest gram (g). The fishes were grouped according to sex and those fish that were too small to identify the sex were considered as combined sex. Samples were collected monthly, thus a total of 2480 specimens of Rhinomugil corsula were collected, where 854 were males, 1482 were females and 144 were unsexed due to small size. Fish total length and weight ranged from 3.3 cm to 29.5 cm and 0.4 g to 235.34g, respectively. Of the total of 2480 fish, 1231 were collected and examined during the first year, from March 2016 to February 2017, and the remaining 1249 during the second year, from March 2017 to February 2018. The L-W data of male and female fishes were used for the study of length-weight relationship separately and also for combined sexes for each year and also for two years together. Sexes of undetermined fishes below 7 cm in length were treated under combined sexes. The length-weight relationships were calculated following the non-linear formula of LeCren (1951): W= aLb, Where W= total body weight (g), L= total body length (cm), a= intercept / constant, b= exponent. This relationship was also expressed in the logarithmic transformation giving a straight-line relationship, Log W= Log a + b Log L. This equation is the same to Y = a + bX, where LogW=dependent variable (Y), Log L = independent variable (X), b= regression coefficient, a= intercept. To get an idea of the relationships (linear, curvilinear or non-linear) between the length and weight, a scattered diagram was drawn, after that arithmetic (curvilinear) and logarithmic (linear) graphs were fitted. To see the effect on body condition due to different length groups and seasons, monthly and length group-wise relative condition factor (Kn) was calculated for male and female sexes separately and also for both the sexes combined, following LeCren (1951): Kn= W/W, where Kn=relative condition factor, W=observed weight, and /W =calculated weight.

Source of Information: Bangladesh J. Zool. 49(1): 91-103, 2021

Article Link (Online Journal): DOI: https://doi.org/10.3329/bjz.v49i1.53685

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

In fisheries management, information on length-weight relationships for a given species could be used in setting up yield equations. In fish yield measurement from a water body and in the regulation of fisheries through the construction of desirable mesh-size of the fishing gear in response to the market demand for a fish not having less than a marketable size and weight–the knowledge of length-weight relationship for a given species could be of practical utility in fisheries practices. A relative condition factor is an important tool for investigating the body condition and the general well-being of a fish in its environment. This may vary with food abundance and the average reproductive stage of the fish stock, and also parasitic infections and physical factors reflecting recent physical and biological circumstances.

Knowledge Management: Journal