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Catch Composition, Seasonal Abundance and Length-weight Relationship of Elasmobranch Species of the Bay of Bengal, Bangladesh

Ehsanul Karim
Bangladesh Fisheries Research Institute, Marine Fisheries & Technology Station, Cox’s Bazar 4700, Bangladesh

M. Zaher
Bangladesh Fisheries Research Institute, Marine Fisheries & Technology Station, Cox’s Bazar 4700, Bangladesh

Sumon Barua
Department of Fisheries, Marine Fisheries Wing, Chittagong

M.J. Rahman
Department of Fisheries, Dhaka

M. Enamul Hoq
Bangladesh Fisheries Research Institute, Mymensingh

Abstract/ Executive Summary:

Present study consists of total catch, catch per unit effort, percentages and trends of catch composition, and length-weight relationship for elasmobranch species for the year of 2010 to 2013 of the Bay of Bengal, Bangladesh. A total of 26 elasmobranch species were identified from the landing data of artisanal fisheries, among them Carchariniformes was 14-18%, Rhinobatiformes was 4-7% and Mylobatiformes, Torpediniformes was 75-80%. Total catch percentage of true sharks varied from 14% to 18%, skates from 4% to 8% and rays from 73% to 80%. Among 26 identified species, length-weight regressions were presented for 24 species. The parameter b ranged from 2.71 for dog shark (Scoliodon laticaudus) to 3.50 for sharp nose guitar fish (Rhyncobatus granulatus) with a mean value of 3.138±0.02 and the median value ranged between 3.06 and 3.34. The coefficient of determination (r²) ranged from 0.890 for black spotted electric ray (Narcine teimlei) to 0.990 for dwarf sting ray (Himuntura walga). Sixteen species had r² values between 0.95 and0.98 while seven r² values were less than 0.95. All regressions were highly significant (p<0.001). Again ten species showed positive allometric growth, five showed negative allometric growth and nine showed isometric growths. It indicates different growth status of different species in different months and also indicates the proper growth of maximum species.

Keywords: Elasmobranchs, Bay of Bengal, Length-weight relationship

Location: Cox’s Bazar, Chittagong, Kuakata and Dubla Island.

Start Date: 00-07-2010

End Date: 00-04-2013

Program Area: Animal Health and Management

Commodity: Marine fish

Objectives:

To provide information about the total catch, catch per unit effort, percentages and trends of catch composition, and length-weight relationship for elasmobranch species

Methodology:

Sharks are mainly caught by artisanal fishery with drift gill net (DGN) which is used mainly for hilsa catching, set bag nets (SBN), long lines/hook and lines (LL/HL) and trammel nets (TN). But now a days, the shark fishing activities are done mostly by newly introduced ‘shark net- a modified gill net’. The study was conducted in the important shark landing sites throughout the coastal belt of Bangladesh including Cox’s Bazar, Chittagong, Kuakata and Dubla Island areas. In each of the areas, all sub-stations were included. For example in Cox’s Bazar the compiled data of the sub-stations i.e. BFDC landing center, Moheskhali, Kutubdia and Teknaf were included. Length-weight data were analyzed for the data collected from July, 2010 to April, 2013 of different artisanal fishing vessels that caught sharks by using various gears like long line, gill net, shark net etc. from above landing stations. Species identification (taxonomic study) and catch composition of shark fisheries: The group name ‘sharks’ were used as per FAO guidelines to include all Elasmobranches, i.e. Cartilaginous fishes with emphasis on the sharks, skates and rays. The landing sites were visited fortnightly in all the seasons considering lunar cycle and all species were recorded together with their respective catch in weight and size in length. Samples were identified and taxonomical study was also conducted with greater efforts. Then the collected information was assessed to evaluate the lunar and seasonal impact. Finally, the annual catch for each of the species were assessed for each of the landing sites. Catch per unit effort (CPUE) and seasonal abundance: The catch of fish, in numbers or in weight, taken by a defined unit of fishing effort is catch per unit effort (CPUE). Also called, catch per effort, fishing success, availability. CPUE is a measure of the density or population size of an animal that is targeted by fishing. Large CPUEs indicate large populations since many individuals are caught for every unit of fishing effort. CPUE standardizes catch data based on the amount of the effort (total time or area sampled) exerted. Considering the scope an effort was done to find out the trends of sharks availability and trend of stock position by analyzing species-wise total catch, contribution of individual species to the total catch and CPUE with the data from the fishing log sheet submitted in BFDC office by the skippers upon arrival from their sea voyages. Length-weight data analysis: All individuals were measured (either total length indicated as TL or fork length indicated as FL) to the nearest cm and weighed to the nearest gram. The weight–length relationship was calculated using the expression W = aL^b, where the W is the weight (g), L the length (cm), a the intercept of the regression and b is the regression coefficient. Parameters a and b of the weight–length relationship were estimated by linear regression analysis based on logarithms log(W)= log(a)+ b log(L). All data were log10 transformed for correction of non-linearity and heterogeneous variances. A t-test was done to compare regression lines whether b was significantly different from 3 (α= 0.05). Normality of the distribution of b was tested using Shapiro-Wilk normality test.

Source of Information: Bangladesh J. Fish. Res., 15-16, 2011-12: 115-124

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

A total of 26 elasmobranch species were identified from the landing data of artisanal fisheries, among them Carchariniformes was 14-18%, Rhinobatiformes was 4-7% and Mylobatiformes, Torpediniformes was 75-80%. Total catch percentage of true sharks varied from 14% to 18%, skates from 4% to 8% and rays from 73% to 80%. Among 26 identified species, length-weight regressions were presented for 24 species.

Knowledge Management: Journal