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Population Dynamics of the Ribbon Fish, Lepturacanthus Savala (cuvier 1829) from the North-eastern Part of the Bay of Bengal

M.G. Mustafa
Bangladesh Fisheries Research Institute, Mymensingh 2201

S.M. Nurul Amin
Bangladesh Fisheries Research Institute, Mymensingh 2201

M.A. Haque
Institute of Marine Science, University of Chittagong, Chittagong4331, Bangladesh

M. Zafar
Institute of Marine Science, University of Chittagong, Chittagong4331, Bangladesh

Abstract/ Executive Summary:

Population parameters of Lepturacanthus savala from the trawl catches in the northeastern part of the Bay of Bengal, Bangladesh were investigated based on length frequency data, using compleat ELEF AN computer program. The asymptotic length (Loo) and growth constant (K) were estimated to be 106.50 em (total length) and 0.80/year respectively. Based on these growth parameters, the total mortality (Z) was estimated to be 1.89. The estimated values for natural mortality (M) and fishing mortality (F) were 1.08 and 0.81 respectively. The estimated value for the exploitation rate (E) using the length converted catch curve was 0.43. The recruitment pattern showed two peaks per year. The estimated sizes of L. savala at 25, 50 and 75 percent probabilities of capture were 57.49, 60.39 and 63.28 cm respectively. The estimated length weight relationship for combined sex was W = 0.00093 TL ^2.97

Keywords: Population dynamics, L. savala, Bay of Bengal

Location: North eastern part of the Bay of Bengal

Start Date: 00-10-1996

End Date: 00-09-1997

Program Area: Animal Health and Management

Commodity: Marine fish

Objectives:

To estimate the population parameters of L. savala to assess its stock in the north-eastern part of the Bay of Bengal.

Methodology:

Collection of data- Length frequency data of L. savala were collected from trawler catches in the northeastern part of the Bay of Bengal during the period October 1996 to September 1997 by making regular fortnight field visits to the major fish landing centres in Firingibazar, Chittagong. More than eight trawlers of Sea Resources Group Companies Ltd. Bangladesh were sampled randomly. On each sampling day, total lengths of 100-200 fish obtained by random sampling were measured to the nearest 0.1cm using a measuring scale. Analysis of data- Monthly length frequency distribution of L. savala for each month was analysed using compleat ELEFAN computer program . The program was also used to estimate the parameters of the von Bertalanffy growth equation. The fitting of the best growth curve was based on the ELEFAN I program, which allows the line to pass through the maximum number of peaks of the length frequency distribution. With the aid of the best growth curve the growth constant (K) and the asymptotic length (Loo) were estimated. Additional estimate of Loo and Z/K value were obtained by plotting L minus L' on L , i.e. L- L' =a+ bL' where, Loo =-alb and Z/K = - (1 + b)/b where L is defined as the mean length, computed from L' upward, in a given length-fre_guency sample while L' is the limit of the first length class used in computing a value of L. The growth performance of L. savala population in terms of length growth was compared using the index. i.e. ' = Log₁₀ K + 2 Log₁₀ Loo. (Where Loo is the asymPtotic length in cm and K is the growth constant per year}. The instantaneous total mortality coefficient (Z) was estimated using the length converted catch curve method which has been incorporated into the compleat ELEFAN computer program. Natural mortality rate (M) was estimated using Pauly's empirical relationship 0 i.e. · Log₁₀ M = - 0.0066 - 0.279 Log₁₀ L= + 0.6543 Log₁₀ K + 0.4634 Log₁₀ T where L= is expressed in em and T, the mean annual environmental water temperature in ac (here it was 28°C). Fishing mortality (F) was obtained by subtracting M (natural mortality) from Z (total mortality) and exploitation rate (E) was obtained from F/Z. Recruitment pattern was obtained by backward projection on the length. Using Pauly's empirical equation for theoretical age at length zero a very approximate estimate of theoretical age at length zero was obtained. The recruitment pattern was also derived using the compleat ELEF AN computer program. The probabilities of capture by length of L. savala were estimated by calculating the ration between the points of the extrapolated descending arm and the corresponding ascending arm of the length converted catch curve. Relative yield-per-recruit Y/R and biomass-per-recruit B/R were obtained from the estimated growth parameters and probabilities of capture by length. The calculations were carried out using the compleat ELEF AN package developed at ICLARM. Length weight relationship was estimated for combined sex using simple linear regression. For this purpose 352 specimens of L. savala were measured and varied from 12.0 em to 32.0 em in total length and 25g to 340g in body weight during one year samples.

Source of Information: Bangladesh J. Fish. Res., 4(2):179-184, 2000

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The asymptotic length (Loo) and growth constant (K) were estimated to be 106.50 cm (total length) and 0.80/year respectively. Based on these growth parameters, the total mortality (Z) was estimated to be 1.89. The estimated values for natural mortality (M) and fishing mortality (F) were 1.08 and 0.81 respectively. The estimated value for the exploitation rate (E) using the length converted catch curve was 0.43. The recruitment pattern showed two peaks per year. The estimated sizes of L. savala at 25, 50 and 75 percent probabilities of capture were 57.49, 60.39 and 63.28 em respectively. The estimated length weight relationship for combined sex was W = 0.00093 TL ^2.97

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