ABSTRACT

A study on the population dynamics of the Jew fish, Johnius coitor (Hamilton-Buchanan, 1822) from the Bay of Bengal, Bangladesh was conducted using FISAT-II program, with the length-frequency data of 1178 fish samples. The growth parameters like asymptotic length (Lα) and growth co-efficient (K) were calculated to be 29 cm and 1.1year-1, respectively. The rate of natural mortality (M), fishing mortality (F) and total mortality (Z) were found to be 1.1, 1.84 and 2.94, respectively. The recruitment pattern of the studied fish has shown the species recruited with one peak during April to July. The probability of capture at L25, L50, L75 were observed to be 13.6, 15.8 and 16.96 cm, respectively. The relative yield per recruit (Y΄/R) and biomass per recruit (Β΄/ R) were found to be 0.544 and 1.0, respectively. The exploitation (E) rate was observed to be 0.63, which indicated the fish was over fished (E> 0.50) in the Bay of Bengal. But the maximum exploitation (Emax) value was 0.731 which was greater than current E (0.63) value. The growth performance index (φ΄) of J. coitor in the Bay of Bengal was moderate (2.97). Emphasize should be given on proper and requisite recruitment to overcome the overfishing and to generate maximum yield per-recruit.

Key words: Jew fish, Johnius coitor, population dynamics, length-frequency data, Bay of Bengal

Introduction

Study of population dynamics of any fish species is very important to develop scientific strategies for their protection, management, conservation and also to get a sustainable yield. Fish population dynamics is the fluctuation in number of fish in a population due to several factors such as reproduction, growth, mortality and recruitment (King, 2007) for which a given population grows or shrinks. Study of different population parameters including the asymptotic length and growth co-efficient, mortality (natural and fishing) rate and exploitation level are essential for planning and management of marine resources (Baset et al., 2020). Lack of knowledge of population structure and exploitation of marine resources demand a detailed study to facilitate better management. The fresh water capture fisheries of Bangladesh are now decreasing day by day due to various natural and manmade causes; thus, it cannot meet the increasing demand. Hence, exploration, exploitation and culture of marine water resources will be the only solution for the existing problem. For this purpose, a comprehensive knowledge about the ecology, biology and population size of marine fishes are very important. Johnius coitor (Family Sciaenidae, a croaker), locally known as “Poa Mach” is one of the most dominant and commercially important Jew fishes in the Bay of Bengal. J. coitor is also captured from the river, estuaries and Kaptai lake (Azadi et al., 1999). The fish is widely distributed in different parts of the world. About 20 species of Jew fish (Poa, Lambu, Kala-datina etc.) are found in the Bay of Bengal, out of which seven species play a vital role in the national economy of Bangladesh. Jew fish comprise almost 12.8% of the total demersal fish stock in the EEZ of Bangladesh, 66.5% of the demersal fishes found in the continental shelf are within 20-meter depth of water (Sarker and Rahman, 1991). Almost one thousand tons of dehydrated and salted Jew fishes are exported annually to the South-East Asian countries (Sarker and Rahman, 1991). Review of literature revealed that except some short works on morphometry (Azadi et al., 1999), reproductive parameters (Sarkar et al., 2018) of Johnius coitor from the Karnaphuli river, Bangladesh and Ganga River, India, respectively no work on length-frequency based and other population analysis of this fish has been done, however, some works on other species of Johnius like J. dussumieri (Chakraborty et al.,1997, Baset et al., 2020) have been done from Indian waters and Pakistani waters. Faizah and Anggawangsa (2019) worked on J. carouna from Indonesian water, Bhuyan et al.