Abstract:
Abstract
of Fish eries, University of Rajshahi, Rajshahi, Bangladesh
Stock (population) identification is very crucial both in the context of monitoring
and conservation of the fish species of marine and open water habitats. The
present research was conducted accordingly in the BoB aiming to determine and
categorize the population of some commercially important marine fish species. In
this study, about one hundred samples of each species of ten marine fishes
namely, Pampus chinensis, Pampus argenteus, Euthynnus affinis, Auxis thazard,
Megalaspis cordyla, Lates calcarifer, Setipinna tenuifilis, Sillaginopsis panijus,
Polynemus paradiseus and Coilia dussumieri were collected between October
2020 and April 2022 mainly from coast and rivers of two ecological regions of
Bangladesh coast: the South - west (Borguna-Khulna-Patuakhali, KP) coastal
zone and the South - east (Chattograrn-Cox 's Bazar, CC) coastal zone based on
different physico-chemical characteristics of habitat (KP: high brackish water area
with dense clayed particle in river and estuaries; CC: sandy area with more clear
saline water river and estuaries) and biology of species (e.g. migratory,
amphidromous, anadromous etc.). For amphidromous or anadromous fish,
samples were collected from coastal rivers. Fish/tissue samples or DNA sequence
data of other seas in the Indian Ocean region (e.g. Southern BoB, Andaman Sea,
Arabian Sea, Persian Gulf etc.) and South-west Pacific (e.g. South China Sea,
Gulf of Thailand etc.) were collected from different researchers and scientific
studies. For genetic diversity and stock study, sequence variations in different
mitochondrial DNA gene regions such as COI, Cytb, and noncoding control
region (d-loop) were examined Polymerase Chain Reaction (PCR) was used to
amplify the target mtDNA region and sequenced by sequence analyzer. Lastly,
the variation in the obtained DNA sequence data of different geographic locations
were analyzed using different bioinformatic softwares such as Geneous,
ARLEQUIN, MEGA-6; TCS etc. For all the genes in every sampling locations of
Bangladesh, the nucleotide diversities (zr) were very low (0.000 I ~ 0.05) but the
haplotype or gene diversities (h) were relatively high, 0.5 ~ 1.0 (i.e. h>O.5)
indicates that the studied fishes have experience population expansion after a
period of low effective population size, except two species, M. cordyla and L.
calcarifer for which the gene diversities were also low (0.14 ~ 0.263). This result implies that these two fish species of Bangladesh have experienced recent
bottleneck. Immediate conservation measures should be immediately taken for
these two species, such as limiting catch, banning on fishing season, making long
term sanctuary or protected area etc. Neutrality tests such as Tajima's D and Fu's
Fs statistics also suggested that most of the studied fish species of Bangladesh
populations have undergone the demographic history of population expansion.
Interestingly, the population statistic FST, and exact test of population
differentiation revealed two different scenarios for two group of fishes. The first
group contains six fish species namely P. chinensis, P. argenteus, E. affinis, A.
thazard, M cordyla, L. calcarifer. Fishes of this group have similar
characteristics. These fishes are mostly oceandromous, pelagic and highly
migratory. They are widely distributed throughout the Indian Ocean and SouthWest
Pacific (or, Indo-west Pacific). These fish showed no significant population
genetic structuring indicating that these six species have single genetic stock and
these are panmictic (random mating among populations) throughout Bangladeshi
marine water. Bangladesh should manage these species as a single conservation
unit. Further, these six migratory fish showed its own genetic structure (FST P
:S0.05) within the BoB when compared with other neighboring seas such as
Arabian Sea, Andaman Sea, Gulf of Thailand, South China Sea etc. So, the BoB
countries should manage and conserve these vital and shared marine species
through cooperation with each other. On the other hand, the second group consists
of four species S. tenuifilis, S. panijus, P. paradiseus and C. dussumieri is
relatively distributed to the narrow or localized geographic area mostly in the BoB
and Andaman Sea. These fishes are mostly off-shore or coast dwelling and
amphidromous (i.e. migrate between fresh and marine water but not to breeding
purpose). For these species, pairwise FST values of mtDNA markers among fish
samples of different sampling rivers showed significant differences (P :S0.05) in
most of the cases. Different rivers showed their own genetic stock for different
fish such as Bishkhali, Kirtankhola and Boleshwar river for P. paradeseus and S.
panjius; Naf and Pashur river for C. dussumieri; Matamuhuri and Bishkhali river
for S. tenuifilis etc. Based on this result, it is recommended that different river
populations of studied fish having unique genetic structures (i.e. genetic stock)
require separate monitoring and management strategies for conservation because
the population of each river is genetically distinct and overfishing in any of the
rivers may cause extinction of its fish stock. So, it is needed to maintain good
water flow and direction such as by regular dredging and also needed to control
water pollution for the good ecosystem to safeguard these fish species.
Keywords: BoB, population, stock status, molecular marker, fisheries management.