Assessment of genetic diversity and population trend of frigate tuna (auxis thazard) in Tanzania marine waters
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Date
2022
Authors
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Journal ISSN
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Publisher
Sokoine University of Agriculture
Abstract
Frigate tuna (Auxis thazard) is one of the most harvested marine fish in Tanzania and
contributes to food security of the people living in the coastal areas and the country’s
economy in general. Frigate tuna is highly exploited due to its better meat property and
there is a danger that, in a long run it will be depleted. Therefore, there is a need to
establish strategies for sustainable utilization and conservation of the overexploited tuna
species. Establishment of conservation strategies requires information on genetic diversity
and the distinctiveness of the species. Furthermore, effective management of frigate tuna
requires information about population size and dynamics. This study assessed the stock
genetic diversity and structure of frigate tuna (Auxis thazard) populations using
mitochondrial displacement loop (mtDNA D-loop) sequences. Furthermore, the study
assessed population trends in Tanzanian marine waters using secondary data obtained from
the Ministry of Livestock and Fisheries and collected for a five-year period from 2015 to
2019. Also, the size of frigate tuna caught in Tanzania’s marine waters was determined.
For the genetic diversity study, a total of 100 frigate tuna were collected from small-scale
fishermen at Dar es Salaam (20 fish), Tanga (30 fish), Mtwara (30 fish) and Zanzibar (20
fish) landing sites. For each fish, a muscle tissue sample (approximately 50 g) was
collected from the area above the lateral line of the fish and near the dorsal fin and put in a
vial containing 95% ethanol, and the vial was labelled. DNA extraction was done using a
genomic DNA mini-extraction kit (Quick-DNA Kit). DNA quantification was done by
using a spectrophotometer. The DNA concentration was adjusted to 50 ng and then stored at 4 o C. The DNA samples were sent to the Agricultural Research Council-Biotechnology
Platform, South Africa for polymerase chain reaction (PCR) amplification and sequencing.
A fragment of 432 bp containing the first half of the mitochondrial DNA control region (D-
loop) was amplified using the following primer set. The forward primer sequence was 5′-
CCGGACGTCGGAGGTTAAAAT-3′
and
reverse
primer
sequence
was
5′-
AGGAACCAAATGCCAGGAATA-3′. Sequencing of the purified PCR fragments was
performed using the same primers. The sequencing was done using an ABI PRISIMTM
3100 Genetic Analyzer (Applied Biosystems). A total of 88 haplotypes were identified in
the four populations. Hyplotype diversity was high in all populations. The haplotype
diversity of Mtwara and Zanzibar populations were the highest (1.000 ± 0.010) while the
Dar es-Salaam and Tanga populations had the lowest haplotype diversity of 0.993 ± 0.021
and 0.992 ± 0.012, respectively. The Tanga population had the highest nucleotide diversity
(0.078 ± 0.018), followed by Zanzibar (0.027 ± 0.014), Mtwara (0.025 ± 0.014) and Dar es
Salaam populations (0.016 ± 0.009). Results from AMOVA indicated that variation within
the populations was higher (90.35%) than the variation among populations (9.64%).
According to the results for F ST , genetic differentiation between populations was greatest
between Tanga and Dar es Salaam (0.17828), followed by Tanga and Zanzibar (0.14633)
and Tanga and Mtwara populations (0.13865). The genetic distance between the Dar es
Salaam population and the Tanga population was the highest (0.01001), followed by the
genetic distance between Tanga and Zanzibar populations (0.00873) and Tanga and
Mtwara (0.00827), while the genetic distance between Mtwara and Zanzibar was the
lowest (0.0004). The results for the rate of migration among the populations showed high
gene flow as revealed by the number of immigrants per generation. Gene flow between
Mtwara and Zanzibar populations showed the highest number of immigrants (Nm = 18.31),
followed by Dar es Salaam and Zanzibar populations (Nm = 15.83), while the lowest
number of immigrants was found between Tanga and Dar es Salaam populations (Nm = 1.18), between Tanga and Zanzibar populations (Nm = 1.47) and between Tanga and
Mtwara (Nm = 1.55). The phylogenetic tree reconstructed based on the 88 haplotypes
grouped the haplotypes into two major clusters. Cluster 1 consisted of nine haplotypes, of
which eight were solely from the Tanga population and one from reference sequences of
Euthynnus affinis (Kawakawa tuna). Cluster 2 included haplotypes from Dar es Salaam,
Mtwara, Tanga, and Zanzibar populations. There were no population-specific sub-clusters.
Population history was assessed using Fu’s Fs and Tajima’s D, and the results indicated
negative values. Population expansion of A. thazard was suggested based on the results of
neutrality tests.
In the second study, data from 4906 tons of frigate tuna were obtained from the Ministry of
Livestock and Fisheries and used to assess the population trend in the four localities for the
period from 2015 to 2019. The data were further subjected to ANOVA to test the
significance of the difference in catches among years and locations. Over a five-year
period, a total of eight species of tuna, namely Auxis thazard, Euthynus affins, Istiompax
indica, Rachcentron canadum, Rastrelliger kanagurta, Scomberomorus plurilineatus,
Thunnus obesus, and Xiphias gladius, were caught in the study locations. The quantity of
catch differed among species (p = 0.001), whereby Rastrelliger kanagurta was the most
caught species (13,473 tons for the period of five years), followed by Scomberomorus
plurilineatus (7,489 tons for five years). In the case of A. thazard, the average mean catch
over a five-year period was 981.16 t, contributing 19.99% of the total tuna species caught.
The catch was higher in 2015 than in any other year afterward in all localities.
A total of 240 frigate tuna (48 fish from each site) were collected from fishermen and
measured for total body length (in cm) and body weight (in g) to assess the size of fish
caught. The total length ranged from 36 to 38.0 cm, while the body weight ranged from
461 to 1612 g. The fish from Mtwara had the smallest mean body weight (792.284 ± 33.092 g) while those from Dar es Salaam had the largest mean weight (977.692 ± 25.841
g).
It is concluded that there is high within population genetic diversity, but the genetic
differentiation of frigate tuna populations is not significant among the four sites, hence,
they can be regarded as a single stock unit for management purposes. Also, the production
trend of A. thazard showed a decreasing trend over the five year period from 2015 to 2019
and the relationship between body length and weight was linear and positive. The length
and weight of A. thazard exhibited isometric growth by which fish grow in weight as
length increases, and the condition factors were > 1 in all sampling sites. The condition
factor greater or equal to one is good, indicating a good level of feeding, and proper
environmental condition.
Description
Dissertation
Keywords
Assessment, Genetic diversity, Population trend, Frigate tuna, Auxis thazard, Tanzania, Marine waters