In the coastal area of Bangladesh, there are about 490 species of fish (Hossain, 1971) and 19 species of prawns or shrimps (Chowdhury and Sanaullah, 1991). It provides an estimate of the number and families in the estuary throughout the year and groups families into temporary aggregations and their spawning season. This research presents a comprehensive overview of larval assemblage abundance in the Bakkhali River estuary on the Cox's Bazar coast.
Spawning takes place in the river, estuary or offshore based on the fish species (Elliott et al., 2007). Gobiid larvae were found throughout the year, with the greatest concentrations in January–March during the northeast monsoon (Arshad et al., 2012). The cumulative number of fish families and densities in the estuary decreased significantly from the lower estuary to the higher estuary according to the distribution pattern of total fish larvae.
The monsoon effect on the abundance of fish larvae in the Pendas River estuary is a characteristic aspect of the larval assemblage. In the northwestern Indian Ocean (Red Sea, Arabian Sea and Persian Gulf), Nellen (1973) identified a total of 102 fish larval families. The changes in the density of fish larvae among the stations were not significant (p>0.05) in the Pendas River Estuary, Malaysia.
Stequert and Marsac (1989) found the largest abundance of tuna larvae in the eastern Indian Ocean in February.
Chapter Three Methodology
- Study Area
- Fish larvae sorting
- Morphological identification of fish larvae
- Determination of number of larvae, diversity indices and constancy of occurrence
- Number of larvae individual per 1000 m 3 Bongo net diameter, d= 0.50m
- Measurement of diversity
- Measurement of species richness
- Determination of the constancy of occurrence
- Statistical Analysis
Meristic traits, such as the number of myomeres, beads, or filament rays, are countable structures that appear in a series. Melanophores are pigment compounds found in different regions of the body and determine a species-specific pigmentation pattern (Rodriguez et al., 2017). Temporal changes of fish larvae were determined by the number of larvae along with diversity indices.
At its most basic level, diversity refers to the number of different species that exist. The number of species (richness) and the relative abundance of these species (evenness) vary among biological communities (Anon, 2021). The Shannon index, which estimates species richness and the proportion of each species within a community, is one of the most popular diversity indices (Arzamani et al., 2018).
Where C = constancy of occurrence of the family (%), P = number of samples in which the family occurs, Q = total number of samples. Based on the monthly abundance of larvae, the spawning season of the identified families was categorized as summer, winter and rain monsoon.
Chapter Four Results
- Fish larval composition and abundance
- Constance of Occurrence
- Top two abundant families
- Clupeidae
- Temporal variation and biodiversity indices
In terms of frequency of occurrence, among the 11 families, the percentages by family of constant:auxiliary:random families were 18:27:55 (Table 1). It indicates that only 18% of the families are detected at a constant rate throughout the year in the Bakkhali River Estuary and usually use it as a nursing area. Most families in this region were not found often and were considered random.
Clupeidae were the most abundant larvae and contributed more than half of the total abundance (56.29%) of the Bakkhali River estuary. This family was found in seven months of the year - from April to August, November and December (Fig. 5). Several commercially important species under the Clupeidae family are found in Cox's Bazar, such as Tenualosa ilisha, T.
This family was found in six consecutive months from May to October and December (Fig. 5). Ambassis dussumieri is the only species of Ambassidae that has been found off the coast of Cox's Bazar. The highest mean total larval density was 94±126.59 observed in June, while no fish larvae were found in January.
Margalef's wealth index also clearly showed two significant peaks, one in November (0.910) and the other in September. Larvae of 11 fish families were collected and their time of emergence was used as an indicator of their spawning season. Most families spawn in the warmer months of the year (March-June) based on the availability of their larvae (Fig. 9).
Among the families, Clupeidae and Ambassidae were confirmed as summer, monsoon and winter spawners as their larvae were found in all three seasons. The Engraulidae family has been recorded to spawn in mid-summer and mid-monsoon, Blenniidae has been documented to spawn in mid-summer, early monsoon and late monsoon, Mugilidae as spawning in mid-summer and mid-monsoon, and Gobiidae as late winter and late summer. Ambassidae May-Oct, Dec Apr-Sep, Nov Summer, Monsoon, Early winter Engraulidae May, Sep, Oct Apr, Aug, Sep Mid summer, Mid monsoon Gobiidae Mar, Jul Feb, Jun Late winter, Late summer.
Chapter Five Discussion
- Fish larval composition and abundance
- Constance of Occurrence
- Temporal density and diversity indices
- Spawning season
Based on the percentage of larvae, two (02) dominant families were identified as Clupeidae and Ambassidae. 2008) recognized Photichthyidae, Myctophidae, Bregmacerotidae, Gonostomatidae, Callionymidae and Carangidae as abundant families in the Bay of Bengal. Another study by Tzeng et al. 1997) at the estuarine stations of Yenliao Bay confirmed Pomacentridae, Apogonidae and Tripterygiidae as dominant families at the rocky stations, and Gobiidae was abundant. 2013) identified the four most common families: Sparidae, Scombridae, Clupeidae and Nemipteridae.
The majority of the families were not found frequently in this region and were considered to be accidental (Table 2). The result of the survey showed a low number of constant families in the Bay of Bengal. This was consistent with the findings of Lirdwitayaprasit et al. 2008), who observed a low number of constant (28) and accessory (22) families in the Bay of Bengal, with 50 families classified as accidental.
However, most fish families were recorded in May, indicating that summer is the most fertile month. While family wealth refers to the number of different families present in a given region, equality refers to the number of individuals of each family present. Pielou uniformity is related to the Shannon-Wiener index as it is determined by dividing the Shannon-Wiener index by the total number of households.
In this study, the evenness index is zero (0) for January, March and April, indicating no evenness. The Shannon-Wiener index obtained in this study matched with Arshad et al. 2012) that indicated significant intra-monsoon and inter-seasonal variability at the Pendas River estuary. Again, a brief study conducted by Zhang et al. 2021) from September to October in 44 different stations in the eastern Indian Ocean showed results similar to this study.
The majority of the families (07) spawn in the summer season, meaning they reproduce from March to June, and their larvae were found between April and May in this estuary. Most of the commercially available species were summer spawners in the Red Sea, matching this study. Clupeoids grow faster in tropical waters and have a shorter life cycle than in temperate environments (Araújo et al demonstrated that rainfall data show significant negative correlation with the distribution and frequency of finfish larvae.
Chapter Six Conclusion
Chapter Seven Recommendations
- Sampling by bongo net Plate 2. Fish larvae sorting from sample
- Larvae identification under stereo microscope
- Larvae labeling and storage
- Clupeidae larvae Plate 6. Ambassidae larvae
- Engraulidae larvae Plate 8. Gobiidae larvae
- Sillaginidae larvae Plate 10. Mugilidae larvae
- Megalopidae larvae Plate 12. Blenniidae larvae
- Terapontidae larvae Plate 14. Sparidae larvae
- Gerreidae larvae
Ocean transport pathways for the early life history stages of offshore spawning flatfish: a case study in the Gulf of Alaska. Study of the diversity and abundance of fish larvae in the southwestern part of the Sea of Oman in 2011-2012. The collection of fish larvae at the mouth of the Kosi Estuary, KwaZulu-Natal, South Africa.
The Commercial Fishes of the Bay of Bengal (Fisheries Development Survey, East Pakistan, Chittagong). Composition and occurrence of fish larvae in mangrove areas along the east coast of Phuket Island, Western Peninsula, Thailand. Temporal variation in species composition and abundance of fish and decapods of a tropical seagrass bed in Cockle Bay, North Queensland, Australia.
Ministry of External Affairs, Press Release: Press statement by the Honorable Foreign Minister on the Arbitral Tribunal/PCA award. Distribution and Abundance of Fish Eggs and Larvae in Arasalar Estuary, Karaikal, South East Coast of India Journal of Environmental Biology. Temporal and spatial distribution of fish and shrimp assemblages in the Bakkhali River Estuary, Bangladesh in relation to some water quality parameters.
Composition and diversity of larval fishes in the Indian Ocean using morphological and molecular methods. Distribution and abundance of larval fishes in the northern Aegean Sea—eastern Mediterranean Sea—in relation to early summer oceanographic conditions. Biophysical processes leading to the invasion of temperate fish larvae into estuarine nursery areas: a review.
The nursery function of the intertidal areas in the western Wadden Sea for 0-group sole Solea solea (L.). Duration of the planktonic larval stage of one hundred species of Pacific and Atlantic wrasses (family Labridae). Seasonal and spatial distribution of fish larvae in the waters over the Northwestern Continental Shelf of Western Australia.
APPENDICES
Brief biography of the author
