2023 field study links fish traits, feeding and mitochondrial DNA in Vietnam’s Tam Giang Lagoon

A 2023 observational field study in Vietnam’s Tam Giang Lagoon characterized the morphology, diets and mitochondrial DNA of three economically important fish species: Terapon jarbua, Acanthopagrus latus and Oxyurichthys ophthalmonema. The researchers found distinct anatomical traits, carnivorous or animal-biased feeding strategies, and measurable variation in mitochondrial sequences among the sampled fish.

The study, conducted from January through December 2023, sampled wild fish at three representative locations in the lagoon system in Hue City using fishing gear operated by local fishermen. The collections included 69 O. ophthalmonema, 37 T. jarbua and 41 A. latus specimens, although the locations were not exclusive to individual species. The work was a biological and genetic characterization of three selected species rather than a lagoon-wide abundance survey, annual-trend analysis or test of a management intervention.

Morphological measurements were taken from freshly caught specimens using a standardized protocol. T. jarbua had an elongated, laterally compressed body, ctenoid scales, a sharp opercular spine and three lateral black stripes; its head length ranged from 26.9% to 36.3% of standard length. A. latus had a deeper, laterally compressed body, a blunt snout, conical anterior teeth and molariform posterior teeth. Its dorsal fin typically contained 11 spines and 9–10 soft rays, and a dark blotch extended from the pectoral-fin base to the lower operculum. O. ophthalmonema possessed ocular cirri, a retractable upper lip, unicuspid upper-jaw teeth, a thin nuchal crest and two dorsal fins.

Microscopic examination of stomach contents showed broad diets with different food-group frequencies. For T. jarbua, crustaceans occurred in 61.54% of examined digestive tracts, phytoplankton in 59.62%, detritus in 42.31%, fish scales in 30.77% and zooplankton in 21.15%, supporting its classification as an ecologically flexible omnivore. A. latus had a carnivorous–omnivorous diet: zooplankton occurred in 30.47% of tracts, small fish in 25.5%, shrimp in 15.47%, algae in 15.86% and small squid in 10.24%, while organic residues were comparatively low at 5.28%.

O. ophthalmonema showed the strongest carnivorous signal among the three species. Crustaceans appeared in 96.15% of examined tracts, whole fish in 80.77% and polychaetes in 23.08%. The fish also consumed algae and organic detritus, with detritus recorded in 53.85% of tracts, so the observed diet was predominantly carnivorous but not exclusively so. The study also noted that T. jarbua’s diet varies with body size, with smaller individuals mainly taking algae and small invertebrates and larger individuals consuming a wider range of animal prey.

The researchers supplemented stomach-content observations with relative gut length, calculated as intestine length divided by total body length. Under the study’s classification, values below 1 indicated carnivory, values above 1 suggested herbivory and values near 1 indicated omnivory. Mean relative gut length was 0.94 and 0.93 in the two O. ophthalmonema size classes, 0.89 and 0.90 in the two T. jarbua classes, and 0.68 for A. latus individuals under 100 mm and 0.92 for those above 100 mm. All three species were therefore classified as carnivorous or omnivorous, with the anatomical index generally consistent with the observed diets.

For genetic identification, the researchers sequenced mitochondrial cytochrome c oxidase subunit I sequences from 30 O. ophthalmonema and 31 T. jarbua specimens, plus approximately 470-base-pair mitochondrial control-region sequences from 30 A. latus specimens—a total of 91 fish for DNA analysis. Comparisons with GenBank references produced 99.41–99.85% identity for O. ophthalmonema and 98.71–99.9% for T. jarbua. A. latus control-region sequences showed 95.32–99.15% homology, a difference the authors attributed to the control region’s faster evolutionary rate. They considered the sequence comparisons sufficient to confirm species identity and support phylogenetic analysis.

Sequence analysis detected 11 polymorphic positions among the 30 O. ophthalmonema COI sequences, with overall similarity to the reference remaining above 99%. T. jarbua showed differences at 15 reported positions, while the 30 A. latus control-region sequences contained variation across 46 listed positions. Maximum-likelihood trees placed the O. ophthalmonema samples close to published sequences of that species, with bootstrap support of at least 84% and up to 100% for the reported principal grouping. T. jarbua samples formed several subgroups with bootstrap values of about 40% to 84%, while A. latus samples occupied multiple branches linked to different GenBank genotypes.

These tree patterns were interpreted as consistent with common haplotypes and possible local maternal lineages in T. jarbua and A. latus, but the study did not directly measure the wider population structure or establish the origins of those lineages. The principal limitations were the single-year sampling period, three locations, modest genetic sample sizes and selection of three target species from a lagoon reported in the source to contain many more fish species. Within those bounds, the combined morphological, dietary and mitochondrial analyses provide field-based descriptions of the three species and information intended to support local fisheries management, biodiversity conservation and aquaculture planning.

Subject of Research: Morphology, feeding ecology and mitochondrial variation in three fish species from Tam Giang Lagoon, Vietnam

Article Title: High-value fish species ( Actinopterygii ) in Tam Giang Lagoon, Vietnam: Biological traits and ecological insights

Article References: Nguyen, T. D. Q., Le, H. D. M., Nguyen, T. N., Vo, Q. V., Le, T. D., Phan, V. T., Pham, N. T. T., Nguyen, H. N. M., Nguyen, T., Nguyen, T. H., Vu, T. P. A., & Tran, V. G. (2026). High-value fish species (Actinopterygii) in Tam Giang Lagoon, Vietnam: Biological traits and ecological insights. Acta Ichthyologica et Piscatoria, 56, 77-89. https://doi.org/10.3897/aiep.56.174662

Image Credits: AI Generated

DOI: 10.3897/aiep.56.174662

Keywords: Tam Giang Lagoon, Vietnam, fish ecology, feeding behavior, morphology, mitochondrial DNA, Terapon jarbua, Acanthopagrus latus, Oxyurichthys ophthalmonema, phylogenetics, High-value, fish