Vertical variability in the diversity and abundance of fish larvae in a shallow tropical estuary in Southern Gulf of Mexico

Abstract

Vertical migration in ichthyoplankton is relevant for its survive and it has been documented in estuaries with more than 10 m depth. In shallow estuaries this process is unknow or not supposed since no environmental stratification is assumed. The objective of this research is to investigate the vertical variability of fish larvae assemblages in shallow tropical estuarine zones. The model study site was the estuarine area of the San Pedro y San Pablo River located east of the Centla Wetland Biosphere Reserve in the Gulf of Mexico. The experimental design includes depth (bottom-surface), habitat type (coast-low complexity and mangrove-high complexity), circadian cycle (sunrise–midday–sunset–night), and season (dry-rainy). The ichthyoplankton were collected using a supporting net (150 cm in length, 50 cm mouth diameter, 400μm mesh size) device for simultaneous collection on the surface and bottom. Circular tows (2.7–4 km/h) were carried out for 5 min. A total of 40,968 fish larvae were collected, belonging to 13 orders, 26 families, and 38 species. The highest larval abundance expressed as mean density (693 ± 916 ind/m3), richness (4.57 ± 3.46), Shannon-Wiener diversity (1.05 ± 48.48), and Pielou evenness (0.88 ± 0.3) were recorded in the bottom. The variance component analysis indicated that the most important factors to explain the larval abundance was the depth (15.55%), the period of the day (11.20 %) and the interaction between all factors (10.51 %). Fish larvae species as Achirus lineatus, Trinectes maculatus, Etropus crossotus, and Symphurus civitatium occupied only the bottom. Despite the low depth, a vertical stratification was observed in the physicochemical parameters, especially in the salinity, being greater in the bottom than in the surface during the two seasons and habitat type and it was related with higher values of abundance, richness, and diversity of ichthyoplankton.