Trophic ecology and habitat use of an overexploited commercial snapper (Lutjanus jocu) in a tropical nursery estuary elucidated by stable isotopes

Abstract

Despite the economic importance and susceptibility to overfishing of the dog snapper Lutjanus jocu, information on their first life stages and the use of nursery habitats remains scarce. Herein, we assessed the seasonal and ontogenetic changes on trophic ecology and habitat use of juveniles in a tropical estuary. Trophic position, prey assimilation, and trophic connectivity across dry and wet seasons and different size classes (< 100 and > 100 mm of total length – TL) were investigated using stable isotope analysis. A total of 57 individuals of 101.8 ± 51.9 mm (mean ± SD) of TL, ranging from 29 mm to 245 mm, had their trophic ecology and habitat use analyzed using dorsal muscle’s carbon (δ13C) and nitrogen (δ15N) stable isotope ratios. Significant differences were observed for δ13C according to the season (ANOVA, p < 0.001), but this pattern was not observed for δ15N (ANOVA, p = 0.471). In contrast, significant differences were observed for δ13C (ANOVA, p = 0.045) and δ15N (ANOVA, p < 0.001) according to the size classes. Trophic position increased according to body size (from 3.01 ± 0.09 to 3.57 ± 0.08 in smaller and larger individuals, respectively), although it remained similar between wet and dry seasons. Mangrove crab (Goniopsis cruentata) was the most assimilated prey across seasons, with a shift towards shrimps for the larger dog snappers. Our Bayesian mixing models revealed that juvenile dog snappers depend predominantly on trophic sources from sheltered estuarine habitats. However, smaller individuals have an isotopic composition more similar to marine sources, therefore, suggesting their recent ingression into the estuarine nursery habitat. In conclusion, our findings may serve as a basis for conservation plans, reinforcing the critical importance of preserving habitat connectivity during the early stages of fish species that use estuaries as nursery grounds and face threats from overexploitation.