Mémoire

Abstract

Tropical lagoons, particularly in Madagascar, play important ecological and socio-economic roles but remain understudied and poorly protected. The Tampolo Lagoon in northeastern Madagascar lacked any prior ichthyological inventory despite its importance to local communities and signs of overfishing. This study aimed to characterize the lagoon and produce an ichthyological inventory to diagnose ecological status and inform sustainable management. We conducted four standardized gillnet surveys at four representative sites (S1–S4) between February and April 2025 using multimesh nets (8–40 mm), complemented by vertical physicochemical profiles and water sampling for environmental DNA (eDNA). The lagoon is shallow (maximum depth 4.3 m) and displays strong stratification, with a halocline (conductivity 10–13 mS·cm⁻¹ at the surface vs. 41.1 mS·cm⁻¹ at depth) and an inverted temperature profile reaching 37.0 °C at 2.7 m, consistent with a heliothermal system. Morphological and genetic identifications documented 17 species and 364 individuals (2.19 kg), unevenly distributed among sites: S1 (46 individuals/6 species), S2 (97/5), S3 (78/5), and S4 (143/7). Assemblages differed among sites in composition and richness; β-diversity was high and dominated by turnover (>90%) despite a shared core of Ambassis ambassis, Ptychochromis grandidieri, and Leiognathus equula. Size structure indicated an absence of large fish: mean relative length (TL/Lmax) ranged from 0.39 to 0.44, below the 0.6 alert threshold; subadults comprised 80–90% of catches, whereas adults were <2% (cf. 30% benchmark). Together, these indicators point to an unbalanced stock. These findings support an overexploitation status that may compromise renewal. Given the unique contribution of each sector to overall biodiversity, spatial management preserving the full habitat continuum is warranted. Regulations including minimum and maximum mesh sizes, seasonal closures, and functional zoning appear necessary to secure the lagoon’s sustainability. All 12 eDNA samples yielded measurable DNA (0.74–5.08 ng μL⁻¹), confirming protocol viability and the suitability of samples for downstream analyses.