Pre-invasion biological baselines for native fish and bivalves from a southern Mediterranean coastal lagoon: El Mellah, Algeria

Mediterranean coastal lagoons are increasingly threatened by biological invasions, yet impact assessments are critically constrained by the lack of pre-invasion biological baselines. This study establishes a biometric reference dataset for five autochthonous species of ecological and fisheries importance from El Mellah, Algeria's only coastal lagoon and a UNESCO Biosphere Reserve, using samples collected 5 years prior to the first observed occurrence of the invasive blue crab Callinectes sapidus. Length-weight relationships and condition indices were estimated for three fish (Anguilla anguilla, Mugil cephalus, Chelon auratus) and two bivalves (Cerastoderma glaucum and Ruditapes decussatus) from 1192 specimens. All regressions were highly significant (p < 0.0001, R² = 0.828–0.966), with isometric growth observed in three species and negative allometry in the remaining two. Physiological conditions were broadly consistent with those reported from Mediterranean transitional ecosystems. By providing the earliest published biometric records for native species from El Mellah, this study establishes a valuable historical benchmark for future comparative assessments of biological change and long-term ecological monitoring following the establishment of C. sapidus and other ecological changes affecting this transitional ecosystem.