Length/weight structure variability and physiological status of Chrysichthys nigrodigitatus (lacepède, 1803) and Chrysichthys auratus (Geoffroy Saint-Hilaire, 1809) specimens collected from Benin and Guinea Republic fresh water ecosystems

Abstract

<h2>Abstract</h2> Length-weight structure and physiological status of fishes depend on abiotic and biotic habitat features. For West African catfishes, the International Union for Conservation of Nature (IUCN) notes major gaps in basic biology, ecology, habitat condition and threats for <i>Chrysichthys nigrodigitatus</i> and <i>C. auratus</i>. We quantified length-weight relationships and condition factors (K) for both species using specimens collected in 2024 from artisanal fisheries in Benin (Tovè River; three sites; n = 211) and Guinea (Tilé River; four sites; n = 196). Following initial sorting by genus in the field, specimens were fixed in 10% formaldehyde, identified to species level using standard ichthyological keys, and then individually measured and weighed. Weight-class distributions (0-25, 25-50, 50-75 and 75-100% of maximum weight) differed sharply between countries for each species (Kolmogorov-Smirnov, p < 1 × 10⁻¹⁰), with larger size classes more frequent in Guinea. Pooled length-weight fits indicated near-isometric growth for both species across countries (t-tests, p > 0.05; mean r² = 0.93 ± 0.07), yet allometric exponents (b) varied geographically: <i>C. auratus</i> b = 3.035 (Benin) and 3.299 (Guinea); <i>C. nigrodigitatus</i> b = 2.995 (Benin) and 2.906 (Guinea). Condition factors differed by country (ANOVA, p < 0.05), with the highest K in <i>C. nigrodigitatus</i> and the lowest in <i>C. auratus</i> from Guinea. Despite a high b (3.299), <i>C. auratus</i> in the Tilé River showed poor condition (<i>t</i>-test for b vs 3: p = 0.184), suggesting suboptimal adaptation to prevailing ecological conditions. Principal Component Analysis (PCA) separated assemblages by geography, with an absence of large specimens in Benin and their presence in Guinea. Our results support the hypothesis that morphological variation and physiological status in <i>Chrysichthys</i> stocks are structured by geographic provenance, with implications for population assessment and river-specific management.