An Evolutionary Ecology Framework for Understanding the Origin and Maintenance of Anadromous Migration in West African Freshwater Clupeid Fishes

Anadromy, the life history strategy involving marine feeding with freshwater spawning, represents one of the most energetically demanding migrations in the animal kingdom, yet its evolutionary origins and maintenance mechanisms in West African freshwater clupeids remain theoretically unexplored. This research develops an evolutionary ecology framework integrating life history theory, phylogenetic comparative analysis, and field-based energetic measurements to understand the selective pressures maintaining anadromy in Ethmalosa fimbriata and related West African clupeid species. Phylogenetic reconstruction was performed on 45 clupeid species using concatenated mitochondrial and nuclear gene sequences. Ancestral life history trait reconstruction tested the polarity of diadromy evolution using Bayesian stochastic mapping. Field-based energetic budgets were constructed for migratory and non-migratory populations of E. fimbriata in the Niger and Cross River systems through bioenergetics modelling parameterised with direct respirometry and field energy intake measurements. Density-dependent habitat quality models were constructed for spawning and feeding habitats under historical and current conditions. Phylogenetic analysis resolved anadromy as derived twice independently within the Ethmalosa clade. Comparative analysis demonstrated that anadromy was significantly associated with river systems experiencing strong wet season nutrient pulses that amplify productivity differentials between freshwater and marine habitats. Energetic modelling confirmed that migration profitability was positive across all parameterised scenarios, but was sensitive to degradation of spawning habitat quality. Life history optimisation theory predicts diadromy collapse when the ratio of freshwater spawning habitat quality to marine feeding habitat quality falls below a critical threshold identified as 0.27 in this study. This framework predicts future anadromy persistence under various habitat and climate scenarios. Keywords: anadromy, life history theory, Ethmalosa fimbriata, diadromy evolution, West Africa.