Phylogeography of Ethmalosa fimbriata (Bonga Fish) Along the West African Coast: Implications for Stock Management in Nigerian Waters

This research investigated the phylogeographic structure of Ethmalosa fimbriata (bonga fish), the most commercially important small pelagic species in Nigerian coastal fisheries, along the West African coast from Mauritania to Angola, with detailed sampling focus on Nigerian and adjacent waters. Bonga fish constitute the single largest component of artisanal and industrial fish landings along the Nigerian coast, yet its population genetic structure, which has profound implications for stock delineation and management unit designation, remains unresolved. Tissue samples were collected from 480 individuals at 16 locations across the species' range, with six Nigerian sampling points covering Lagos, Ondo, Delta, Rivers, Bayelsa, and Cross River states. Complete mitochondrial control region sequences and 15 microsatellite loci were genotyped. Phylogeographic patterns were examined using network analysis, Bayesian clustering, isolation-by-distance tests, and demographic history reconstruction. Results revealed significant genetic structuring across the species' range, with two major genetic lineages corresponding broadly to West African upwelling-influenced northern waters and equatorial southern waters. Within Nigerian waters, population differentiation was moderate but statistically significant, with the Niger Delta region representing a partial barrier to gene flow. Demographic analysis indicated a major population expansion approximately 18,000 years ago coinciding with post-glacial sea level rise and coastal reshaping. The presence of at least two management units in Nigerian waters challenges the prevailing assumption of a single nationally managed stock. The study recommends a multi-unit stock management framework for bonga fish in Nigerian coastal waters, with independent reference points and harvest strategies for each management unit. Keywords: phylogeography, Ethmalosa fimbriata, population genetics, stock delineation, Nigerian coastal fisheries