Reconstructing Historical Fish Community Baselines and Shifting Baselines Syndrome in Nigerian Freshwater Systems Using Archival and Paleolimnological Approaches

The absence of long-term biological baseline data for most Nigerian freshwater systems creates a shifting baselines problem that severely limits conservation target-setting and accurate assessment of anthropogenic change. This study reconstructs historical fish community baselines for four major Nigerian river systems using complementary archival and paleolimnological approaches, creating a multi-century temporal perspective on freshwater fish biodiversity change. Colonial-era naturalist collection records from museum databases in the United Kingdom and Germany were digitised and georeferenced to reconstruct fish species occurrence patterns for the period 1880 to 1950. Paleolimnological analyses of diatom assemblages, fish bone fragments, and organic biomarkers from dated sediment cores from four reservoir and lake systems provided ecological proxies extending the temporal record beyond the archival period. Interviews with elderly fishers applied a structured recall methodology to reconstruct community knowledge of fish community change over the past 50 years. Synthesis of these three temporal data streams revealed cumulative species loss of 28 to 41 percent from historical peak richness across the four river systems, with losses accelerating dramatically after 1980. Species targeted by commercial and artisanal fisheries showed the steepest declines, while detritivores and small-bodied generalists increased in relative abundance. The study documents the extent of freshwater biodiversity change obscured by institutional memory loss and establishes historically grounded conservation reference conditions. Recommendations include developing a national freshwater biodiversity historical data recovery programme drawing on museum archives and paleolimnological capacity. Keywords: historical baselines, shifting baselines, fish biodiversity, paleolimnology, Nigeria.