Prey body size and anthropogenic resource depression: The decline of prehistoric fishing at Chelechol ra Orrak, Palau

• Marine resource shifts at Chelechol ra Orrak, Palau, imply declining foraging efficiency.
• Possible resource depression is tested using median size of parrotfish genera.
• Scarus size is unchanged; Chlorurus size increases over 1500 years.
• Overfishing is rejected as a cause of resource shifts.
• The significance of prey size change in human paleoecology is critically evaluated.

Prior investigation at the Chelechol ra Orrak site (3000/1700–0 BP) in Palau’s Rock Islands revealed a decline in fishing and increased reliance on small-bodied, inshore and littoral molluscs, commensurate with evidence for declining foraging efficiency and prey switching that signal potential resource depression. Yet, standard markers for ‘overfishing’, such as diet-breadth expansion, increased taxonomic richness, and a switch to exploitation of offshore waters, are lacking at the site, undermining the case for anthropogenic resource (exploitation) depression as a cause of the observed patterning. Broad scale climate change similarly fails to account for these shifts. To investigate these conflicting patterns we performed a mean/median size analysis of two parrotfish (Scaridae) taxa, Scarus and Chlorurus, among the most commonly recovered fish at the site. Results indicate that Scarus size remains unchanged through 1500 years of exploitation, while Chlorurus become larger, substantiating previous findings for sustainable resource use at Orrak. With these results in mind, we critically evaluate prey size change as a metric for anthropogenic exploitation depression, noting that size diminution, in particular, may arise epiphenomenally due to multiple causes unrelated to human predation pressure. Results have broader implications for the detection and attribution of resource depression in studies of human paleoecology.