Observation of high proportions of unmated females in kuruma prawn Penaeus japonicus in the Seto Inland Sea: implications for a mate-finding Allee effect

Abstract The abundance of kuruma prawn Penaeus japonicus in Japanese waters has declined markedly since the mid-1990s. Despite this long-term decline, the underlying mechanisms remain poorly understood, limiting the development of effective stock management strategies. One consequence of reduced population density may be lower encounter rates between mates, potentially suppressing the species’ reproductive rate through a mate-finding Allee effect, in which the probability of mating decreases as population density declines. Yet, this mechanism has received little attention in conventional fisheries management. In this study, we investigated whether a mate-finding Allee effect occurs in P. japonicus populations in the Seto Inland Sea, using market-based sampling. Our results revealed a high proportion of unmated females (> 30%), even among those sampled immediately before or after spawning. As a result of Bayesian generalized linear mixed models fitting for the probability that a female was unmated, interannual variation in population abundance, used as a proxy for individual density, was considered a possible explanation for this high proportion, implicating occurrence of a mate-finding Allee effect. This study provides valuable insights into mate-finding Allee effects in Penaeus japonicus , which may contribute to reduced mating success and the recent population decline, with implications for stock assessment and management.