Multidecadal changes in the demersal community structure in an urban bay: Shift from a bottom- to a top-heavy pyramid

Abstract

Fish community structures in urban bays have changed over time as a result of anthropogenic activities, leading to changes in species richness, diversity, fish assemblage composition, and abundance. Tokyo Bay is one of the most eutrophic urban bays in Japan, and hypoxia occurs every summer in the bottom water of the bay. Accordingly, the demersal community in the bay has undergone drastic changes in taxonomic composition over the past several decades. We analyzed data from a bottom trawl survey spanning 35 years to characterize multidecadal changes in the slopes of body size spectra of the demersal community in Tokyo Bay, as a reliable index to represent shifts in community structure. We found that the community shifted from bottom-heavy pyramids in the past (1977–1995) to top-heavy pyramids in recent years (2003–2019) due to a decrease in the biomass of small-sized crustaceans and an increase in the biomass of large-sized elasmobranchs. Human-induced hypoxia can promote the formation of top-heavy pyramids, as it decimates the number of small-sized individuals. The existence of energy subsidies to large-sized individuals from the inner-shallow and outer bay areas as well as the pelagic community provides a possible explanation for the persistence of top-heavy pyramids.