Mapping Past Distributions of Marine Mollusks Using Shell Death Assemblages

Marine species assessments rely heavily on baseline surveys conducted after the 1960s, long after many anthropogenic pressures began, which could lead to misinformed management decisions and poor conservation outcomes. In this study, we collaborated with Florida Fish and Wildlife to conduct stock assessments for mollusks of the west Florida shelf that incorporate shell death assemblages. One of our first assessments was of the Florida Fighting Conch, Strombus alatus, an abundant gastropod that is also under consideration as a replacement fishery for the threatened Queen Conch. Live and dead shells were collected from >300 dredge tows between 2008-2018 covering the entire west Florida shelf. Shells were age-partitioned by 14C- and AAR-calibrated taphonomic criteria. Counts were converted to densities per m2. Inverse distance weighting interpolation of S. alatus death assemblages reveals multiple population centers along the coast and a rapid decrease in density with depth from 25-120 m. In contrast, live conchs were absent in our dredge samples from shelf depths deeper than 40 m. These differences are confirmed by single-visit occupancy methods that account for variation in detectability across the samples. Live-dead differences in spatial distribution are probably influenced by time averaging in death assemblages, which increases detectability of conchs in deeper habitats, where they may be too rare to be sampled alive. However, extirpation of offshore populations was also indicated by independent natural history collection occurrence records, which show numerous live-collected conchs from 1940-1980 but none afterwards, despite an increase in sampling effort. These results suggest that live-dead comparisons can reveal biodiversity loss at the scale of large marine ecosystems.