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Marine habitats are in decline due to increasing anthropogenic pressures, but baseline data on species distributions needed to manage and conserve populations are lacking. Incorporating death assemblages into species assessments can create a more accurate understanding of pre-anthropogenic communities than survey records alone. In this study, we conducted a live-dead analysis on mollusks from a new 2008-2018 dredge survey in the eastern Gulf of Mexico. We selected the predatory banded tulip snail, Cinctura hunteria, as a test case for assessment because this species is one of several designated by the Florida Fish and Wildlife as a species of concern. Using spatial count data for shells in our samples, we estimated density values for each taxonomic grade over the sampled area using IDW spatial interpolation. These maps reveal large areas of occupation across the west Florida shelf for two taxonomic grades of dead shells but loss of offshore occurrence for live records. One explanation for the lack of occurrences in offshore habitats is that, unlike dead shell records, there is no time averaging accumulation of live shells. Time averaging increases detectability of species in habitats where they are rare. However, independent fisheries data from live-only animal surveys not only mirror our live-dead results but suggest that habitat loss in our live-dead comparisons was rapid and occurred in the late 1980s or early 1990s. Thus, live-dead comparisons reveal both natural baselines as well as anthropogenic changes in distribution without being significantly distorted by time-averaging biases. Including live-dead data can greatly improve species assessments when long-term survey records are unavailable and provide a key tool in combatting biodiversity loss across marine ecosystems. 

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.