A field test of the “graveyard hypothesis” reveals avoidance of chemical but not visual cues in Bahamian queen conch (Aliger gigas)

Abstract

Queen conch (Aliger gigas) are large gastropod molluscs harvested for their meat, shells, and pearls and as they are generally easy to collect by hand, they are vulnerable to overfishing. In The Bahamas, fishers often clean (or “knock”) their catch and dispose of the shells away from collection sites, forming midden heaps or “graveyards”. Although queen conch are motile and found throughout shallow water habitats, live animals are rarely observed in the vicinity of middens, giving rise to a common belief that conch actively avoid graveyards, possibly by moving offshore. Here, we experimentally evaluated avoidance behaviours of queen conch to chemical (tissue homogenate) and visual (shells) cues indicative of harvesting activity using replicated aggregations of six size-selected small (< 14 cm shell length) and large (> 14 cm) conch at Eleuthera Island. Large conch were consistently more likely to move, and to move farther, than small conch, independent of treatment. Small conch, however, demonstrated greater occurrence of movement in response to chemical cues vs seawater controls, while conch of both sizes demonstrated equivocal responses to visual cues. Collectively, these observations suggest that more economically desirable large conch may be less vulnerable to capture during successive harvest events than smaller juveniles due to their greater propensity to move, and that chemical cues consistent with damage-released alarm cues may play a greater role in eliciting avoidance behaviour than the visual cues typically associated with queen conch graveyards. Data AvailabilityData and R code are archived and freely available at Open Science Framework (https://osf.io/x8t7p/; DOI: 10.17605/OSF.IO/X8T7P).