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

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 AVAILABILITY: Data and R code are archived and freely available at Open Science Framework (https://osf.io/x8t7p/; DOI: 10.17605/OSF.IO/X8T7P).

Capture severity, infectious disease processes and sex influence post-release mortality of sockeye salmon bycatch.

Bycatch is a common occurrence in heavily fished areas such as the Fraser River, British Columbia, where fisheries target returning adult Pacific salmon (Oncorhynchus spp.) en route to spawning grounds. The extent to which these encounters reduce fish survival through injury and physiological impairment depends on multiple factors including capture severity, river temperature and infectious agents. In an effort to characterize the mechanisms of post-release mortality and address fishery and managerial concerns regarding specific regulations, wild-caught Early Stuart sockeye salmon (Oncorhynchus nerka) were exposed to either mild (20 s) or severe (20 min) gillnet entanglement and then held at ecologically relevant temperatures throughout their period of river migration (mid-late July) and spawning (early August). Individuals were biopsy sampled immediately after entanglement and at death to measure indicators of stress and immunity, and the infection intensity of 44 potential pathogens. Biopsy alone increased mortality (males: 33%, females: 60%) when compared with non-biopsied controls (males: 7%, females: 15%), indicating high sensitivity to any handling during river migration, especially among females. Mortality did not occur until 5-10 days after entanglement, with severe entanglement resulting in the greatest mortality (males: 62%, females: 90%), followed by mild entanglement (males: 44%, females: 70%). Infection intensities of Flavobacterium psychrophilum and Ceratonova shasta measured at death were greater in fish that died sooner. Physiological indicators of host stress and immunity also differed depending on longevity, and indicated anaerobic metabolism, osmoregulatory failure and altered immune gene regulation in premature mortalities. Together, these results implicate latent effects of entanglement, especially among females, resulting in mortality days or weeks after release. Although any entanglement is potentially detrimental, reducing entanglement durations can improve post-release survival.