Deep-Sea Fish Avoid The Dead

Coryphaenoides armatus arrive to feed on bait (mackerel) set by ROBIO. Photo from here

A recent study by Barry and Drazen (open access)notes that some deep-sea fishes avoid the odor of dead conspecifics. Coryphaenoides armatus, the Pacific grenadier, is a prominent scavenger and typically one of the first fish to arrive at a food fall of a carcass. However, in caging experiments to test the effects of ocean acidification on seafloor organisms, cages where C. armatus died potentially due to cage-related stress, predation, or exposure to acidic waters, did not attract C. armatus. Figure 6 (below) from the paper demonstrates this clearly showing the number of individuals that visit per time unit. At time 0 hours, C. armatus dies in the cage. For over a week (200 hours) there are no visits of C. armatus despite the appearance of other scanvengers (the octopus Bentoctopus sp. and the snubnose eelpout Pachycara bulbiceps).

Why did grenadiers avoid the cage? The authors suggest four possible hypotheses: 1) sensitivity to the acidification experiment, 2) inability to detect a weak odor plume or the odor was unappealing to scavengers, 3) quick departure from inaccessible a carcass (the dead grenadier was caged), and potentially the most interesting 4) avoidance of dead or dying individuals of the same species. 1 is unlikely as grenadiers were present at the initiation of the experiment. 2 is unlikely as other scavengers arrived at the cage. 3 is also unlikely because grenadiers were present at the cage when the original bait placed. Thus the questions is why do grenadiers avoid the dead and dying? One explanation is that the behaviour represents an adaptation to avoid potentially dangerous situations such as predation events, physiological taxing habitats, or diseased individuals.