SEMENIUK, C.A.D.*; DILL, L.M.D.; Simon Fraser University, Burnaby, BC, Canada; Simon Fraser University, Burnaby, BC, Canada: Costs and benefits of grouped and solitary resting in the cowtail stingray, Pastinachus sephen, in Shark Bay, Western Australia

Resting is an important and necessary behaviour for many animals, but can also be a risky one. Unless a safe refuge is found where predation threats are negligible, resting may pose a considerable risk to the animal, necessitating risk management strategies. The adult cowtail stingray (Pastinachus sephen), of Shark Bay, Western Australia, is a solitarily foraging animal that uses shallow, inshore subtidal sandflats for resting. Our study revealed that when visibility is below a certain threshold (due to turbidity or low ambient light levels), cowtail stingrays primarily rest in small groups (2-3). Above this visibility threshold, stingrays mainly rest alone. Tests of flight initiation distances to a mock predator reveal the stingray’s vision to be severely limited in low visibility conditions. We suggest that grouping is beneficial when visibility is reduced, owing to risk dilution, early warning and predator confusion. Possible reasons why cowtail stingrays do not group under all environmental conditions were explored in terms of increased search costs for conspecifics, decreased escape speeds of rays in groups, and increased delays in initiating flight. Results from focal follows eliminate increased search costs as an explanation. Data from mock predatory attacks suggest that the relative cost of decreased escape efficiency of grouped rays (decreased speeds and lengthened delays) is increased when vision is unconstrained, since the ray already stands a good chance at escaping a predator on its own. The interplay between costs and benefits of grouped and solitary resting is easily influenced by environmental conditions, and thus is subject to anthropogenic impacts. Conservation implications of this study are also explored.