We measured resting routine oxygen consumption rates (MO2, in mg O2 · h−1 · kg−0.67) of bat rays (Myliobatis californica) under two seawater (33, 35 ppt) and two brackish (15, 25 ppt) salinity conditions. A significant increase in MO2 was observed for rays in 15 and 25 ppt brackish water, compared with those in 33 or 35 ppt, presumably due to increased osmoregulatory costs. Our results suggest that energetic factors associated with seasonal decreases in salinity may influence the out-migration of bat rays from Tomales Bay (California) during the winter months.

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