|Title||Lunar cycles affect common dolphin Delphinus delphis foraging in the Southern California Bight|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||Simonis A.E, Roch M.A, Bailey B., Barlow J, Clemesha R.ES, Iacobellis S., Hildebrand JA, Baumann-Pickering S.|
|Journal||Marine Ecology Progress Series|
In the Southern California Bight, the common dolphin Delphinus delphis is the most abundant dolphin species and preys upon small pelagic fish, mesopelagic fish, and cephalopods. Mesopelagic fish and many cephalopods are available throughout the year, and they form deep scattering layers, some of which characteristically undergo strong diel vertical migrations. The extent of vertical migration depends on the degree of sea surface solar and lunar illumination. At their daytime depth, mesopelagic prey are beyond the range of shallow-diving dolphins. Autonomous acoustic recorders were used to monitor dolphin echolocation at 2 offshore recording locations from 2009 to 2014. Manual and automated classification techniques were used to identify periods of high echolocation activity, indicative of common dolphin foraging. Clear lunar patterns existed in cool months, when echolocation activity was highest during the darkest periods of the night and lunar month, indicating times when dolphins were foraging, possibly on mesopelagic prey. Echolocation was more abundant during warm months, but diel and lunar patterns in echolocation were weaker. Generalized additive mixed models show that the observed patterns in echolocation activity are correlated with lunar day and position of the moon in the night sky. Seasonal patterns may represent geographic shifts in common dolphin populations, shoaling scattering layers, or prey switching behavior during the warm months, whereby dolphins target small pelagic fish not associated with the deep scattering layers. Overall, dolphin foraging activity declined from 2009 to 2014 during warm months, which may be related to a declining abundance of small pelagic fish.
Here we report a relationship between common dolphin echolocation activity and the lunar cycle throughout the year at 2 offshore locations in the SCB. The relationship between echolocation and the lunar cycle is strongest during cooler months and seems to match the expected vertical migration of organisms within deep scattering layers. We believe that the strong relationship between lunar cycles and echolocation behavior corresponds to the importance of mesopelagic prey during cooler months. Seasonal differences in acoustic behavior may be related to dynamic upwelling and dissolved oxygen conditions, the migration of CPS, geographic shifts in common dolphin populations, and changes in the foraging strategies of dolphins. While some behaviors linked to the lunar cycle may be based on an endogenous rhythm, the dolphins also demonstrated a response to unpredictable changes in the light environments due to cloud cover. The ability to respond to changes in oceanographic conditions and community composition on seasonal timescales may buffer dolphin populations from the dramatic historical cycles observed in CPS populations.