Winter habitat use and temporal segregation by dolphins and porpoises off Maryland, USA
Offshore wind farms provide a source of clean energy. However, noise from wind farm construction has the potential to physically harm protected marine mammal species and cause behavioral disturbance that could interfere with vital activities such as navigation and foraging. We used passive acoustic monitoring to gather information about little-known patterns in winter distribution of small cetaceans (dolphins and porpoises) in and around the site of the proposed Maryland Wind Energy Area (20-40 km off Ocean City, Maryland, USA). We deployed acoustic devices called C-PODs from November 2014 to April 2015 at three sites within the area that could potentially be impacted by offshore wind farm construction noise (approximately 15, 35 and 50 km offshore). C-PODs continuously monitor the 20-160 kHz frequency range and use digital waveform characterization to select and log cetacean echolocation clicks. We analyzed these detections to determine the occurrence of small cetaceans and to identify temporal and spatial patterns.
The site furthest offshore had the highest number of total detection positive hours (DPH) of both dolphins and porpoises (547 and 562 hours respectively) and detections decreased closer to shore. Dolphins were detected most frequently in November, December and April, whereas porpoise detections peaked in January to March. This seasonal pattern is consistent with previous studies on bottlenose dolphin (Tursiops truncatus) movements. They are known to violently attack harbor porpoises (Phocoena phocoena) and the observed temporal segregation may represent a strategy by porpoises to reduce interactions. The increasing number of dolphin detections offshore may represent an offshore movement by bottlenose dolphins and/or the occurrence of common dolphins (Delphinus delphis). There was a significant diel pattern with dolphin detections peaking after sunset and before sunrise. At all three sites, dolphins and porpoises were regularly detected (19-87% and 48-52% of days respectively), but they tended to be present only for short periods (median daily DPH 3 and 1 respectively). Thus the potential impacts of wind development off Maryland on small cetaceans during the winter may be relatively low, although this could increase in the summer when dolphin sightings peak. Identifying seasonal patterns of habitat use is critical for understanding potential human-wildlife conflicts.