Abundance, Residency, And Potential Hazards For Coastal Bottlenose Dolphins (Tursiops Truncutus) Off A Productive Lagoon In The Gulf Of Mexico

Movements of many cetacean species are commonly related to temporal and spatial variations in food resources and human activities. Worldwide evidence shows that anthropogenic pressures faced by coastal dolphin populations are increasing; however, the lack of reliable baseline infonnation generally prevents the assessment of such interactions. We studied the temporal dynamics in abundance, site fidelity, and residency of bottlenose dolphins (Tursiops truncatus) off the productive Alvarado lagoon in the Gulf of Mexico, and we assessed the potential hazards posed by human activities and natural predators. This 2-y study (2006 to 2008) was based on the photographic identification of 174 individuals from 871 high-quality dorsal fin photographs obtained during 41 surveys totaling 225.4 h of observation. Overall monthly abundance averaged 125 (SD = 52) dolphins, whereas naturally marked individuals averaged 106 (SD = 25); abundance values were somewhat consistent within and between years, but the community was composed of different dolphins at any given time. Seasonal site fidelity and residency were higher during the dry (March to June) and rainy (July to October) seasons. Previous studies from Alvarado and elsewhere suggest long-term residency (up to 7 y) but also widespread movements (100 to 300 km) for some individuals. Physical evidence of attacks by large predators was exclusively found in nonresident adult dolphins (3.5%), suggesting a seasonal incursion to the area by individuals from deeper waters. Also, dolphins bearing marks of interactions with fisheries were more common in adult residents (11.5%). Despite these threats, dolphins are recurrent in the area, possibly due to high prey abundance and availability, which may constitute the main factors driving their distribution and abundance.