Abstract
Basic demographic data such as size variation in a stranded population can provide critical information about population dynamics, particularly for highly mobile marine mammals that can be difficult or costly to directly monitor. The common bottlenose dolphin (Tursiops truncatus) is found in coastal and nearshore environments throughout the world and is the most commonly stranded marine mammal along the northern Gulf of Mexico (GOM) coast. Size frequency distributions of dolphin populations and relationships to established age classes (i.e.; perinate, yearling, calf, subadult, adult) are poorly documented, but have potential to vary among geographic regions, making these data essential to set baselines and detect changes in population demographics through time. To determine the difference in size distributions of stranded dolphins in the GOM, we analyzed straight length and location data for dolphins stranded between 2008 and 2019 for the US portion of the GOM as a whole and individually for each GOM state (Texas, Louisiana, Mississippi, Alabama, Florida Gulf Coast). Chi-squared tests and mixture distribution analyses were performed on data in 8 cm bins. Louisiana, Mississippi, and Florida had different frequency distributions compared to the whole GOM (Chi-squared; p< 0.001). Texas, Mississippi, and Alabama had a greater proportion of perinate and neonate strandings than the whole GOM, suggesting lower reproductive success rates in these states. Three normal distributions were fit to the size frequency data for each state with means of 102.5±3.6, 197.7±7.4, and 249.8±4.6 cm, providing evidence that size within age class varies little between states. These findings indicate population demographics of dolphins spatially vary within the GOM but not along a continuous geographic spectrum. Future research is needed to determine which environmental factors most contribute to the consistency (size at age) and variation (stranding frequency within an age class) in GOM-wide demographic patterns of stranded dolphins.
