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.

Mortality investigation for bottlenose dolphins (Tursiops truncatus) stranded on the Alabama coast increased from 2010 to 2014 during the northern Gulf of Mexico unusual mortality event (UME) associated with the Deepwater Horizon oil spill (DWHOS). Cause of death (COD) patterns since that event have not been published and baseline prevalence of Brucella and morbillivirus, two infectious diseases previously reported in bottlenose dolphins in this region, are lacking for this population. We analyzed stranding records (n=225) of bottlenose dolphins in Alabama from 2015 to 2020 to determine COD and prevalence of Brucella spp. and morbillivirus spp. Cause of death was determined using gross necropsy and histological findings. To determine prevalence of Brucella and morbillivirus infections in stranded animals, a subset of individuals was selected for molecular testing. Necropsies were completed on 181 dolphins, and histology data were available for 63 of those animals to date. Cause of death was grouped into 7 general categories, including human and fisheries interaction, infectious, organ failure, prolonged freshwater exposure, trauma, multifactorial, and unknown. Unknown was further divided into those with poor and good body condition and those with evidence of fetal distress. Advanced decomposition limited COD determination in many individuals, however, fisheries interaction (n=8) was the most often confirmed cause. Morbillivirus was detected in 0% of samples tested (n=68), and Brucella was detected in 20% of samples tested (n=70). Brucella was detected in some moderately to severely decomposed carcasses, indicating that it may be feasible to test a broader range of stranded animals than previously assumed. This study provides valuable information on COD trends in bottlenose dolphins in Alabama since the DWHOS and is the first to establish baseline prevalence of two common infectious diseases in stranded animals from this population.

Mobile, apex predators are commonly assumed to stabilize food webs through trophic coupling across spatially distinct habitats. The assumption that trophic coupling is common remains largely untested, despite evidence that individual behaviors of these predators might limit trophic coupling. We used stable isotope data from common bottlenose dolphins across the northern and eastern Gulf of Mexico to determine if these apex predators coupled estuarine and adjacent, nearshore marine habitats. 13C values differed among the sites, likely driven by environmental factors that varied at each site, such as freshwater input and seagrass cover. Within most sites, 13C values differed such that dolphins sampled inside and in the upper reaches of embayments had values indicative of estuarine habitats while those sampled outside or in lower reaches of embayments had values indicative of marine habitats. 15N values were more similar among and within sites than 13C values. Data from multiple tissues within individuals corroborated that most dolphins consistently used a narrow range of habitats but fed at similar trophic levels in estuarine and marine habitats. Individual habitat specialization in these dolphins maintained trophic compartments between estuarine and adjacent marine habitats at a regional scale, challenging the notion that trophic coupling by mobile, apex predators is widespread and common.

The West Indian manatee, which lives at the interface of freshwater and marine systems, can serve as a representative model for potential vulnerability of nearshore species to in-water construction activities. In coastal Alabama, planned construction projects including the Mobile River Bridge and Bayway widening and Mobile ship channel expansion may impact individual manatees as well as habitat resources and migration pathways. Direct impacts to manatees such as vessel interactions, entanglement or ingestion of construction materials, and entrainment may result in acute physical injury or mortality. Indirect impacts from construction such as habitat obstruction or degradation and increased noise from construction activities can alter behavior and intra-species communication and reduce access to essential resources. While permitting requirements for in-water construction projects help to mitigate risks, manatees may be particularly vulnerable in areas like the northern Gulf of Mexico where manatee occurrence has increased in recent years and relatively few data are available on abundance and distribution. Some impacts of construction may be immediately difficult to quantify, but planned operations can implement and evaluate a variety of mitigation strategies pre-, during, and post-construction to prevent large-scale negative outcomes. As human populations increasingly occupy coastal zones across the globe, effective planning of coastal development and in-water construction of bridges, marinas, boat launches, and other infrastructure will be essential to support conservation efforts for manatees and other species at-risk in affected areas.

An increased number of bottlenose dolphins (Tursiops truncatus) with skin lesions indicative of freshwater exposure stranded in Alabama during Spring 2020. Little is known about the salinity threshold and length of exposure required for lesions to appear. Grossly, these lesions present acutely as combinations of pale, proliferative skin with areas of ulceration and erosions. These lesions progress to multifocal to coalescing erosions with blubber involvement and algal matting in presumably more chronic cases. In this study, stranding location, sex, age class, and presence/absence of skin lesions were analyzed for freshly dead and moderately decomposed dolphins (n=39) stranded between 1 Jan to 31 Aug 2020. Sixteen animals were not included in this study due to advanced state of decomposition. Discharge from the Alabama and Tombigbee Rivers and salinity data from Dauphin Island were examined during the same period to identify freshwater influx trends. A 40-year flood occurred in Mobile Bay between February and March, with the highest rate of discharge occurring in February, 2.4x greater than the 6-month average, and the lowest salinity occurring in March, 8 psu lower than the 6-month average. Fifty-one percent (20/39) of T. truncatus examined had evidence of freshwater-associated lesions. Prevalence of lesions was highest in adults (13 adults, 6 subadults, 1 calf), but did not differ between sexes. Individuals with freshwater lesions primarily stranded in April (n=13) and were more likely to strand in Mobile Bay (n=17) than in the Gulf of Mexico (n=1). The prevalence of freshwater-associated lesions on dolphins stranded in the Mobile Bay estuary, the fourth largest freshwater drainage system in the country, demands further investigation and highlights the importance of stranding response and skin lesion documentation, especially in times of environmental disruptions.

Vessel strikes are one of the leading causes of human-related mortality for large whales. In the Gulf of Mexico, there is a high volume of vessel traffic from commercial shipping, fishing, recreation, oil and gas, and military vessels. This puts the sperm whale and the Gulf of Mexico Bryde's whale, both of which are listed as endangered under the U.S. Endangered Species Act, at risk of vessel strikes. To date, the magnitude and spatial extent of this risk is poorly understood. We quantified spatially-explicit vessel strike risk in the Gulf of Mexico for these two large whale species by combining several years of Automatic Identification System (AIS) vessel data with modeled species distribution data. Our analysis of the AIS data confirmed a large amount of vessel traffic in the Gulf of Mexico, a substantial portion of which was attributed to the oil and gas industry. Spatial analyses of the co-occurrence of vessel traffic and species distribution data indicated that Bryde’s whales face considerable vessel strike risk near De Soto Canyon and on the West Florida shelf, while sperm whales face elevated vessel strike risk in shipping lanes associated with the ports of Galveston and Port Arthur, Texas. For both species, the risk of lethal vessel strike is also high off the coast of Louisiana, near the Mississippi Canyon. This work represents one of the first attempts to quantify the spatially-explicit vessel strike risk for the Gulf of Mexico’s two resident large whales species. The identified areas of higher risk can help managers prioritize where mitigation measures can be implemented to reduce the risk of vessel strikes for both sperm whales and the Gulf of Mexico Bryde's whale.