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An unusual number of near term and neonatal bottlenose dolphin (Tursiops truncatus) mortalities occurred in the northern Gulf of Mexico (nGOM) in 2011, during the first calving season after two well documented environmental perturbations; sustained cold weather in 2010 and the Deepwater Horizon oil spill (DWHOS). Preceding the stranding event, large volumes of cold freshwater entered the nGOM due to unusually large snowmelt on the adjacent watershed, providing a third potential stressor. We consider the possibility that this extreme cold and freshwater event contributed to the pattern of perinatal dolphin strandings along the nGOM coast. During the 4-month period starting January 2011, 186 bottlenose dolphins, including 46% perinatal calves (nearly double the percentage for the same time period from 2003-2010) washed ashore from Louisiana to western Florida. Comparison of the frequency distribution of strandings to flow rates and water temperature at a monitoring buoy outside Mobile Bay, Alabama (the 4(th) largest freshwater drainage in the U.S.) and along the nGOM coast showed that dolphin strandings peaked in Julian weeks 5, 8, and 12 (February and March), following water temperature minima by 2-3 weeks. If dolphin condition was already poor due to depleted food resources, bacterial infection, or other factors, it is plausible that the spring freshet contributed to the timing and location of the unique stranding event in early 2011. These data provide strong observational evidence to assess links between the timing of the DWHOS, other local environmental stressors, and mortality of a top local predator. Targeted analyses of tissues from stranded dolphins will be essential to define a cause of death, and our findings highlight the importance of considering environmental data along with biological samples to interpret stranding patterns during and after an unusual mortality event.

This study provides regional-scale data on drivers of horseshoe crab ( Limulus polyphemus ) presence along the northcentral Gulf of Mexico coast and has implications for understanding habitat suitability for sparse horseshoe crab populations of conservation concern worldwide. To collect baseline data on the relationship between environmental factors and presence of horseshoe crabs, we surveyed four sites from the Fort Morgan peninsula of Mobile Bay, Alabama (AL) to Horn Island, Mississippi (MS). We documented number, size and sex of live animals, molts, and carcasses as metrics of horseshoe crab presence and demographics for two years. Data were compared to in situ and remotely sensed environmental attributes to assess environmental drivers of occurrence during the time of study. Overall, greater evidence of horseshoe crab presence was found at western sites (Petit Bois and Horn Islands) compared to eastern sites (Dauphin Island, Fort Morgan peninsula), mediated by a combination of distance from areas of high freshwater discharge and interannual variation in weather. Higher sex ratios also were found associated with higher occurrence, west of Mobile Bay. Land cover, particularly Bare Land and Estuarine Emergent Wetland classes that are common to western sites, was most predictive of live animal and to some extent carcass occurrence. Our findings suggest that small-scale variation in habitat quality can affect occurrence of horseshoe crabs in sparse populations where density is not a limiting factor. Data from molts and carcasses were informative to supplement live animal data and may be useful to enhance ecological assessment and support conservation and management in regions with sparse populations.

The Deepwater Horizon oil spill was unprecedented in total loading of petroleum hydrocarbons accidentally released to a marine ecosystem. Controversial application of chemical dispersants presumably accelerated microbial consumption of oil components, especially in warm Gulf of Mexico surface waters. We employed δ13C as a tracer of oil-derived carbon to resolve two periods of isotopic carbon depletion in two plankton size classes. Carbon depletion was coincident with the arrival of surface oil slicks in the far northern Gulf, and demonstrated that subsurface oil carbon was incorporated into the plankton food web.

The frequency and intensity of anoxic and hypoxic events are increasing worldwide, creating stress on the organisms that inhabit affected waters. To understand the effects of low dissolved oxygen stress on oysters, hatchery-reared oysters were placed in cages and deployed along with continuously recording environmental data sondes at a reef site in Mobile Bay, AL that typically experiences low oxygen conditions. To detect and measure sublethal stress, we measured growth and survival of oysters as well as expression of three biomarkers, heat shock protein 70 (HSP70), hypoxia inducible factor (HIF) and phospho-p38 MAP kinase, in tissues from juvenile and adult oysters. Survival rates were high for both juvenile and adult oysters. Expression levels of each of the 3 isoforms of HSP 70 were negatively correlated to dissolved oxygen (DO) concentrations, suggesting that HSP 70 is useful to quantify sublethal effects of DO stress. Results for HIF and phospho-p38 MAP kinase were inconclusive. Test deployments of oysters to assess expression of HSP 70 relative to environmental conditions will be useful, in addition to measuring abiotic factors, to identify appropriate sites for restoration, particularly to capture negative effects of habitat quality on biota before lethal impacts are incurred.

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 might limit trophic coupling. We used stable isotope data from common bottlenose dolphins across the Gulf of Mexico to determine if these apex predators coupled estuarine and adjacent, nearshore marine habitats. δ 13 C 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, δ 13 C values differed such that dolphins sampled 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. δ 15 N values were more similar among and within sites than δ 13 C 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. Because these dolphins exhibited individual habitat specialization, they likely do not contribute to trophic coupling between estuarine and adjacent marine habitats at a regional scale, suggesting that not all mobile, apex predators trophically couple adjacent habitats.

Predation of fish on their scyphozoan hosts has not been clearly defined using analysis of gut contents because gelatinous prey are difficult to visually detect and are dissolved by fixative solutions. Therefore, scyphomedusae have been generally considered not relevant in fish diet. To determine the contribution of their scyphozoan host tissue to the assimilated diet of age-0 Chloroscombrus chrysurus, we determined δ¹³C and δ¹⁵N of fish, their hosts (scyphomedusae Aurelia sp. and Drymonema larsoni) and their potential prey—small plankton (<200 μm) and mesozooplankton (>200 μm)—in the coastal waters of Alabama, USA. The diet of C. chrysurus was defined using the Bayesian mixing model Stable Isotope Analysis in R (SIAR). Models indicated that the scyphozoan hosts contributed on average ~90 % to fish assimilated diet. In contrast with previous dietary assessments based on analysis of gut contents, these results highlight that scyphozoans are important to the diet of fish associated with them. Because several ecologically and economically important fish species live in association with scyphomedusae, a redefinition of trophic links in marine food webs may be needed in light of the findings in this study.

Human activity affects marine mammal stranding rates in two major ways; through human interaction (HI) that may lead to mortality and through search and response efforts that enable carcass detection. To better quantify the combined effects of these interacting human influences, we analyzed strandings for bottlenose dolphins ( Tursiops truncatus ) in the northern Gulf of Mexico (nGOM), an area of high cetacean strandings. To identify hotspots of human influence, we first determined the number of total and HI-related bottlenose dolphin strandings normalized to shoreline length in each nGOM U.S. state, which represent major response areas. To estimate the effects of response effort on stranding numbers (for HI and non-HI strandings), we used the Deepwater Horizon oil spill (DWHOS) as an established benchmark to compare periods of lower (pre-spill) and higher (post-spill) response effort. Strandings in Alabama waters were used as a case study to detail spatial and temporal variation due to human influences during the 39-year period of retrospective study. We found an increase in strandings from Louisiana through Alabama following the DWHOS. Non-oil related HI strandings increased in total number in AL, and they increased as a proportion of total strandings in Alabama (16%) and the Florida panhandle (12%). The increase in HI-related strandings in Alabama was driven by mortality of many types, but particularly fishery interactions and cases of apparent intentional harm. The Alabama case study clearly detected lower stranding numbers during periods of low or intermittent response coverage. Our findings are consistent with an overall increase in stranding numbers due to a combination of increased stranding occurrence and response effort following the DWHOS. Importantly, we provide evidence that HI-related standings increased independently from the DWHOS, with ongoing increases in at least one hotspot (Alabama). These findings provide a first step to parsing out different effects of human influences on stranding data for a common coastal cetacean. Our approach establishes baselines for future damage and recovery assessments, identifies areas where resources can be focused for management and education, and highlights the power of response and monitoring agencies to positively influence stranding datasets.

Stable isotope analysis (SIA) provides ecological data that can be safely and efficiently collected on endangered, threatened, and cryptic species. Marine mammals are an ecologically important group for which economical and logistical constraints can make data collection challenging. Stranded marine mammals are often used in research, but the causes of strandings and subsequent tissue decomposition could affect SIA. We conducted a three‐part study to test the validity of using δ13C and δ15N values from tissues of stranded bottlenose dolphins (Tursiops truncatus) and West Indian manatees (Trichechus manatus) for ecological studies. First, we quantified isotopic overlap using ellipses based on 95% of the data to compare isotope values in skin between stranded and live‐captured animals. Second, we compared stable isotope values from liver, skin, and muscle of animals that had stranded and were sampled at different decomposition stages. Third, we experimentally exposed each tissue to environmental conditions and sampled tissues as they decomposed. For both dolphins and manatees, isotopic ellipses from skin of stranded carcasses were similar to live‐captured individuals. Among individuals recovered at different decomposition stages, more advanced decomposition affected δ13C values in dolphin liver and skin but not in manatee tissues and had no effect on δ15N values in any tissue for either species. In the experimental manipulation, decomposition resulted in depleted δ13C values, enriched δ15N values, and increased C:N in liver for both species. Skin and muscle from stranded dolphins and manatees are representative of their corresponding live populations and can be used for SIA with appropriate caution. To facilitate the use of tissues from stranded animals, tissues should be dried or frozen for storage as soon as possible after sampling. We recommend liver from stranded animals only be used for SIA when researchers need tissues with short turnover times and can access fresh samples. Without consideration of decomposition effects on isotope values, ecologists may make inaccurate inferences about habitat use, diet, and community structure. Careful use of SIA on tissues from stranded animals can help researchers provide better quality information for managers and policy makers.

Common bottlenose dolphins ( exposed to freshwater or low salinity (<10 practical salinity units; PSU) for prolonged periods of time have been documented to develop skin lesions, corneal edema and electrolyte abnormalities, and in some instances they have died. Here we review a case of an out-of-habitat subadult, female common bottlenose dolphin that remained in a freshwater lake in Seminole, Alabama for at least 32 days. Due to concerns for the dolphin's health a rescue was initiated. At the time of rescue bloodwork results indicated minor electrolyte abnormalities (hyponatremia, hypochloremia, hypoosmolality). Renal function was not affected (normal creatinine and urea nitrogen) and all other bloodwork parameters (hemogram; serum biochemistry analytes) were within normal limits. The dolphin was deemed healthy enough for immediate relocation and release. A satellite-linked tag was attached to the dorsal fin to track the dolphin following its relocation to a nearby brackish water bay (Perdido Bay, AL), a known habitat for bottlenose dolphins. Twelve weeks following release, the dolphin was found dead as a result of a fisheries interaction (peracute underwater entrapment). A full necropsy was conducted and there was complete resolution of the skin pallor and skin lesions and no evidence of chronic renal or central nervous system lesions. Post-mortem analysis of vitreous humor (used as a proxy for serum analytes and to determine post-mortem interval) was challenging to interpret and has not been validated in dolphins. This supports the need for future research in cetaceans to establish a species-specific approach. Elevated barium (Ba) concentrations in tooth dentin corresponded to increased seasonal freshwater discharge patterns, confirming repeated annual exposure to low salinity conditions prior to death and indicating freshwater exposure may pose an ongoing threat to dolphins in the region. This case provides a unique opportunity to follow the progression of prolonged freshwater exposure and recovery in a bottlenose dolphin and highlights that dolphins in nearshore habitats face a combination of persistent natural and human associated threats.

Ecosystems are experiencing elevated levels of disturbance, and species with narrower niches are often more vulnerable to disturbances. Niche breadth is often measured in terms of either diet or habitat use but diet and spatial use are infrequently considered in tandem. These different aspects of niche breadth potentially expose species to different types of disturbances; species with narrow dietary niches may be more affected by disturbances that alter trophic relationships, while species with narrow habitat niches may be more vulnerable to habitat loss and point‐source pollutants. We examined dietary and habitat niche breadth of common bottlenose dolphins, Tursiops truncatus truncatus, from three different nearshore sites in the Gulf of Mexico (GoM). Using stable isotopes, we determined proportional contributions of different prey groups to dolphin diets at each site and through time at one site. We used satellite‐linked telemetry at two sites to determine habitat use and site fidelity. Additionally, we examined the literature on cetacean diet, habitat use, movement, and IUCN status to determine relationships between niche breadth and population status for different species. Dolphin diets varied among sites as available prey varied, but Perciformes fish were the most frequently consumed prey. At the site for which we had temporal data, dolphins consumed more cephalopods in 2015 and 2018 but otherwise consumed primarily Perciformes fish. Dolphins had small utilization distributions and exhibited high site fidelity. Data from 31 cetacean species revealed that most species with vulnerable, threatened, or endangered IUCN statuses not only have specialized diets but also exhibit high site fidelity. Dolphins had diet characteristic of flexible generalists but were habitat specialists with high site fidelity. Dolphin populations in the GoM may have altered their diets in response to environmental changes that have altered community composition and trophic dynamics. On the other hand, their high site fidelity has exposed them to point‐source pollutants, such as oil spills, persistent organic pollutants, and freshwater. Our broader analysis of cetaceans confirmed that species with specialized diets and high site fidelity were the most vulnerable to disturbances, providing a framework to predict which nearshore dolphin populations, and cetaceans in general, are most vulnerable to environmental changes.