Exploring variability in the diet of depredating sperm whales in the Gulf of Alaska through stable isotope analysis

Isotope-based inferences of the trophic niche of short-finned pilot whales in the Webbnesia

Knowledge of predator-prey interactions is key in ecological studies and understanding ecosystem function, yet this is still poorly explored in the deep-sea environment. Carbon (δ13C: 13C/12C) and nitrogen (δ15N: 15N/14N) stable isotope ratios of a deep-diving species, the short-finned pilot whale (Globicephala macrorhynchus), were used to explore knowledge gaps on its ecological niche and foraging habitats in the Webbnesia marine ecoregion (Tenerife Island, n = 27 animals vs. Madeira, n = 31; 500 km apart) where animals display distinct levels of site fidelity. Specifically, we tested whether intraspecific isotopic variation results from differences between geographic areas (due to possible foraging plasticity between regions), sexes, and/or years (2015-2020) using Generalized Linear Models. In general, significant differences (p < 0.05) were found in the stable isotope profiles of pilot whales between the two archipelagos, which were also reflected in their isotopic niche. The higher mean and wider range of δ15N values in Tenerife suggest that pilot whales consume prey of higher trophic levels and more diverse than Madeira. The higher mean and wider range of δ13C values in Madeira suggest that in that island, pilot whales rely on prey from more diverse habitats. There was significant variation between some years, but not between sexes. Finally, we discuss pilot whales' foraging strategies worldwide and infer the reliance on benthic or benthopelagic food sources in the Webbnesia.

Understanding the relationship between the Bering Sea Cold Pool and vocal presence of odontocetes in the context of climate changea)

The Cold Pool is a subsurface layer with water temperatures below 2 °C that is formed in the eastern Bering Sea. This oceanographic feature of relatively cooler bottom temperature impacts zooplankton and forage fish dynamics, driving different energetic pathways dependent upon Bering Sea climatic regime. Odontocetes echolocate to find prey, so tracking foraging vocalizations acoustically provides information to understand the implications of climate change on Cold Pool variability influencing regional food web processes. Vocal foraging dynamics of ice-associated and seasonally migrant marine mammal species suggest that sperm whales spend more time searching for prey in warm years when the Cold Pool is reduced but are more successful at capturing prey during cold years when the Cold Pool is stronger. Beluga whale foraging vocal activity was relatively consistent across climate regimes but peaked during the warm regime. Killer whale foraging vocal activity peaked in both warm and cold regimes with indicators of different ecotypes exploiting changing prey conditions across climate regimes. Foraging activity of odontocete apex predators may serve as a sentinel indicator of future ecosystem change related to prey availability that is linked to a diminishing Cold Pool as water temperatures rise and seasonal sea ice decreases due to climate change.

Diet variation in a critically endangered marine predator revealed with stable isotope analysis

Understanding the foraging ecology of animals gives insights into their trophic relationships and habitat use. We used stable isotope analysis to understand the foraging ecology of a critically endangered marine predator, the Māui dolphin. We analysed carbon and nitrogen isotope ratios of skin samples (n = 101) collected from 1993 to 2021 to investigate temporal changes in diet and niche space. Genetic monitoring associated each sample with a DNA profile which allowed us to assess individual and population level changes in diet. Potential prey and trophic level indicator samples were also collected (n = 166; 15 species) and incorporated in Bayesian mixing models to estimate importance of prey types to Māui dolphin diet. We found isotopic niche space had decreased over time, particularly since the 2008 implementation of a Marine Mammal Sanctuary. We observed a decreasing trend in ∂¹³C and ∂¹⁵N values, but this was not linear and several fluctuations in isotope values occurred over time. The largest variation in isotope values occurred during an El Niño event, suggesting that prey is influenced by climate-driven oceanographic variables. Mixing models indicated relative importance of prey remained constant since 2008. The isotopic variability observed here is not consistent with individual specialization, rather it occurs at the population level.

Effect of trophic position on mercury concentrations in bottlenose dolphins (Tursiops truncatus) from the northern Gulf of Mexico

Marine species from the Gulf of Mexico often have higher mercury (Hg) concentrations than conspecifics in the Atlantic Ocean. Spatial differences in Hg sources, environmental conditions, and microbial communities influence both Hg methylation rates and the bioavailability of Hg to organisms at the base of the food web. Mercury bioaccumulates within organisms and biomagnifies in marine food webs, and therefore reaches the greatest concentrations in long-lived marine carnivores, such as dolphins. In this study, we explored whether differences in trophic position and foraging habitat among bottlenose dolphins (Tursiops truncatus) from the northern Gulf of Mexico (nGoM) contributed to the observed variation in skin total Hg (THg) concentrations. Using the δ¹³C and δ³⁴S values in dolphin skin, we assigned deceased stranded dolphins from Florida (FL; n = 29) and Louisiana (LA; n = 72) to habitats (estuarine, barrier island, and coastal) east and west of the Mississippi River Delta (MRD). We estimated the mean trophic position of dolphins from each habitat using δ¹⁵N values from stranded dolphin skin and tissues of primary consumers taken from the literature following a Bayesian framework. Finally, we compared trophic positions and THg concentrations among dolphins from each habitat, accounting for sex and body length. Estimated marginal mean THg concentrations (μg/g dry weight) were greatest in dolphins assigned to the coastal habitat and estuarine habitats east of the MRD (range: 2.59-4.81), and lowest in dolphins assigned to estuarine and barrier island habitats west of the MRD (range: 0.675-0.993). On average, dolphins from habitats with greater THg concentrations also had higher estimated trophic positions, except for coastal dolphins. Our results suggest that differences in trophic positions and foraging habitats contribute to spatial variability in skin THg concentrations among nGoM bottlenose dolphins, however, the relative influence of these factors on THg concentrations are not easily partitioned.

Metal content in stranded pelagic vs deep-diving cetaceans in the Canary Islands

The Canary Islands are home to many cetacean species, many of which are resident species. The present work aims to analyze, for the first time to the best of the authors' knowledge, the macronutrients, micronutrients and trace elements and toxic heavy metals in muscle and liver tissue of six species of stranded cetaceans in the Canary Islands. The study species were: Tursiops truncatus, Stenella frontalis, Delphinus delphis, Grampus griseus, Globicephala macrorynchus and Physeter macrocephalus. Statistical analysis studied the significant differences between the concentrations in muscle and liver tissues, with the differences in element content depending on the type of diving and length of the species. The results indicate that there are differences between muscle and liver for Ca, Cd, Co, Cu, K, Mg, Mn, Mo, Ni, Pb, Sr, V and Zn. Deep-diving animals differ in their concentrations of Cr, Cu, Mg, Mn, Mo, and Zn with respect to shallow-diving animals in muscle and in liver in Al, B, Cr, K, Mn and Mo. As for the differences between sex, the males present differences in their concentrations of B, Cd, K and Mg in muscle tissue with respect to the females, while differences in the liver were only detected in the Fe content. The study of the correlations shows that as the size of the animal increases, the concentration of Cd increases while the concentrations of Al, Cu and Zn decrease. The specimens foraging in shallower waters had the highest concentration of the macronutrient.