Same, but different: distinguishing blacktip shark populations in the Gulf of Mexico
One of the largest populations of the blacktip shark – a Vulnerable species on the IUCN Red List – occurs in the Gulf of Mexico, where it is managed as a single stock. However, Addie is exploring the variability in the blacktip populations that live in different parts of the gulf with a view to refining the management of different subpopulations. She’s using novel techniques to understand these fine-scale differences. Improving the management of the gulf’s blacktip sharks, especially more sensitive subpopulations in critical areas, is crucial for the population’s overall health.
Addie Binstock
I am a Master’s candidate in marine science, an open-water scuba instructor and an elasmobranch enthusiast. After graduating with a BSc in animal science and wildlife biology from a land-locked state in the USA, I knew I wanted to get back to the water and get my career on track for using my generalised education to address marine conservation issues. With this in mind, I began travelling and working for different conservation non-profit organisations worldwide, combining my water skills in boat handling and scuba diving with my passion for research and science. This is where my captivation with...
Assessing blacktip shark population structure in the Gulf of Mexico with stable isotope analysis
The primary objective of this project is to explore variability in the life-history characteristics of blacktip sharks that reside in different regions of the Gulf of Mexico using stable isotope analysis. This information can be used to improve the management of different subpopulations, which will mitigate the risk of overexploitation of sharks in more sensitive groups.
The Gulf of Mexico supports one of the largest populations of Atlantic blacktip sharks, but it is currently managed as a single population, which doesn’t take into account the geographic and ecological isolation of sharks in different regions of the Gulf. Providing evidence of regional differentiation will help to improve management guidelines for smaller subpopulations. As blacktips are classified as Vulnerable by the IUCN, mitigating the risk of overexploitation of sharks in more sensitive subpopulations will be critical for the population as a whole.
According to the most recent assessment of Atlantic blacktip sharks in the Gulf of Mexico, the entire population is managed as a single unit, despite evidence suggesting that subpopulations exist within different regions of the Gulf. Due to the socio-economic importance of the Gulf’s blacktips, exploring differences between subpopulations clustered in different regions is critical to ensuring the effective management of the species. Studies have demonstrated variability between blacktip sharks within different parts of the Gulf, but stable isotope analysis has yet to be applied to characterise the life-history differences of the various subpopulations.
Stable isotopes vary in concentrations across different regions due to variable biogeochemical and oceanographic processes, providing unique location-based isotopic tracers. Common tissues used for isotopic analysis are metabolically active and change over time. In contrast, elasmobranch vertebrae are formed by compounding concentric layers that are metabolically inert after synthesis. Isotopic signatures found in discrete vertebral bands provide a ‘snapshot’ of local biogeochemistry and dietary sources assimilated at the time the band was formed. An entire vertebra thus provides an unaltered chronological record of isotope dynamics over an organism’s lifetime, where the growing vertebral edge represents recent history and the band-pair closest to the birth band represents local chemistry and diet encountered in the year following birth. By comparing isotopic composition from the time of capture and the time of birth, inferences about diet composition, trophic levels and foraging habitats over an organism’s lifetime can be made. Comparing trends for sharks in different regions will clarify variations between different subpopulations’ life-history characteristics. Understanding differentiation within the Gulf of Mexico’s blacktip shark population will inform efforts to improve the management of this species in one of its most critical habitats.
This study aims to provide the first stable isotope assessment of blacktip shark vertebrae, providing a novel method of exploring inter-population variations in the Gulf of Mexico’s blacktips.
- We will evaluate and compare the isotopic trends found in blacktip shark vertebrae from different regions of the Gulf that relate to foraging habitats, trophic position and isotopic niche breadth, with respect to both ‘recent history’ and ‘past history’ characteristics by exploring different parts of the vertebrae.
- The results of this study will provide further insight into the ecological population structure of blacktips within the Gulf of Mexico. Knowledge of discrete ecological groupings or life-history variations within the Gulf’s blacktip shark population will inform efforts to improve management of the Atlantic blacktip shark population.