Brett Sweezey

Who I am

As a child, I remember sitting on the floor wide-eyed, watching BBC Earth’s The Trials of Life series and the episode ‘Hunting and Escaping’, where a pod of orcas deliberately beached themselves on the shoreline of Patagonia to capture a lone sea lion. My interest and excitement in marine predators naturally led me to the fascinating world of shark ecology. After I received my undergraduate degree in biology from Old Dominion University in Virginia, USA, I volunteered at the Bimini Biological Field Station in The Bahamas, where I was introduced to the technological advances of acoustic transmitters and satellite tags. I became interested in the techniques for tagging animals and how even the simplest tags could be used to collect movement, depth and water temperature data.

My fascination for utilising electronic tags led me to graduate school at the University of New England, where I used acoustic transmitters and satellite tags to estimate discard mortality by analysing the movement (or lack thereof) of Atlantic cod and dusky sharks, respectively. However, my thought process relating to movement ecology shifted as I became curious about how the distributions of predator populations could be associated with foraging behaviour.

I am now pursuing my PhD in marine biology at Texas A&M University at Galveston to estimate the habitat use and ecological connectivity of scalloped hammerhead, silky and sandbar sharks at the Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico. It is my plan to combine electronic tagging techniques with stable isotope analysis to estimate the movement patterns and trophic ecology of shark species in this region. These data can be used to inform future ecosystem-based fisheries management strategies at this remote and isolated marine sanctuary.

Where I work

The outer continental shelf of the Gulf of Mexico is composed of a complex network of salt domes that stretch from west to east along the Texas–Louisiana shelf towards the Mississippi River. These salt domes, commonly referred to as ‘banks’, provide variable natural habitat comprising small underwater mountains, ridges and patches of hard bottom. Whereas most of the banks support a variety of mixed invertebrate communities such as sponges, corals and gorgonians, the shallowest bank structures can exceed 60% hard coral structure. The Flower Garden Banks sanctuary is one of the healthiest and most northerly coral reef ecosystems in the western Atlantic Ocean. It is remote in nature, approximately 160 kilometres (100 miles) from the coastline of the USA, and embraces individual banks that peak at about 16–40 metres (52–132 feet) deep.

Flower Garden Banks was first recognised as a federal sanctuary in 1992. Two banks, East Flower Gardens and West Flower Gardens, were specified in the designation and it was expanded to include Stetson Bank in 1996, making the area of the sanctuary approximately 90 square kilometres (35 square miles). Since its creation there has been growing evidence that ecological activity and connectivity within the banks and to artificial reefs beyond their boundaries may be substantially higher than previously thought. In 2021, the USA again expanded the sanctuary’s boundaries to include an additional 14 banks composed of hard-bottom, mud flats and artificial reefs, increasing its coverage to 260 square kilometres (100 square miles). Although boosting the sanctuary’s area may be beneficial, our understanding of species’ connectivity to and among the banks remains limited and hampers our ability to evaluate whether size or gear restrictions implemented within Flower Garden Banks are efficient enough to protect the vulnerable habitats and species that live there.

What I do

Sharks are facing rapid population declines worldwide due to fishing pressure, habitat destruction and climate change. These challenges are exacerbated by the animals’ slow growth, late maturity and relatively small litter sizes. Under increasing natural and human-induced stressors, many sharks are susceptible to mortality when they are discarded after being captured. It is imperative that the techniques used to study these animals minimise stress by using less invasive sampling methods to increase the chance of survival when an individual is released back into the wild.
Stable isotope analysis is a popular technique used to estimate the feeding relationships among species within a specific ecological habitat. This process is done by collecting biological samples from an individual such as blood or muscle tissue, which are required to be frozen and can be difficult to preserve when collected from remote or isolated field sites. In addition, the capture-and-handling process necessary to obtain these samples can cause injuries or stress to an individual that scientists should try to avoid.

A technique called dried blood spot sampling was recently utilised to determine the trophic position of bony fish with only a few drops of blood. I plan to test this methodology on scalloped hammerhead, silky and sandbar sharks within the Flower Garden Banks sanctuary to see if this is a technique that could be suitable for shark species so as to decrease the introduced stressors associated with traditional stable isotope analysis sampling. Moreover, the dried blood spot methodology requires less biological material to be collected for sampling. Dried blood spots can be preserved at ambient temperature for up to a year, unlike whole blood and muscle tissue collected for traditional stable isotope analysis techniques. This method could provide an efficient and alternative sampling model for remote study sites, reduce financial challenges and increase our understanding of foraging strategies in areas of high ecological importance.