Gustav Hellström
Who I am
Growing up amid the forests, marshes and mountains of central Sweden, I wasn’t initially drawn to the sea. As a child, armed with a fishing rod I would explore the creeks and lakes near my home, developing a profound fascination for fish. Peering into the waters, I often wondered about the lives of these creatures in the depths below. Unfortunately, an early fear of water – an unusual trait for a budding marine biologist – hindered me from exploring these wonders until later, during my PhD studies on fish behaviour and the introduction to fish-tracking technology. This technology allowed me to monitor where the fish were and what they were doing, sparking an intense passion for transmitters, sensors, loggers and the like. Having started my research on fish in lakes and rivers, I now primarily work in marine environments, studying everything from tiny gobies in small harbours to large, ocean-crossing apex predators. Deep-sea sharks captivate me as the most elusive and fascinating animals on the planet, and it is a lifelong dream to track their behaviour and migrations down there in the dark, cold depths.
Where I work
Skagerrak is a strait between the North and Baltic seas, bordered by Sweden, Norway and Denmark. It is one of the most intensely bottom-trawled regions in the world, and also one of the busiest shipping lanes. Many demersal fish stocks have declined or collapsed in the region, including many shark and ray populations. However, recent discoveries in the northern reaches of Skagerrak have unveiled deep, restricted areas that host a rich biodiversity. These regions, characterised by deep canyons, rifts and underwater mountains, seemingly provide a sanctuary for numerous fish species, including giant Greenland sharks and sailrays, offering them refuge from the intense demersal trawling going on around them.
What I do
The project focuses on understanding behaviour, migration and habitat use of Greenland sharks and sailrays in the deep regions of Skagerrak. Studying and tracking deep-sea elasmobranchs is challenging due to the high-pressure environment and the vast expanse of the sea. The first problem is always to catch them. Here we work with experienced specialist deep-sea recreational anglers who help us to catch enough fish to tag. The weather usually allows our tagging boat and our small fleet of sport-fishing boats only a few days per year to venture far out to the deepest parts of Skagerrak.
Once on site, the tagging boat is positioned in the middle of the fleet and we wait for the teams to start catching and delivering fish for us to tag. This wait can take sometimes minutes, sometimes hours. Once we have a shark or ray at the surface, the tagging team springs into action, swiftly yet carefully attaching tags, measuring, and collecting samples of tissue, DNA, blood and parasites. Once everything is done, releasing the shark back into the depths is always a moment of relief and satisfaction as we watch the majestic creature slowly return to the dark waters.
We tag the sharks with pop-up satellite tags and acoustic tags, the latter being the focus of this project. These tags enable us to monitor the sharks’ behaviour and movements within a network of acoustic receivers deployed down to 700 metres (2,300 feet) in a large array covering our study area. If a shark moves beyond our network, we can access global detection databases like ETN or OTN, utilising the coverage of other receiver networks across the Atlantic. We retrieve and maintain the receivers annually – downloading data, changing batteries and redeploying them. The journey home, laden with fresh data, is filled with a mix of intense anticipation and anxiety, eager to uncover new insights into the mysterious lives of these deep-sea inhabitants.