Reflecting on the journeys of our satellite-tagged juvenile loggerhead turtles with Upwell

For the past six months, researchers at Upwell and Two Oceans Aquarium Foundation have had an opportunity that is still novel in the world of sea turtle research: To track juvenile turtles at sea. As part of Upwell’s Lost Year’s Initiative, our collaborative team deployed micro-satellite tags on 12 loggerhead turtles rehabilitated at the Turtle Conservation Centre. We watched their progress with many questions and hypotheses. Are they actively swimming or just riding the currents? If they are swimming, would they swim towards the Atlantic or Indian Ocean? How long would these rehabilitated turtles survive, if at all?

While it will take far longer than six months to answer any of these questions fully, we made some fascinating observations that allow us to ask even more questions!

All 12 turtles were released into the Agulhas Current at iSimangaliso Wetland Park. Kagiso left the current for around two weeks before rejoining, and Tao spent almost the entire two months that his tag transmitted swimming through the eddies to the east of the current. Parker joined Tao in the eddies upon returning from the retroflection zone. Both Tao and Parker exhibited active swimming in an area where upwelling (water movement relative to the continental shelf) generates “high productivity”. These areas host robust food webs that support the growth of many kinds of invertebrates which loggerhead turtles love to snack on.

All the other turtles reached the Agulhas Current retroflection zone where the current doubles back towards the Indian Ocean. The 40-degree south latitude line is a hard barrier where the current turns back on itself, creating a dynamic ocean environment with fast-moving eddies, temperature breaks, and strong winds. This is where both the temperature and productivity change abruptly and the turtles go from warm, fast-flowing waters to cold, rough waters.

Many of the turtles meandered around the retroflection zone until their tags ceased transmitting, but three turtles took off into the Atlantic Ocean. Bailey stayed offshore while Taylor and Rory headed northwards closer to the coast. Interestingly, many adult turtles of varying species tagged by the Turtle Conservation Centre have frequented the cold waters of South Africa’s West Coast and Namibia. This stretch of coastline offers nutrient-rich waters, abundant foraging grounds, and unique geological markers to which turtles may respond.

In addition to showing the loggerhead turtles’ geographic distribution, the tags also gave us insight into another dimension of their oceanic home: The water column. Most of the turtles utilised the first five metres of the water column, but Benji made daily dives deeper than 30 metres, exhibiting a much more multifaceted way of using the water column than previously assumed.

While we cannot draw definite conclusions about juvenile loggerhead habitat use in the region with such a small data set, these tracks add to a growing body of evidence that juvenile turtles are not just going with the flow but are actively swimming to seek out favourable habitats.

The data from these turtles also helps us improve our ocean modelling techniques to more accurately predict where juvenile loggerheads will go depending on distinct ocean conditions. These models predict juvenile turtles’ movements based on data about currents and environmental cues that signal good turtle habitat (for example, sea surface temperature, chlorophyll levels, and ocean productivity).

“We are only beginning to understand what happens to our rehabilitated young loggerheads after release. I’m incredibly grateful to Upwell for their partnership in this important project and deeply excited to not only uncover where our turtles spend their ‘lost years’, but also to learn how we might improve our release methods and locations to give them the best possible chance of survival in the wild,” says Talitha Noble-Trull, Turtle Conservation Centre Manager.

Fifteen years ago, collecting data from a juvenile loggerhead weighing several hundred grams as it journeyed through the open ocean was still out of reach. Improved miniaturisation technology (thank you to our collaborators at Lotek Wireless!) has made it possible to tag tiny turtles, and our novel research is still just at its beginning. Between the first release in January 2024 and the second in October 2024, we improved our experimental design and increased tag transmission durations from a maximum of two months to six months.

Arrival at concrete conclusions will require tagging many more turtles to generate a larger and more comprehensive interannual data set. Upwell Executive Director Dr George Shillinger is looking forward to continuing the partnership, saying, “We are thrilled to have the Turtle Conservation Centre as a collaborator within our global Lost Years Initiative. The findings from this important tracking study support the efficacy of the Turtle Conservation Centre’s efforts to rehabilitate and release stranded juvenile loggerhead turtles. They will also help to shed light on the dispersal and habitat use of loggerhead turtles as they embark on their lost years journey into the oceanographically dynamic ecologically rich habitats surrounding the Cape of South Africa and beyond. We look forward to continuing this collaboration with further releases that shed light on the early-stage dispersal and movements of sea turtle populations worldwide.”