How some marine species navigate

The way marine animals navigate has long fascinated scientists. Now, two new tracking studies have highlighted some complicated logistics. 

In the first, Australian researchers found that green sea turtles (Chelonia mydas) pull off their impressive navigation feats with only a rough map to guide them to their target location.

Published in the journal Current Biology, it highlights that while turtles often stray off course, in some cases by hundreds of kilometres, they are able to re-orient themselves in open water. 

“While their navigation is not perfect, we showed that turtles can make course corrections in the open ocean when they are heading off-route,” says lead author Graeme Hayes from Deakin University.

“These findings support the suggestion, from previous laboratory work, that turtles use a crude true navigation system in the open ocean, possibly using the earth’s geomagnetic field.”

To track their movements, the researchers attached satellite tags to 33 nesting turtles before they moved from Diego Garcia to their foraging grounds in other islands across the western Indian Ocean.

Comparing these migration tracks with navigational models, the researchers found that 28 of the 33 turtles ended up off course – travelling more than 4000 kilometres to the east coast of the African continent, as far north as Somalia. However, despite straying, they still arrived at their target location, albeit a few weeks later than planned.

The researchers say the next generation of tag technology will allow them to assess the compass of migrating turtles more directly.

“Then we can directly assess how ocean currents carry turtles off-course and gain further insight into the mechanisms that allow turtles to complete such prodigious feats of navigation,” says Hayes.

In the second study, researchers at Florida Atlantic University, US, reveal that the movements of tiger sharks (Galeocerdo cuvier) in the Gulf of Mexico (GoM) varies depending on age, gender and season.

Matthew Ajemian and colleagues tagged 56 sharks with tags that give off a signal every time their fins breached the surface of the water, allowing the team to track their movements via satellite.

Over eight years, they found that larger juvenile and adult sharks had higher rates of movement and travelled further into deeper warms and away from the GoM shelf, especially in winter and autumn.

“Larger juvenile and adult sharks may venture more frequently off-shelf to partake in seasonal migrations or to access unique and ephemeral food resources that may arise in open-ocean habitats such as sea turtles,” they write in the journal PLOS ONE.

Smaller juvenile sharks on the other hand tended to favour shallow areas closer to shore, though some did venture out into deeper waters between April and June and then again between September and November.

Medium-sized sharks were found to stick with smaller juveniles, although they too ventured out into deeper waters between August and December.

Another notable finding was that, after accounting for size, females tended to move more quickly than males.

The researchers note the difference in behaviours could be explained by changes in fin morphology as sharks age, which likely improves long-distance swimming performance, or changes in age-specific needs.

They suggest their findings have implications for mapping the impact of climate change, oil spills and other environmental factors. However, they do note more studies are needed with larger sample sizes and improved tracking strategies.