Diel variation in diving behaviour of rehabilitated grey seal juveniles (Halichoerus grypus) in the Baltic Sea

Imagine the ocean as a giant, three-dimensional supermarket. For the creatures living there, the most important rule of survival is knowing exactly where the food is on the shelves. Some animals hunt near the ceiling, others dive to the floor, and many have to time their shopping trips perfectly to catch their prey when it moves. This is the world of "optimal foraging," a fancy way of saying that animals are constantly trying to get the most calories for the least amount of energy. To understand how they do this, scientists look at "diving behavior." Think of a dive like a shopping trip: how deep did you go? How long did you stay in the aisle? And how hard did you have to work to find what you needed? By tracking these patterns, researchers can figure out if an animal is having a great day at the market or struggling to find a snack.

This story is about a group of young grey seals in the Baltic Sea who had a very rough start to life. These aren't just any seals; they were "rehabilitated" juveniles. Picture them as kids who had to go to a special hospital to recover from being sick, hungry, or injured before being sent back to the wild. The big question for scientists was: once these seals were released back into the ocean, could they remember how to be good hunters? Could they figure out the daily rhythm of the sea, knowing when to dive deep and when to stay shallow? This paper dives into that question, using high-tech satellite tags to watch how these young seals behaved day and night after leaving the safety of the rehabilitation center.

The Story of the Seals' Return

The researchers followed thirteen young Baltic grey seals that had been nursed back to health at the Lithuanian Sea Museum. These seals were about six to seven months old when they were released back into the Baltic Sea, 20 kilometers off the coast of Lithuania. To keep an eye on them, scientists attached small satellite tags to their fur. These tags acted like tiny backpacks that recorded every dive the seals made, sending data back to the researchers about how deep they went, how long they stayed underwater, and how often they popped up for air.

The study looked at the seals' behavior over a period ranging from 20 days to over seven months. The goal was to see if the seals could adapt to the wild and if their diving habits changed between day and night. The results showed that these rehabilitated seals demonstrated successful post-release behavioural adaptation, rapidly adjusting their habits to the wild environment.

Day vs. Night: The Seals' Shopping Schedule

The most exciting discovery was a clear difference between how the seals acted during the day versus at night. It's like the seals had two different shopping lists for the two different times of day.

During the Day:
When the sun was up, the seals went on "deep dive" missions. They would plunge down deeper, with an average maximum depth of about 72.2 meters, and they would stay down for shorter bursts. Think of this as a quick, targeted trip to the bottom of the ocean floor. The data suggests they were hunting for fish that were hiding deep down, perhaps in tight schools near the seabed.

During the Night:
When the sun went down, the strategy flipped. The seals stayed much closer to the surface, with average maximum depths dropping to around 58.4 meters. However, they stayed underwater for longer periods at these shallower depths. It's as if they were leisurely browsing the upper shelves, taking their time to look around. Interestingly, the seals also seemed to spend more time resting on the surface (or "hauling out") during the night than during the day.

The Mystery of the "Extra Effort"

Here is where it gets a little tricky and fascinating. The scientists used a special math tool to measure "diving effort." Imagine you are walking up a hill. If you walk up a steep hill quickly, you are working hard. If you walk up a gentle hill slowly, you might be working just as hard, or maybe even harder, because you are taking your time.

The study found that at night, even though the seals were diving shallower, they were spending more time underwater for each dive than the math would predict for that depth. This is called a "positive dive residual." It suggests that at night, the seals were working harder or searching longer for their food.

Why would they do this? The paper suggests a few possibilities. Maybe the fish they were hunting were spread out and harder to find at night, forcing the seals to search longer. Or, perhaps the seals were just being extra careful. The researchers note that because the seals also spent more time resting on the surface at night, this "extra effort" might not mean they were having a feast. It could mean they were hunting in areas where the food was scattered and harder to catch. The study highlights that interpreting this effort is complex, as these patterns could arise under both favorable and suboptimal foraging conditions.

Growing into the Wild

One of the most important parts of the story is how the seals changed over time. When they were first released, they were a bit like tourists in a new city, exploring everything. They dove deeper immediately after release, perhaps just checking out the deep waters. But very quickly—within a few days—they adjusted. Their diving patterns stabilized, and they settled into a routine that matched the local environment perfectly.

The study also looked at whether male and female seals behaved differently. The answer was a clear "no." Both boys and girls followed the same day-and-night patterns, suggesting that this behavior is about survival and the environment, not about gender.

The Verdict

So, what does this all mean? The paper concludes that these rehabilitated seals were a success story in terms of adaptation. They didn't just survive; they rapidly learned to read the ocean's daily rhythms. They adjusted their diving habits to match where the fish were moving, diving deep during the day and staying shallower at night.

The researchers are careful to say that while the data suggests the seals were hunting effectively, they can't see exactly what the seals were catching. The "extra effort" at night is a bit of a mystery—it could mean great hunting or a tough search. But one thing is certain: these young seals, after being sick and rescued, were able to go back to the wild and rapidly adjust their diving behaviour to local prey and environmental conditions, proving that rehabilitation can help these marine predators regain their place in the ecosystem.