Wind speed and dissolved oxygen predict mullet jumping in a subtropical estuary

In the shallow, sun-drenched waters of subtropical estuaries, fish face a constant, invisible challenge: the amount of oxygen dissolved in the water can shift dramatically. Unlike humans who breathe air freely, most fish rely entirely on gills to extract oxygen from the water, a process that becomes difficult when the water is warm, stagnant, or crowded with algae. Some fish have evolved specialized ways to cope, but one of the most famous and puzzling behaviors belongs to the mullet. These common, silver-scaled fish are known for their spectacular leaps, launching themselves completely out of the water. For decades, scientists have debated why they do this. Some theories suggest the jumps are a way to escape predators, shake off parasites, or simply play. However, a long-standing idea called the "internal diving bell" hypothesis proposes a more urgent reason: that the fish are leaping to gulp air because the water below them lacks enough oxygen to keep them alive. This theory suggests that mullet have a special pocket in their throat that can trap air, allowing them to breathe directly from the atmosphere when the water fails them.

To test whether this breathing theory holds up in the wild, researchers set out to observe mullet in Big Sarasota Bay, a shallow estuary on the Gulf coast of Florida. The question was not just whether low oxygen levels trigger jumps, but whether the wind plays a hidden role. In shallow bays, wind does more than just ripple the surface; it mixes the water, bringing oxygen-rich air down into the depths and preventing the water from becoming stagnant. The researchers wanted to know if calm days, which allow the water to sit still and potentially lose oxygen, would lead to more jumping, even if the oxygen levels measured at that exact moment seemed acceptable. They also wondered if there was a specific point where the wind becomes strong enough to stop the jumping entirely, acting as a natural switch that keeps the water oxygenated enough for the fish to stay submerged.

Over a four-month period in 2025, a team of observers stood on a dock and watched the bay for one-hour intervals, recording every time a mullet leaped from the water. Alongside the jumps, they measured the dissolved oxygen in the water, the speed of the wind, the temperature, and other environmental factors. They collected data on 126 separate observation periods, creating a detailed picture of the bay's daily rhythm. The goal was to see if the number of jumps could be predicted by the wind and the oxygen levels, and if these two factors worked together or separately to influence the fish's behavior.

The results painted a clear picture of the fish's behavior. The researchers found that mullet jumped most frequently when the water was calm and the oxygen levels were lower. This confirmed the basic idea that the fish are reacting to a lack of oxygen in the water. However, the study revealed something more nuanced: the wind mattered just as much as the oxygen itself. Even when the oxygen levels were the same, the fish jumped significantly more on calm days than on windy ones. This suggests that the wind is not just a background factor but a key driver of the behavior. When the wind blows, it mixes the water and replenishes the oxygen, reducing the need for the fish to leap for air. When the wind dies down, the water becomes still, oxygen levels can drop in localized spots, and the mullet begin to jump.

The data showed a specific threshold where this behavior changed. The researchers identified a wind speed of about 6.3 as a turning point. Below this speed, as the wind got calmer, the number of jumps increased sharply. Above this speed, the jumping remained low and steady, regardless of how much stronger the wind blew. This indicates that very light breezes are not enough to keep the water fully mixed, but once the wind reaches a moderate strength, it is sufficient to prevent the oxygen-deprived conditions that trigger the jumps. The study also found that wind speed and dissolved oxygen were only weakly related to each other, meaning that a calm day did not always guarantee low oxygen, and a windy day did not always guarantee high oxygen. Yet, both factors independently predicted the jumping, showing that the fish are responding to a combination of still air and the resulting water conditions.

While the findings strongly support the idea that mullet jump to breathe air when the water is poor, the researchers were careful to note that this is not the only reason fish leap. Predators, such as sharks and birds, are common in the bay, and jumping is a known escape tactic. The study observed predators during some of the observation periods, but because the data was collected from the shore, it was difficult to distinguish between a jump caused by a predator and one caused by a lack of oxygen. The researchers suggest that future studies using underwater cameras or drones could help separate these causes. Additionally, the study did not find evidence of extreme oxygen starvation, known as hypoxia, where oxygen levels drop to dangerous lows. Instead, the jumping appeared to be a response to moderate reductions in oxygen, suggesting that the fish are sensitive to even small changes in their environment.

This research extends our understanding of how fish interact with their environment, moving beyond a simple link between oxygen and behavior to include the physical forces that shape the water. It suggests that the wind-driven mixing of the water is a critical factor in determining when mullet need to use their unique ability to breathe air. As coastal waters continue to warm and become more prone to oxygen fluctuations due to climate change and pollution, behaviors like this may become more common. The study does not claim that mullet jumping is a perfect indicator of water quality, nor does it prove that every jump is a gasp for air. However, it provides strong evidence that in the quiet, still moments of a subtropical bay, these fish are likely taking a breath of the sky to survive the water below.