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Mysteries of the Bait Ball

Bail Ball

Bait balls, living works of visual art, can produce spectacular fishing, but they also pose deep biological and philosophical questions. For example, in his new book on akule (also known as big-eyed scad in Hawaii), Wayne Levin poses this question: "Which is the individual – the fish or the school?" He applies the same question to flocks of starlings, which move in similar ways in reaction to predators.

Among highly social mammals and insects, this question is easily answered. Even though both may live in large groups, and individuals may even sacrifice themselves for the benefit of the group, the society is highly stratified with different individuals having different roles and social status. There is generally a clear leader – be it the alpha male, the president, or the queen. In flocks of migrating birds, the leadership role may shift frequently from one bird to the next, whereas in shoals of fish there may be no clear leader, and it often seems that the fish act collectively rather than as individuals. Somehow each member of the school must guide its own actions, but this is clearly done in reference to all the nearby individuals. Working with minnows, Brian Partridge showed that when only two fish were present in a tank, one was a leader and the other a follower. When a third fish was added a school was formed and each fish adjusted its behavior according to that of its nearest neighbors.

These decisions appear to be guided in large part by the miraculous organ known as the lateral line – a string of sensory pits down the side of the fish that is so sensitive to minute changes in water pressure that it can literally feel the other fish at a distance – a sense sometimes referred to as "distant touch." Of course vision also comes into play. It appears that the eyes tell a fish when its neighbors are getting too far away, and the lateral line tells it when its neighbors are getting too close. This still leaves open the question of what prompts a school to suddenly change direction or take other decisive action.

Recent research indicates that a decision may be triggered at any moment by the individual with the lowest level of tolerance. The fish that is the hungriest or the most fearful may start the move that is almost simultaneously mirrored by the rest of the school. In this way the society may be governed by the weakest link – in contrast to how we at least like to think of our mammalian societies.

Could this type of social organization result in group actions that are maladaptive? Biologists tell us that by participating in a school, a fish maximizes its own survival chances in two ways. Mathematically, the presence of other individuals reduces the odds of any particular individual being chosen by a predator. Also, by blending into a super-organism, the schooling fish are able to dazzle and confuse a predator and make it difficult for it to target a single individual. Sean Neil and Michael Cullen, researchers at the University of Oxford, showed that, as the number of fish in a school of prey is increased, both the attack rate and the success rate of predators are reduced, benefiting all the members of the school.

Scientists studying schooling behavior of fish are split into two camps regarding the amount of cooperation between individuals in the school. One camp views predator response patterns as cooperative behaviors for group benefit, while the other camp views the response patterns as a sort of accidental result of each fish attempting to hide behind the other fish in the school and keep a certain distance away from predators. This is termed the "selfish herd" theory.

Researchers have found that while schooling increases the survival rate of fish under attack by smaller predators that target individual fish, it may actually work to the detriment of fish that come under attack by larger predators that target whole schools of fish. We often see that large predatory fish or marine mammals seem to work the schooling behavior of their prey to their own benefit. They may carve a bait ball of manageable size out of a larger shoal of fish and corral it – keeping it as a sort of traveling cookie jar, from which they pluck individuals until the last morsel is gone. The shape-shifting tactics of the bait fish do frustrate many feeding charges, but the final outcome seems almost predetermined – the bait ball is slowly whittled down to the last individual. Wouldn't the fish be better off by scattering and taking their chances as individuals?

In fact, we usually see that in the final stage of disintegration of the ball. At some point the bait ball is reduced below critical mass and the fish suddenly scatter at high speed in all directions. As predicted, this makes them easier to target as individuals, and the predators are faster – there are generally no survivors. So the question then becomes whether they would have a better chance of survival if they scattered much earlier, when there were more prey than predators.

Typically, most bait balls vanish in a slow progression into the stomachs of predators. There have been occasions, however, such as those recounted by experienced skipper Anthony Mendillo, where a school of sailfish had become satiated and left the bait ball only half eaten. I have likewise observed situations where predators have been scared off by a passing boat or other stimulus, and the bait ball has dived and escaped. So, until the end is reached, a schooling fish may always have some hope that it can survive along with some of its schoolmates. Unfortunately, as an individual alone in the ocean it stands little chance.

The behavior of the predators can be as thought-provoking as that of the prey. To what extent are they cooperating for the good of the group, and to what extent are they merely acting as self-interested individuals whose actions may also indirectly benefit the other individuals? The more we study marlin, sailfish, and even predators traditionally viewed as loners, such as great white sharks, the more we see them as social animals that engage in pack hunting, rather than as individuals that happen to find themselves exploiting the same food source. Some predators may be social when they are feeding, but much less so at other times. Humpback whales, for example, have not been shown to maintain long-term associations on their breeding grounds, where most of the research has been conducted. However, when they return to their feeding areas, pods re-form into tightly-organized feeding teams, where each whale may maintain a specific position and role over a period of years.

The question of sociality also plays into the long-standing debate about how billfish use their bills. Do they slash their prey, spear their prey, or merely gobble their prey (with the bill acting perhaps as a primarily sensory or defensive organ)? The answer, of course, is "all of the above," but this does not solve the question of intent. It has been observed that a marlin that slashes a baitfish rarely succeeds in consuming it. Another marlin or other predator usually swallows the prize before the killer can turn and snatch it. However, if marlin feed as a social unit, with reciprocal obligations, each member of the team can expect to benefit by helping its teammates.

Dr. Guy Harvey, after filming many hours of video of billfish feeding on bait balls, has concluded that marlin and sailfish use their bills in completely different ways. According to his analysis, sailfish frequently use their bills to bat fish out of the school before consuming them, whereas striped marlin charge into the school and snatch fish directly with their jaws. My own photos show striped marlin with sardines impaled on their bills as well as slashing sardines in half, but I cannot argue that these were not accidents that occurred as the marlin blasted through the swirling school of fish. Dr. Michael Domeier, who has conducted extensive studies on striped marlin states that, "Certainly we can't rule out that striped marlin deliberately strike prey with their bill, but it does seem clear that it is not the primary feeding mode."

Even more mysterious are the Bryde's whales that sometimes feed on bait balls along with marlin, sea lions, sharks, dolphin, and other predators. Everything scatters out of the way when these 40-ton missiles come blasting through the water. Marlin, sea lions, dolphins, and sharks generally clear the area first (although I have seen a whale swerve to avoid a clueless marlin, and there is a video online showing a shark nearly going into a Bryde's whale's mouth), but my photos show that most of the bait fish also manage to evade the gargantuan mouth. Can a mere handful of sardines replace the energy required to propel a giant feeding machine through the water at such velocity? Or does the unexpected presence of humans in its path disturb the whale's feeding methodology? Could it be that Bryde's whales use bait balls that are quite small in relation to their own size for target practice, or as snacks, while gaining their real nourishment elsewhere? As with much of what transpires in the limitless blue of the open ocean, the answer awaits further study.

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