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Shark Side of the Moon

 

The behavior of many animals, including humans, are influenced by the cycle of the moon. For example, many fishes time their reproductive events with the full moon.

Together with Chris & Monique Fallows, I have been fortunate enough to study predator-prey interactions between great white sharks and cape fur seals at Seal Island in False Bay, off the coast of South Africa.

For many years we have wondered if and how the full moon may impact the hunting behavior of white sharks at Seal Island. So, we conducted a study to find this out… and the answer surprised us!

We have found that at Seal Island, great whites primarily attack seals within an hour of sunrise when it is still dark, making it extremely difficult for seals to visually detect the sharks stalking from below through deep dark water. But, the small amount of twilight provides just enough light for the sharks to see the seals silhouetted against the water surface above them. So, during low-light conditions = visual advantage to the sharks. However, when the sun begins to rise over the horizon, the extra sunlight makes the sharks easier to detect by the seals and they can outmaneuver and avoid a hunting white shark. So, during high-light conditions = visual advantage to the seals. So, we wondering if there would be a difference in shark attack rates and capture success of seals during new versus full moons when lunar illumination make effects the ability of sharks and seals to detect one another.

To do this, we evaluated our data that we had collected over 16 years, comparing white shark attack rates and hunting success during periods of low lunar illumination (0-10 %) during new moons and high lunar illumination (90-100%) during full moons.

Analysis of 1,476 natural attacks by white sharks on seals within an hour of sunrise revealed that shark attack frequency and seal capture success was significantly higher at sunrise during periods of low lunar illumination (0–10 %) as compared to high lunar illumination (90–100 %). In fact, we found that shark attacks on seals during new moons at sunrise were about double that during full moons. Similarly, great white capture success rate was about double at sunrise during low lunar illumination compared to high lunar illumination.

To explain these results, we hypothesized that during full moon periods, with high lunar illumination, great whites hunting during the night may be a at a visual and tactical advantage over seals which are silhouetted at the surface in the moonlight and thus easier to visually isolate in darkness. In contrast, white sharks at night remain camouflaged hunting from below through deep water. So, night hunting during full moon = visual advantage to the sharks. However, at sunrise, we hypothesize the advantage shifts to seals as the added lunar illumination, combined with emerging sunlight, may decrease shark stealth and increase the ability of seals to detect and avoid sharks. Thus, the significantly lower frequency and success rate of white sharks at sunrise during full moon periods (high lunar illumination) could be related to sharks being satiated from hunting the previous night, as well as, the increase in lunar illumination the following morning at sunrise making sharks less efficient hunters. So as a result, the sharks hunt less at sunrise during full moons, and when they do hunt, they are less successful. So, sunrise hunting during full moon = visual advantage to the seals.

Photo sequence of an unsuccessful predation attempt by a white shark on a Cape fur seal at sunrise during a full moon. In (panel A), the seal was able to avoid the advancing white shark, potentially due to added lunar illumination (moon in top right corner of panel B) that enabled the seal to visually detect the approaching shark from below. Image Source: Fallows et al. 2016 (Env Biol Fish).

It is likely that such patterns differ at other locations where white sharks hunt seals, but for now, we have answered a question we had for many years at Seal Island in False Bay, South Africa. Nature is amazing!

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