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Feeding Ecology of Hammerhead Sharks

Measuring a freshly caught hammerhead from Kane'ohe Bay    There has long been an interest in what sharks eat -- many studies list the stomach contents of sharks in an effort to understand their diet.   But very few studies have attempted to quantify the amount of food that sharks consume on a daily basis.  Since sharks are at the top of their food chain, they may influence the population structure of species lower in that food chain.   Understanding exactly what and how much sharks eat can give us a much better understanding of how they interact with and affect their environment.  Due to the abundance of juvenile scalloped hammerhead sharks (Sphyrna lewini) in Kaneohe Bay, it was possible to conduct a study that examined the feeding ecology of this species.

Collecting gut contents from a newborn shark    Some sharks, including scalloped hammerheads, regurgitate by everting their stomach (turning it inside-out).  Taking advantage of this facet of the animal's biology provides an easy, non-lethal method for retrieving and studying stomach contents.

    Sharks were caught, anesthetized, measured, weighed, and stomach contents retrieved.  Most of the sharks caught were between 50-60cm and weighed around 600g.   Larger sharks were caught on rare occasions:

A large adult hammerhead next to our handling vessel - photo by Bruce Nyden

      Stomach contents were preserved in ethanol and taken back to the lab to be sorted, identified, and weighed.  The most common food items were a species of snapping shrimp (Alpheus malabaricus), and a gobie (Oxyuricthys lonchotus).  This low diversity of the diet can be attributed to the low species diversity of the bay floor, where the pups live and feed.

Common prey items of Sphyrna lewini

      Sharks were also kept in captivity to study the rate of digestion.  Rate of digestion was then combined with diet data to estimate the rate of food consumption of these sharks in their natural habitat.
    Initial estimates indicate that sharks in Kaneohe Bay are consuming less than 1% of their body weight per day, a suprisingly low rate of consumption.   Sampling of prey density suggests that food abundance on the bay floor is low.   This suggests that food is a limiting factor in the growth and survival of juvenile scalloped hammerheads in Kaneohe Bay.

This site was created by Timothy Fitzgerald and is maintained Nick Whitney
Last updated October 28, 2004 04:20 PM HST