Dive Dry with Dr. Bill

#636: Death Defying... NOT!

Last week I talked about the "happy hookers" in Palau, the divers who used reef hooks in strong currents to stabilize themselves so they could observe the marine life drifting by in the swift currents. I've met a number of divers who like to power kick their way through a dive site without stopping to check out the critters. I sometimes wonder why they bother diving at all... why not use those legs to sprint in a foot race topside instead? Of course the "happy hookers" are somewhat stationary and let the critters do the running... er, swimming. And what a show it is.

Certainly one of the top attractions at places like Blue Corner is the abundance of sharks. Dozens of sharks drift by your eyes, sometimes within just a few feet as they check out the "happy hookers." And that doesn't include the land sharks that may be attached to the reef next to you. I'm no expert on sharks... or fish in general. However, there was no giant kelp in Palau (nor is there much left in our unusually warm waters) so I had to focus on learning the fish to keep my brain sharp as a dull tack. The two species we observed most were the gray reef shark and the whitetip reef shark. There are bull sharks and tiger sharks in Palau but they usually stay outside the reef in open water where they are safe from Dr. Bill's camera.

The gray reef shark (Carcharhinus amblyrhynchos) is perhaps the most common species in the Indo Pacific region. It generally is no longer than eight feet and reaches weights of a paltry 80 pounds. I'm way out of its weight class so it wouldn't be a fair fight if it came to that. These sharks are not aggressive. They spend their time mostly in the shallows around coral reefs although they will dive deeper than 2,600 ft. They are reportedly quite social and adopt a dominance hierarchy, although I saw few obvious interactions between individuals.

I often joke with lady divers that sharks are man eaters and don't go for women. This species prefers fish and cephalopods such as octopus and squid. It uses its extremely sensitive sense of smell to detect prey from a distance. They may also work in groups to trap fish against steep walls. Like most reef sharks, the eggs of this species develop inside the female and one to six young are born live after a 9-14 month pregnancy. Sadly this species only mates every other year... but then that's still more often than me!

The gray reef shark is not very threatening to humans although attacks have occurred, mostly against spearfishers. They are threatened by humans though as this is one of the species taken for shark fin soup and fish meal. Palau and other nations in Micronesia have created an extensive shark sanctuary to protect them, recognizing both their value to the ecosystem and for tourism.

The other species I observed with frequency was the whitetip reef shark (Triaenodon obesus). You'd think with a species name like obesus it would be a fatso, but actually this shark is very slender. It may reach the same length as the gray reef shark (although they are usually shorter) but is about half its weight. The white tip on the dorsal and tail fins is characteristic as is the unusual manner it swims. The whitetip is not a deep thinker. It usually swims no further down than recreational diving limits (130 ft). This species is not the same as the oceanic whitetip which is considered dangerous.

The whitetip is often seen resting on the bottom during the day, sometimes in fairly large groups. Unlike other sharks in the requiem shark family, this species does not need to swim to pass oxygenated water across its gills.

Whitetips are slow and somewhat awkward swimmers which limits their ability to pursue food during the day. Therefore they hunt mostly at night while their munchies are sleeping. Their slender shape allows them to get into the nooks and crannies of the muffin... er, reef... easier than other sharks. Among their victims are eels, smaller bony fish, octopus and crustaceans such as crabs and lobster. Perhaps that helps explain why I saw very few of these animals on my dives in Palau. Apparently these sharks are fished, but I've read they are toxic to humans.

When it comes to mating, the male pursues the female... often for an extended period. I can relate. The male initiates contact by grasping the female's right pectoral fin to get them in the proper position. Hmmm... never tried that! Based on observation of captive individuals, the heads of both are driven into the bottom and the tails are kept elevated. Doesn't sound like fun to me! After a 10-13 month pregnancy, females give birth to one to six live young.

The last shark species I saw in Palau was the zebra or leopard shark (Stegostoma fasciatum). The first name comes from the fact the young are striped and the second because the adults become spotted. We only saw one of these and it hightailed it off the bottom as I approached so I didn't get very good video. Sources suggested they are mainly bottom dwellers (and are called carpet sharks by some) and can breathe by pumping water past their gills.

However, back in 2001 I was diving the Great Barrier Reef in Australia and they were swimming in midwater. In fact on one of my night dives there, I stood on the dive step of the liveaboard and readied myself to take the plunge. I did my giant stride entry... and landed right on top of a pretty good sized (about 10 ft) zebra shark. The poor shark took off like a missile! I had thoughts of the film "Urban Cowboy" although I was riding a zebra shark instead of a mechanical bull. Speaking of which, I was quite happy I didn't run into bull sharks in Palau!

© 2015 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

To return to the list of ALL of Dr. Bill's "Dive Dry" newspaper columns, click here.

Gray reef shark approaching and one with missing fin, probably due to mating;
whitetip reef shark and one resting on the bottom.

This document maintained by Dr. Bill Bushing.
Material and images © 2014 Star Thrower Educational Multimedia