Stan Riggs (left) and Martin Posey on the deck of the research 

 vessel Edwin Link. 



Though the large, branching reef 

 corals are absent, many of the fish 

 and invertebrates common to tropical 

 reef systems — including the four- 



level. 



The edges 

 of these 

 channels are 

 like cliffs, and 

 the scarps — 

 or cliff faces — 

 actually 

 produce the 

 most produc- 

 tive of hard- 

 bottom habi- 

 tats. As the 

 rock beneath 

 the limestone- 

 capped mesas 

 is undercut by 

 boring marine animals, the edges of 

 the top rock break off into slabs. 

 These huge chunks of rock form a 

 "rubble ramp" and provide more 



Some of the rocky hardbottoms are veritable oases 

 covered with algal meadows, sponges, soft whip corals, 



tropical fishes and territorial and predatory animals. 

 These habitats provide shelter and food to sustain valuable 

 commercial and recreational fish such as grouper and snapper, 

 worth millions of dollars to the state's economy. 



eyed butterfly fish and the blue 

 damsel fish — are present. The main 

 difference is that coral reefs are alive 

 and accreting; hardbottoms are 

 literally crumbling down, eroded by 

 the action of boring organisms. 



Swim over much of Onslow Bay, 

 and the seafloor resembles a desert 

 — as far as the eye can see. But 

 brush aside an inch or more of sand 

 and you'll discover a hard rock floor 

 beneath. Almost 90 percent of 

 Onslow Bay is rock bottom, criss- 

 crossed at wide intervals with the 

 channels of ancient rivers that carved 

 the mesas thousands of years ago. 

 This part of the continental shelf lay 

 exposed during periods of low sea 



surface area for increased habitat. 



"It's nothing to have rubble 

 blocks 5 to 10 meters in diameter 

 broken off out there in front — 

 sometimes 100 meters out in front of 

 a rock scarp," says Riggs. 



The nooks and crannies in and 

 around the rubble become hiding 

 places for many reef fish and inverte- 

 brates such as arrow crabs and spiny 

 lobsters; seaweeds such as brown 

 sargassum or green calcareous algae 

 attach to the rock surfaces, and 

 boring animals gnaw into the 

 substrate. And with the right combi- 

 nation of crevices, ledges and 

 overhangs come the grouper, black 

 sea bass and other large predators, 



says Riggs. In the 1960s, before 

 every fisherman and his brother had 

 learned to scout out the high-profile 

 scarps, Riggs commonly spotted 

 packs of "freight-train" grouper with 

 girths large enough to wrap your 

 arms around. 



"Without (the bioerosion) you 

 just get a vertical wall; you don't get 

 any grouper," says Riggs. "But if you 

 get the right shape of overhang on 

 that complex, which is dependent 

 upon the geology of the different 

 hardbottom beds, you get the grouper 

 ledges. If you want to think in terms 

 of farming out there or habitat 

 construction or modification, you 

 have to understand these kinds of 

 processes first." 



High-relief scarps — which rise 

 from 10 to as much as 25 feet above 

 the seafloor — are just one type of 

 hardbottom community. Any place a 

 rock substrate can get as little as 6 

 inches relief above the abrasive, 

 mobile bottom sands, some organ- 

 isms can attach and prosper; but the 

 higher the relief, the greater the 

 productivity. Only about 10 percent 

 of the floor of Onslow Bay is 

 exposed reef rock, says Riggs. 



"Most of it is sand with abundant 

 worms, some burrowing clams and 

 vagrant animals such as starfish and 

 sea cucumbers," he says. "The only 

 place where it looks like a forest ... is 

 where you get some topography and 



The research team boards the Johnson 

 Sea-Link submersible. 



4 MARCH/ APRIL 1993 



