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Pre-Visit Activities : Reefs Background MAIN | OBJECTIVES | STANDARDS | BACKGROUND | PROCEDURES | ASSESSMENT | RESOURCES
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Hard bottom reefs are areas of rock and ancient coral outcrops; places where the underlying rock sticks up through the sand. This rock can stick up above the surface only a few inches or as high as several feet. The hard surfaces provide structure to which sessile organisms, such as sponges and sea whips, attach themselves. Sessile organisms are animals that attach to a surface and remain there their entire lives. These sessile organisms cover hard bottom areas. Because of this luxuriant animal growth, these reefs are also known as live bottom reefs. In South Carolina, approximately 20% of the bottom area is made up of hard bottom reefs, the rest being a sandy bottom. Natural live bottoms can be classified into three zones according to their location on the continental shelf. The zone closest to the shore is known locally as the "Blackfish Banks". They are found in depths of 49 to 98 feet of water. Generally the outcrop in this area affords less than three to four feet of relief from the surrounding sea floor. Relief is the geologic term for difference in elevation, so the top of these livebottom reefs is only three or four feet higher than the surrounding sand. As the name implies, the blackfish or black sea bass (Centropristis striata) is the most abundant fish species found in this zone of live bottom habitat. In addition to black sea bass, this area is also home to sharks, sheepshead (Archosargus porbatocephalus) tripletail (Lobotes surinamensis), greater amberjack (Seriola dumerili), Atlantic spadefish (Chaetodipterus faber), great barracuda (Sphyraena barracuda), and king mackerel (Scomberomorus cavalla). The invertebrates found attached in this area include worm tubes, sponges, barnacles and soft corals. Because they are in relatively shallow water, the Blackfish Banks are more susceptible to sudden change than the other live bottom zones. When these areas are heated in the summer and cooled in the winter the water is more likely to undergo temperature changes. Deeper water, because of its proximity to the warm waters of the Gulf Stream, can maintain a consistent temperature throughout the year. Blackfish Banks are also vulnerable to storms and hurricanes. Powerful storms stir up the bottom in these areas and can bury these live bottom reefs in sand, as well as all the animals attached to them. For this reason it unusual to see large sessile invertebrates in these reefs. The Blackfish Banks have been fished commercially since the 19th Century and have been seriously depleted by over-fishing in recent years. The second live bottom zone, the "Snapper Banks" occurs in depths of 82 to 180 feet of water. The outcrops of this zone have relief up to eight feet above the ocean floor. This live bottom habitat features sessile invertebrates such as soft corals, seafans, sponges, barnacles and seawhips. Here, too, one fish species, the red porgy (Pagrus sedecim), not a true snapper, dominates over all others. Red porgies constitute more than 70 percent of the fish on this area, but other bottom species such as Nassau grouper (Epinephelus striatus), gag grouper (Mycteroperca microlepis), speckled hind grouper (Epinephelus drummondhayi), red snapper (Lutjanus campechanus) and vermilion snapper (Rhomboplites aurorubens) begin to appear. In warm months, highly migratory species such as greater amberjack, jack crevalle (Caranx hippos), sharks, king mackerel and Spanish mackerel (Scomberomorus maculatus) move into this habitat from Florida and Caribbean waters. This zone is covered by the Gulf Stream. Since the warm water of the Gulf Stream remains constant, without the dramatic seasonal changes of inshore waters, Caribbean organisms such as those common in Florida now exist farther north than expected. Such exotic species as yellow edge grouper (Epinephelus flavolimbatus), snowy groupers (Epinephelus niveatus), hogfish (Lachnolaimus maximus) and the longfin scorpionfish (Scorpaena agassizi) inhabit this deep live bottom. Fish species populations that utilize live bottoms are not always permanent, and will change seasonally as water temperatures vary. This is particularly true of the shallow water live bottoms where the low water temperatures that occur in winter can stress fish. The turbulence of winter storms can also strip a live bottom of much of its invertebrate life leaving less food for fish. Increased water depth decreases storm stress, and reduces the variability in water temperature. Near the Gulf Stream, the water temperature remains almost constant throughout the year resulting in increased diversity and less change in species composition. The movement of highly migratory fish species, particularly the higher predators, also seasonally affects live bottoms near shore. As the days become longer and the water warms, sharks move closer inshore and fish such as jack crevalle, cobia (Rachycentron canadum), Spanish and king mackerel, amberjack, and barracuda concentrate around live bottoms where prey abounds. When the water starts to cool in the late fall, the migratory fish move south, and fish formally living in nearshore waters then move farther offshore to occupy the live bottom. Bluefish (Pomatomus saltatrix), sheepshead, black drum (Pogonias cromis), red drum (Sciaenops ocellata), flounder (Paralichthys spp.) and spotted seatrout (Cynoscion nebulosus) find cover and food in water warmer than the beach and estuarine areas they inhabit during the rest of the year. Some reef fish are dependant on the health of the estuaries for their survival. When these fish spawn, the current and flood tides carry the larval fish inshore to the estuaries and coastal wetlands. Approximately 80% of recreationally and commercially important marine organisms found in South Carolina coastal waters are dependent on estuarine waters at least part of its life cycle. This occurs because estuaries are prime nurseries that offer larval and juvenile fish food, shelter and protection from predators. Estuaries are integrally tied to inland watersheds, as they are often the last stop for these watersheds before they discharge into the ocean. For this reason some estuaries in South Carolina are receiving water literally drained from across the state. This means they also can receive pollutants from across the state as well. For example, a chemical spill in Spartanburg can have a negative effect on the salt marshes around the Santee Delta as the pollutants may be carried downstream through the watershed to this area. This in turn will have a negative effect on the juvenile fish and other populations found in the estuaries. Impacting the juvenile populations will eventually impact the adult populations as not as many of the young fish will reach adulthood to reproduce. A good indicator of water quality contamination in saltwater estuaries is the health of mollusks such as the oyster. Oysters are animals found in estuaries throughout South Carolina. Oysters are filter feeders that pump water through their bodies to strain food from them. In the process of doing this, any contaminants in the water will also be collected in the oyster’s body. Scientists can examine the oyster to determine if the contaminants collected in the oyster’s bodies have reached a dangerous level. This in turn is an indicator of the water quality in the surrounding estuary. Currently in the United States, over 33% of oyster beds have been closed because of dangerous contamination levels. Another way that reef habitats are tied to watersheds is the nutrients the watersheds carry into the ocean. As the streams and rivers flow across the land they pick up minerals and decaying organic material, known as detritus, and carry them to the ocean. This is why the coastal waters of South Carolina are so murky, because they are so abundant in nutrients. Contrary to what tourists think, this murkiness is very beneficial to the ocean ecosystem, because it can support a variety and abundance of life not found in the Caribbean. The nutrients in the water support a large phytoplankton, microscopic plants, population, which in turn support a large zooplankton, microscopic animal, population which in turn support all the rest of the animal populations in the South Carolina ocean waters. Without the nutrients being constantly brought into the ocean by watersheds, there would not be enough food to support the same size populations that can currently be found on hardbottom reef habitats. Spotlight Species – Gag Grouper (Mycteroperca microlepis)
Female gag groupers mature sexually at five to six years of age and spawn in water over 200 feet deep. The eggs and planktonic larvae that hatch are carried inshore to the estuaries by eddies spinning off the Gulf Stream. Juvenile gag move inshore into oyster reefs to feed, primarily on grass shrimp, and mature. Within a few months they will reach lengths of up to twelve inches and will begin to swim offshore to the hardbottom reefs. As with most groupers, gags change sex with age. All gags start off as females. As it requires more energy to produces eggs than sperm, it is necessary to have a larger number of females in the population than males. A handful of male gags can produce enough sperm to fertilize the eggs of a large number of female gags. For this reason, only 5 to 20% of a gag population are males. When a female reaches a size where they are among the largest fish in the population, they will change their sex and become males. In a gag, this change occurs between 10 and 11 years of age. Typical of the grouper family, this fish has a long life expectancy and tends to remain in its preferred habitat, perhaps behaving territorially. This trait makes it vulnerable to heavy fishing pressure. Gag are also vulnerable to over-fishing because they concentrate together when they spawn. When ready to spawn, many gags will migrate to southern Florida in a narrow area of ocean water. They do this consistently every year to spawn during the full moons of February and April. As fishermen are aware of this yearly event, they often take advantage of the fishing opportunities it offers. As large numbers of gag can be taken during a short period of time, this can have a negative effect on the gag populations all over the Southeast. The survival of the gag grouper is in serious danger because of over-fishing. Population size and genetic diversity have been in decline. Gag caught today are smaller in size and mature much sooner than gags caught 20 years ago. To ensure the gag’s survival, new management practices, such as marine reserves where no fish may be caught, may need to be instated. Unfortunately, because of the dependence of fishermen on fishing for their livelihood and because of the long held belief that the fish in the ocean are an unlimited resource, it is hard to gain acceptance for such new practices. |
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When
thinking of ocean reefs, one tends to think of the coral reefs
of places such as the Caribbean, not places such as the offshore
waters of South Carolina, yet many reef habitats can be found
there. The reef habitats of South Carolina, known as hard bottom
reefs, are distinct from coral reef habitats and are home to
a variety of sea life species. This activity will introduce
students to the hard bottom reef habitats and show how they
too are dependent on watersheds.
One
of a dozen grouper species inhabiting tropical and subtropical
American waters, the gag ranges from North Carolina to the Yucatan.
Gag live more than thirty miles offshore in warm waters. The
gag averages two to four pounds although it can reach 50 pounds.
Favored habitat includes rocky live bottoms, generally in water
from 122 to 300 feet deep. The gag feeds mainly on small fish,
particularly grunts, and reef dwelling crustaceans such as crabs,
shrimp, and squid.
