Shrimp In SC
SHRIMP IN SC
Shrimp are America’s most valuable and most popular seafood. South Carolina is home to three species of the penaeid shrimp: brown shrimp (Farfantepenaeus aztecus), white shrimp (Litopenaeus setiferus), and pink shrimp (Litopenaeus duorarum). Brown and white shrimp are more common than pink shrimp. All three taste the same.
In South Carolina waters, other small species of shrimp, such as grass shrimp, are easily confused with juveniles of penaeid shrimp but have no commercial or recreational value. Rock shrimp, named for their thick, hard shells, occur in South Carolina’s offshore waters and are commercially fished off Florida’s East Coast. The mantis shrimp (not a true shrimp) is a flattened, inshore crustacean sometimes incorrectly called “rock shrimp.” Mantis shrimp can be eaten, but have little meat of poor quality.
IDENTIFICATION OF SHRIMP SPECIES
All three of the edible shrimp species look very similar, but you can tell them apart if you look closely. White shrimp has a lighter color than brown or pink shrimp. Tail flippers (uropods) on white shrimp are black near the base with bright yellow and green along the edges. Brown shrimp tails are red, dark green, and occasionally light blue in color. Pink shrimp have a light purplish-blue tail, and usually a dark red spot on the side of the abdomen. Brown and pink shrimp have grooves along the upper midline of the head and the upper midline of the lower region of the abdomen. The grooves on pink shrimp are slightly narrower than those of brown shrimp. White shrimp do not have grooves and typically have much longer antennae and a long rostrum (horn).
Brown, white, and pink shrimps have similar life cycles. Spawning usually occurs in the ocean, ranging from near the beaches to several miles offshore. A single female produces between 500,000 and 1,000,000 eggs and may spawn several times. Brown shrimp spawn during October and November, and occasionally later. White shrimp typically spawn in the spring and early summer. The exact timing of the spawning period depends on water temperature.
During mating, the male transfers a packet of sperm, called a spermatophore, to the female. Brown and pink shrimp mate when the female’s exoskeleton is soft, immediately after molting. The spermatophore is covered by two “plates” that hold it in place. Females may spawn days later. White shrimp mate between molts when the exoskeleton is hard. The spermatophore is glued to the underside of the female, and spawning occurs almost immediately. Eggs of both species are fertilized as they are ejected past the spermatophore, and sink to the ocean floor. After about 12 to 24 hours, they hatch into tiny larvae that rise into the water column.
The initial larval stage, during which the shrimp looks like a tiny mite, is followed by about 10 larval phases before reaching the post-larval stage after about two weeks. Postlarvae look like miniature adult shrimp. Brown shrimp postlarvae remain in the ocean bottom sediments during the winter. As the ocean warms in late February and March, these postlarvae become active and ride tidal currents into the estuaries. White shrimp postlarvae move into the estuaries about two weeks after spawning, usually in late May and June, moving further in with each high tide. Death rates are very high for larval and juvenile shrimp; less than one or two percent of the eggs spawned survive to adults.
Postlarval shrimp settle out in the shallow waters in the upper ends of salt marsh tidal creeks. Shrimp stay in this “nursery habitat” for about two or three months, growing to about four inches long. During high tide, juveniles move into the marsh grass to feed and escape predators. At low tide, when the water level is below the salt marsh grass, shrimp gather in creek beds. The smallest shrimp stay close to the creek bank while larger juveniles prefer deeper water. In unusually clear water, shrimp seek the deepest areas available to avoid predatory birds, fish, and crabs.
Both brown and white shrimp prefer muddy bottom, but pink shrimp do best on a sand/shell bottom. In South Carolina’s mostly muddy estuaries juvenile and adult pink shrimp are scarce, although their post larvae are relatively common.
Young shrimp grow quickly, up to 2.5 inches per month, molting their exoskeleton as they grow. Small shrimp molt several times per week, but molting slows as they become larger. Shrimp can tolerate a wide range of salinities. The ideal nursery habitat has salinity about 25 to 40 percent seawater for white shrimp and 35 to 65 percent seawater for brown shrimp. Shrimp can do well, however, in salinities near 100 percent seawater (such as in Murrell’s Inlet) or in 10 percent seawater (such as the Cooper River near Charleston).
Shrimp move three different ways, using either their walking legs, swimming legs, or with a sudden tail snap. While shrimp can walk short distances, when migrating long distances, they swim as much as two to five miles a day. To escape predators, a shrimp contracts its abdominal muscles, which causes the tail to snap, and propels the shrimp backwards. White shrimp commonly use this method to jump from the water.
As shrimp become larger, they leave the nursery area and move toward the ocean on the outgoing tide, particularly at night. Shrimp move from the shallow estuary creeks into coastal rivers when they are about four inches long. They continue to grow as they move into the lower reaches of sounds, bays and river mouths where they gather just before moving into the ocean.
When white shrimp are in the staging areas, they feed in nearby shallow areas at night. Brown shrimp, on the other hand, prefer to stay in deeper waters at night. In years when shrimp are abundant, they migrate into the ocean when they are between four and five inches long. When the population is smaller, however, shrimp may be six inches or more before they leave the estuaries. When shrimp are more concentrated in the tidal creeks, growth rates slow due to competition for limited food, or each shrimp spending more time protecting its space instead of feeding. Low salinities due to heavy rainfall cause juvenile shrimp to leave nursery areas early, reducing growth and survival.
In a wet year, the majority of white shrimp move into the ocean in August, about a month early. This can result in a poor shrimp baiting season and a poor harvest by commercial trawlers in October, which is normally one of the better months for shrimping. Areas most severely affected are Charleston Harbor and Winyah Bay, which receive relatively large amounts of upstate river discharge.
If there is not significant rainfall and/or river discharge during fall, white shrimp remain in the estuaries until water temperature falls to about 60-65 degrees. Migration into the ocean occurs during the large tides associated with new and full moons.
Shrimp seldom live more than eight or nine months. The record white shrimp (just over ten inches) was caught by a commercial shrimper off Seabrook Island in July 1979. That shrimp was probably about 14 months old.
Shrimp are bottom-feeding omnivores, eating most organic materials – animal or plant – they encounter at the bottom. Smaller shrimp pick food off the sediment while larger shrimp become predators, feeding on polychaete worms, amphipods, nematodes, crustacean larvae, isopods, copepods, small fishes, grass shrimp, fiddler crabs, and squareback crabs. Shrimp also eat other shrimp.
Several diseases affect shrimp. One of the most common, cotton disease, is caused by a single cell protozoan parasite called a microspordian. These tiny animals invade various tissues of the shrimp, turning the affected areas grey or white. Cotton disease may affect only the head or the reproductive system but often spreads throughout the shrimp’s entire muscular system. Cotton disease is most common in large white shrimp, but can affect small white shrimp, as well as other species. At times, up to 15 percent of the population of white shrimp has cotton disease. Minor infection (white shrimp having small specks of diseased tissue) may reach levels of 70 or 80 percent, but these specks are usually isolated to the head. Large individuals with cotton disease often have a dark blue or black band across the abdomen. Shrimp with cotton disease are not harmful if eaten, but the disease affects the texture and flavor of the meat.
A second condition common along the Atlantic coast is called black gill or brown gill disease, first seen in 1999. This disease is caused by another single celled protozoan known as apostome.
The swarming stage of this parasite, called a tomite, apparently attaches to and penetrates the shrimp’s gills, turning them brown or black. The shrimp is affected until it molts and casts off its old shell, which includes the outer covering of the gill.
Black gill disease usually begins in mid August, peaks in September, and slowly disappears in October. The disease does not directly cause mortality, but impairs respiration, making the shrimp more vulnerable to predators and temperature extremes. The parasite is not a danger to humans.
In the 1980s massive die offs of shrimp in mariculture farm ponds around the world led to new understanding of the impacts of viruses on shrimp. These viruses are relatively common and can cause mortalities in intensive shrimp farming operations. Shrimp viruses pose no danger to humans and have not negatively affected wild shrimp in South Carolina. However, DNR requires that live shrimp imported for shrimp farming be certified as free of diseases.
As shrimp become larger, they leave the brackish waters and move gradually toward the higher salinity waters of the ocean. Most shrimp probably leave the marsh creeks during ebb tides and this may be more pronounced at night. Shrimp usually begin moving into coastal rivers when they reach about 4 inches in length. Further growth occurs in the rivers until the shrimp are ready to move into the lower reaches of sounds, bays and river mouths. These lower reaches, termed staging areas by some biologists, serve to accumulate shrimp just prior to dispersal into the ocean. When white shrimp are in the staging areas, many will move into the shallow peripheral areas to feed at night. Brown shrimp do not appear to do this to the extent of white shrimp, preferring to remain in deeper waters at night. In years when shrimp are very abundant, they may migrate into the ocean at a size of about 4 to 5 inches in length. When not abundant, however, average size of shrimp may be 6 inches or more before they leave the estuaries. The difference in size between the years of high stock abundance and low abundance seems to be related to what is called densitydependent growth. When large concentrations of shrimp are in the tidal creeks, growth rates are reduced. This may be caused by competition for limited food resources or each shrimp may be spending more time protecting its space instead of feeding. Heavy rainfall, resulting in very low salinities, can force juvenile shrimp from nursery areas. When forced into the inhospitable open-water areas, growth and survival rates are poorer because of less available food and suitable habitat.
Extreme environmental conditions such as droughts or unusually warm fall weather may result in delaying emigration of white shrimp into the ocean. Tagged white shrimp released into coastal waters of South Carolina in September have been observed to remain in the estuaries for two months or more before moving seaward. Heavy rainfall or river discharge along with the accompanying drops in water salinity (salt content of the water) have been known to cause shrimp to move into the ocean prematurely.
In a wet year, the majority of the white shrimp may move into the ocean in August, about a month ahead of normal. The result would be a poor shrimp baiting season and poor harvest by commercial trawlers in October, normally one of the better months for shrimping. The areas typically most severely affected are Charleston Harbor and Winyah Bay, which receive relatively large amounts of upstate river discharge.
Without significant rainfall and/or river discharge during fall, white shrimp appear to remain in the estuaries until water temperature falls to about 60-65˚F and then migration seems to occur primarily during the large tides associated with new and full moons. Some experienced shrimpers claim that the condition known as red legs is indicative of the onset of migration. This phenomenon has not been investigated scientifically.
THE COMMERCIAL FISHERY
The commercial shrimp trawling fishery has three seasons. The first is the so-called roe shrimp season in May or June. This season is opened when the DNR determines that an adequate supply of eggs has been spawned to produce a good fall harvest. The roe shrimp season is usually less than a month long and landings (catches) are dependent upon the severity of the previous winter. Following mild winters, heads-off landings are often 400 to 600 thousand pounds. After severe winters, landings of roe shrimp are usually less than 50 thousand pounds and often zero.
The second season is for brown shrimp. This fishery usually begins in June and ends in August, although significant quantities of brown shrimp have been landed in October during years when the population of brown shrimp was high. Good years for brown shrimp have landings of 1.3 to 2.0 million pounds (heads off).
The fall white shrimp season typically produces the largest catch. These shrimp are the offspring of the spring spawn. Landings of young white shrimp by the commercial fleet usually begin in August and peak in September and October. The season usually lasts through December and sometimes into January.
The channel net or set net fishery occurs in Winyah and North Santee bays. This limited fishery usually begins in September and continues until December 15 if shrimp size and abundance are adequate for commercial harvest. This fishery involves the use of anchored nets that are very similar to shrimp trawls. They are held open at the mouth by long wooden poles and capture shrimp as the tide carries them seaward. In some years, this can be a very effective fishery, with relatively high catch rates at low operating costs.
THE RECREATIONAL FISHERY
The recreational harvest of brown shrimp by cast nets and seines takes place in the state’s tidal creeks, usually starting in early June. White shrimp are first caught in the creeks in late July or early August and have usually moved into the ocean by late October. The shrimp baiting fishery, which targets white shrimp, is set by law to last 60 days and opens at noon on the last Friday on or before September 15. Shrimp are also harvested recreationally by drop nets from docks and seawalls during the fall as larger white shrimp are moving seaward. Unlike seines and cast nets, drop nets require bait. See DNR regulations for the types of nets that can be used for recreational shrimping.
CHANGES IN THE SHRIMP POPULATION SIZE
Shrimp populations experience relatively dramatic fluctuations. Annual commercial shrimp landings have ranged from 1.3 to 6.8 million pounds. White shrimp population, more weather dependent, fluctuates more than the brown shrimp population. During late fall, white shrimp not caught by recreational or commercial fishermen migrate south as far as Cape Canaveral, Florida, and do not return. Therefore, South Carolina depends upon the small white shrimp that overwinter in our estuaries to be our primary spring spawning stock. When winter water temperature falls to 46 degrees or below for seven or more days, most of the overwintering brood stock is wiped out. Following severe winters, the roe shrimp harvest is usually less than 50,000 pounds, and with so few spawners, fall commercial landings also suffer.
Another important factor for the white shrimp abundance is salinity in the nursery habitat during the late summer months. Dry summers, which result in higher salinities, produce smaller white shrimp populations. However, unusually wet summers can also impact white shrimp. Moderate rainfall and normal levels discharge of freshwater from rivers seems to create ideal conditions for white shrimp.
The number of spawners does not seem to be a problem with brown shrimp since the inshore movement of post larvae every year remains relatively constant. The best years for brown shrimp are those with relatively mild springs that allow brown shrimp to begin growing soon after moving into the nursery habitat. Brown shrimp grow and survive best in salinities slightly higher than half strength seawater. Unusually wet spring and early summer weather has detrimental effects on brown shrimp.
Many other factors such as abundance of predators and food availability affect both species. Good habitat and clean water are also important for good shrimp production. Unfavorable winds could transport larvae away from the coast, or heavy predation on larvae by a concentration of jellyfish, for example, could have serious effects.
Scientists in South Carolina are among the world’s leaders in developing techniques to culture marine shrimp. Some of the first studies in the United States were conducted in the 1950s at the old Bears Bluff laboratories on Wadmalaw Island. Today, researchers at the Waddell Mariculture Center near Bluffton provide technical assistance to several companies growing shrimp in ponds in the state. The preferred species for culture is the Pacific coast white shrimp, Penaeus vannamei which grows faster than local shrimp species.
As more and more people move to the coast, pressure on the shrimp resource will continue to increase. To ensure that both commercial and recreational fishermen will continue to have an ample supply of shrimp, everyone must take steps to conserve the resource. It is important that no shrimp be wasted. Shrimpers should utilize all shrimp that they catch, and if shrimp are too small, larger mesh nets should be used or shrimping activities should be postponed until shrimp grow to a useable size. Those individuals who catch more than their fair share of the resource not only violate the law but may force fishery managers to create tighter restrictions for all users. The legal daily limit for recreational shrimpers is 48 quarts (heads-on) or 29 quarts (heads-off) per boat or seining party.
Identifying South Carolina Shrimp
This publication was made possible in part with funds from the sale of the South Carolina Saltwater Recreational Fishing License and the U.S. Fish and Wildlife Service Sportfish Restoration Fund. The South Carolina Department of Natural Resources publishes an annual Rules and Regulations booklet that lists all saltwater fishing regulations. Have an enjoyable fishing trip by reading these requirements before you fish.
Author credentials: J. David Whitaker, Peter Kingsley-Smith of the Marine Resources Division
The above information on shrimp is available in a brochure, please download the Sea Science – Shrimp information pamphlet which is in the Adobe PDF file format. Adobe® Reader® is required to open the files and is available as a free download from the Adobe® Web site.