Shrimp in SC
Shrimp are America’s most valuable and probably most popular seafood. Whole cultures and maritime communities are based solely on these crustaceans. South Carolina has two important penaeid shrimp species, brown shrimp (Farfantepenaeus aztecus) and white shrimp (Litopenaeus setiferus). A third species, the pink shrimp (Farfantepenaeus duorarum), is relatively scarce. Methods of harvest range from large commercial shrimp trawlers to cast nets and drop nets.
Some long-time residents of the Lowcountry of South Carolina may state that they prefer the flavor of one species over another, but taste tests would probably show that few people can distinguish one species from another by taste alone. Some experts claim that white shrimp taste better than the other species, but the difference is subtle.
On the East Coast of the United States, the edible shrimp species found in South Carolina occur in Chesapeake Bay and farther north, but the northernmost shrimp fishery is in Pamlico Sound, North Carolina. Other small shrimps are very abundant in South Carolina’s waters but are of no commercial or recreational value. Among the most common of these are grass shrimp, or hardbacks, which are often seen jumping in tidal creeks and easily confused with the small juveniles of the food shrimps. A species called the rock shrimp occurs in South Carolina’s offshore waters. Rock shrimp, so named because of their thick, hard shells, grow to a relatively large size, reaching five inches in total length. The species occasionally supports a large commercial fishery off Florida’s East Coast, but since they usually occur in 15 to 30 fathoms (90 to 180 ft.), they are of no recreational value. An inshore crustacean called the mantis shrimp (it’s not a true shrimp) is frequently identified incorrectly as the rock shrimp. The mantis shrimp, or stomatopod, is a flattened crustacean that has front legs modified into long, stabbing appendages. Though mantis shrimps may be eaten, the meat yield is small and of rather poor quality.
All three of the edible shrimp species look very similar but can be distinguished by careful examination of tail colors and subtle body features. Placed side by side, fresh white shrimp appear lighter in body color than brown or pink shrimps, thus the origin of the common names. Tail flippers (uropods) on white shrimp are typically black near the base with bright yellow and green margins, while brown shrimp tails have red, dark green and occasionally light blue pigmentations. Pink shrimp almost always have an azure color on the tail and they usually have a dark red spot on the side of the abdomen.
Brown and pink shrimps 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).
All penaeid shrimps have much the same life cycle. Spawning usually occurs in the ocean 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.
During mating, the male transfers a packet of sperm, called a spermatophore, to the female. Brown and pink shrimps mate when the female’s exoskeleton is still soft immediately after molting. The spermatophore is covered over by two “plates” that hold it in place. Spawning in the species may take place days later. Mating of white shrimp occurs between two “intermolt” (hard exoskeleton) individuals. The spermatophore is glued to the underside of the females and spawning occurs almost immediately. Eggs, which are fertilized as they are ejected past the spermatophore, are thought to sink to the ocean floor and after about 12 to 24 hours hatch into minute larvae that move into the water column.
Large concentrations of spawning brown shrimp have never been observed, although some commercial shrimpers have reported finding dozens and sometimes hundreds of “roe” brown shrimp in catches during October and November, which appears to be the primary spawning period. Some spawning seems to occur during late winter or early spring because of the scattered presence of a few postlarvae in May and June.
The spawning season for white shrimp during spring is obvious by the large catches of mature shrimp by the commercial fleet. The exact timing of the spawning period seems to be set by water temperature during spring, but white shrimp typically spawn during May and early June with a few individuals spawning as late as July and early August.
The initial larval stage, which looks like a tiny mite, is followed by about 10 larval phases, reaching the post-larval stage after about two weeks. Postlarvae look like miniature versions of adult shrimp. White and pink postlarvae are about 1/4 inch and brown postlarvae are about 1/2 inch in length when they enter the coastal inlets. Brown shrimp postlarvae are thought to remain in the ocean bottom sediments during the winter. As ocean temperatures rise in late February and March, these postlarvae apparently become active and ride tidal currents into the estuaries. White shrimp postlarvae move into the estuaries about two weeks following spawning, usually in late May and June. The transport mechanisms that carry postlarvae into the estuaries are not fully understood, but it seems that the postlarvae will become active and move up into the water column during flood tide and settle to the bottom during ebb tide. Thus, this behavior moves postlarvae farther inshore every tidal cycle until they settle out in the nutrient-rich tidal creeks and marshes.
Natural mortality rates are extremely high for larval and juvenile shrimp. Probably less than one or two percent of the eggs spawned will survive to be adult shrimp.
Post larval shrimp seem to settle out in the shallow waters in the upper ends of saltmarsh tidal creeks. Shrimp will remain in this “nursery habitat” about two or three months until they are about four inches in length. During high tide, juveniles move into the marsh grass to feed and escape predators. At low tide, when the water level is below the saltmarsh grass, shrimp concentrate in creek beds. The smallest shrimp remain near the creek bank while larger juveniles tend to be in deeper creek waters. If the water is unusually clear, shrimp will seek the deepest areas available, presumably to avoid predatory birds, fish and crabs. In other regions of the Southeast where tides are very small or nonexistent, juvenile white shrimp are known to remain in the saltmarsh grass until they are ready to move seaward.
Both brown and white shrimp seem to prefer muddy bottom but pink shrimp appear to do best on a sand/shell bottom. The predominance of mud bottoms in South Carolina’s estuaries is probably why pink shrimp, juveniles and adults, are scarce, although their postlarvae are relatively common.
Growth is very rapid while in the nursery habitat, up to 2 to 2 1/2 inches per month. Shrimp, like all other arthropods, must molt (shed the old exoskeleton) in order to grow. Small shrimp may molt several times per week, but as they become larger, time between molts becomes greater. Ideal nursery habitat has brackish water that is about 25 to 40 percent sea water for white shrimp and 35 to 65 percent sea water for brown shrimp. Shrimp have been known to do well, however, in water that was near 100 percent sea water (such as in Murrell’s Inlet) or in 10 percent sea water (such as in the Cooper River near Charleston).
Of the shrimp that reach an adult size, most die before they’re eight or nine months old. The size record for white shrimp (just over ten inches) was caught by a commercial shrimper off Seabrook Island in July 1979. This individual had probably been spawned in May the previous year making it 14 months old. It’s doubtful that any shrimp live longer than two years.
Stomach contents of shrimp are difficult to identify because the food is torn and shredded by the mouth parts. Shrimp are generally termed bottom-feeding omnivores, meaning that they will eat most organic materials - animal or plant - that they encounter at the bottom. One study suggested that small shrimp, less than about two inches in length, indiscriminately ingest the top sediment layer. Shrimp 2 to 2 1/2 inches select the organic portion of the sediment and larger shrimp become more active predators, feeding on small animals. Animals commonly listed as food are polychaete worms, amphipods, nematodes, crustacean larvae, isopods, copepods, small fishes, grass shrimp, fiddler crabs and square-back crabs. Shrimp are also known to be cannibalistic. One researcher speculated that bacteria are the main living component in the food of shrimp. Shrimp are thought to scavenge dead animals that may be leftovers from other predators. The effectiveness of bait, particularly oily fish, indicates that shrimp are able to home in on odors.
Several diseases are known to affect shrimp. One of the most common is called cotton disease, which is caused by a single cell protozoan parasite called a microsporidian. These tiny animals invade various tissues of the shrimp and can cause the affected areas to appear grey or white. The affected areas may be confined only to the head or the reproductive system. However, the disease is often in the entire musculature resulting in what is commonly called the cotton condition. Cotton disease has been noted in white and brown shrimp but seems to be most common in white shrimp. It is also usually limited to larger individuals, but small shrimp have also become infected. At times the infection rate for severe cases may reach 10 or 15 percent. Minor infection, with 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 what appears to be dark blue or black bands across the abdomen. This seems to be an artifact of the disease.
Shrimp with cotton disease are not thought to be harmful if eaten by humans. Because the texture and possibly the flavor of a diseased shrimp is impaired, it is probably best to discard these shrimp.
A second condition that has been common along the Atlantic Coast, beginning in about 1999, is called black gill or brown gill disease. This disease is also caused by a single celled protozoan known as an apostome. The swarming stage of this parasite, called a tomite, apparently attaches and penetrates the gills in the trophont stage. The shrimp gill responds by turning brown or black, becoming darker with greater infection. The condition persists in the shrimp until it molts and casts off its old shell, which includes the outer covering of the gill. Black gill disease usually shows up in mid August, peaks in September and slowly wanes in October. The disease apparently does not directly cause mortality, although laboratory studies have confirmed that it impairs respiration thus reducing the shrimp’s endurance. This may, in turn, make the shrimp more vulnerable to predators or temperature extremes. The parasite is not a danger to humans but may cause shrimp to appear less appealing.
In the 1980s massive die offs of shrimp in mariculture farm ponds around the world led to new understanding about the impacts of pathogenic viruses on shrimp. It is now known that viruses are relatively common and can cause mortalities. These problems, however, seem to be most acute when they occur in intensive shrimp farming operations. Shrimp viruses pose no danger to humans and no evidence indicates that the wild stocks of South Carolina have been negatively affected by viruses. However, DNR requires that any imported live shrimp must be certified as free of diseases of concern.
Shrimp have three primary modes of locomotion. While feeding or resting on the bottom, shrimp will use their walking legs (periopods) for moving short distances. While migrating long distances, shrimp will use their swimming legs (pleopods). These appendages are located under the abdomen and beat in unison as the shrimp swims. Studies using tags suggest that shrimp may be able to swim two to five miles in a day. The third form of movement is the tail flex. This is a rapid contraction of the strong abdominal muscles that results in a powerful and rapid snap to the tail propelling the shrimp backwards. White shrimp commonly use this method to jump from the water. The tail snap or flick is a defensive mechanism allowing a shrimp to quickly evade predators.
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 in South Carolina is dominated by shrimp trawlers, which may range in length from 17 to 85 feet. The larger boats are the most recognizable and account for most of the shrimp caught in the fishery. Trawling is allowed only in the ocean, except for limited periods during fall when trawlers may work in the lower areas of Winyah and North Santee Bays. Most shrimpers work within three or four miles of the beach.
The commercial shrimp trawling fishery has three basic seasons. The first is the so-called roe shrimp season in May or June.This season is opened when management biologists determine that an adequate supply of eggs has been spawned. The roe shrimp season is usually less than a month in duration and landings are dependent upon the severity of the previous winter. Following mild winters, heads-off landings are often 400 to 600 thousand pounds. Following 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 when stock abundance was very high. Good years for brown shrimp have landings of 1.3 to 2.0 million pounds (heads off).
The fall white shrimp season is typically the largest except in years following severe winters. These shrimp are the offspring of the spring spawn. Landings of youngof- the-year white shrimp by the commercial fleet usually begin in August and peak in September and October. The season usually lasts through December and into January in some years.
A seasonal fishery called the channel net or set net fishery occurs in Winyah and North Santee bays. This limited fishery usually begins in September and may be extended 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. The nets are placed perpendicular to the tidal current and shrimp are captured as the tidal current carries them seaward. In some years, this can be a very effective fishery, with relatively high catch rates at low operating costs. By-catch of fish is usually relatively low in this fishery.
The recreational harvest of brown shrimp by cast nets and seines usually starts in early June in the state’s tidal creeks. White shrimp are first caught in the creeks in late July or early August and have usually disappeared 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. Harvesting by drop nets from docks and seawalls is most popular during the fall as larger white shrimp are moving seaward.
Seining for shrimp can be expensive, but used properly, seines are very effective. Seines, may not exceed 40 feet in length and webbing must be 1/2- inch square mesh (1-inch stretch) or larger for nylon nets. For cotton nets, the minimum mesh size is 9/16-inch square. Law also forbids the blockage of more than one-half the width of any slough, creek or other waterway on any tidal stage. Seines with or without tailbags must be pulled by hand and cannot be pulled by any enginepowered boat or staked to poles. Pulling a seine by boat or staking is interpreted as using commercial methods in a sanctuary and can result in severe fines including confiscation of boats and fishing gear.
The most popular and effective way to pull a seine is to pull with the falling tide. Pulling against the tide is very difficult and will not cover much bottom. Although brown and white shrimp are most common in mud-bottom areas, it is often impractical or impossible to seine in such areas. Sand/shell bottoms are the most popular areas for seining and several productive areas of this type exist along the coast. An ideal seining area contains suitable banks or sandbars for pulling nets ashore to remove shrimp.
Cast nets are used more commonly than seines. Cast nets used over bait must have a mesh size of 1/2-inch bar mesh (oneinch stretch), but nets used without bait have no restrictions. A study found that a 1/2-inch mesh net was just as effective in capturing usable shrimp as a 3/8-inch mesh net. The smaller mesh net was found to retain a large number of shrimp considered too small for food. A 3/8-inch mesh net works well for capturing shrimp to be used as fish bait. Casting seems to be most effective in creeks with mud bottom during low tide. Deeper areas, over 3 to 4 feet, usually produce larger shrimp during daylight since shrimp avoid light.
Most cast nets are made of nylon or monofilament. The mono nets are more effective than the nylon nets but are more susceptible to damage from oyster shells. One may use a cast net from a boat, creek bank, pier or even while wading a creek.
From time to time, shrimp populations have relatively dramatic fluctuations in size. This has been reflected in annual commercial shrimp landings which have ranged from 1.5 to 5.7 million pounds. We’ll probably never be able to totally explain the fluctuations in shrimp stocks or those of any marine fishery, but we have determined a few of the most important relationships.
White shrimp abundance fluctuates more than that of brown shrimp. The primary cause of these large fluctuations is the occasional near-total loss of spawning stocks. The white shrimp is a subtropical species and, being such, is susceptible to cold temperatures. During late fall, larger white shrimp that aren’t caught by recreational or commercial fishermen migrate south as far as Cape Canaveral, Florida. This has been repeatedly documented by tagging studies. Unfortunately, most of these shrimp are caught before they have an opportunity to return north the next spring (assuming they would if allowed). Therefore, we in South Carolina are dependent upon the small white shrimp that overwinter in our estuaries to be our primary spawning stock. During winters in which water temperature falls to 46˚F or below for seven or more days, most of the overwintering brood stock are wiped out. In some years, cold-related mortalities have been noted as far south as the Georgia-Florida border. Following cold kills, the roe shrimp harvest is usually less than 50,000 pounds and often zero. Fall commercial landings also suffer, being less than 20 percent of the long-term average.
If an adequate number of spawners is present, the next most important factor for white shrimp abundance seems to be water salinity in the nursery habitat in August and perhaps July. Low landings seem to be related to unusually dry summers resulting in higher than average salinity values. However, unusually wet summers can be detrimental also. Moderate rainfall and river discharge appear to create ideal conditions for white shrimp in most of the state’s coastal marshes.
The number of spawners does not seem to be a problem with brown shrimp since the immigration of postlarvae every year seems to be relatively constant. The best years for brown shrimp seem to be those with relatively mild spring temperatures that allow brown shrimp to begin growing soon after moving into the nursery habitat. Brown shrimp are thought to grow and survive best in water salinities slightly higher than half strength seawater. Unusually wet spring and early summer weather probably has detrimental effects on brown shrimp.
Many other factors may affect abundance of both species. These may include abundance of predators or food availability. Some fish species and blue crabs are known to feed heavily upon shrimp. Availability of healthy habitat and clean water are also thought to be 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 investigations into the culture of marine shrimp. Some of the first studies in the United States were conducted in the 1950s at the old Bears Bluff laboratories. Today, researchers at the Waddell Mariculture Center near Bluffton are providing technical assistance to several companies that are growing shrimp in ponds in the state. The preferred species for culture is the Pacific Coast white shrimp, Penaeus vannamei. This species grows faster than the local shrimp species, but creation of a local hatchery could entice farmers to use the native species.
As more and more people move to the coast, pressure on our shrimp resource will continue to increase. To ensure that citizens, both commercial and recreational, 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 shrimp is 48 quarts (heads-on) or 29 quarts (heads-off) per boat or seining party.
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, 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.