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Common Name:
Brown shrimp; Northern brown shrimp

This species exhibits sexual dimorphism, with females growing larger than males. Generally, males attain only 3/5 of female weight, and 5/6 of female length. Females are further distinguished by the presence of a closed thelycum located on the ventral sternum of the thorax, while males are identified by the presence of the pentasma.

Synonymy:
Penaeus brasiliensis aztecus
Penaeus aztecus aztecus

Other Taxonomic Groupings:
Subphylum: Crustacea
Subclass: Eumalocostraca
Suborder: Dendobranchiata

Potentially Misidentified Species:
P. brasiliensis

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Brown shrimp occur along the Western Atlantic coast from approximately Martha’s Vineyard, MA through Florida and the Gulf of Mexico to the lower Yucatan Peninsula (Williams 1984).

IRL Distribution:
P. aztecus occurs throughout the Indian River Lagoon.

III.  LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Large specimens of P. aztecus males attain an adult size of approximatley195 mm in length, while large female specimens may reach 236 mm in length (Williams 1955). Growth rates for juvenile P. aztecus in North Carolina were estimated by Williams (1955) to be 46 mm per month. In Louisiana, Wilson (1969) estimated growth rates ranging from 45 – 50 mm per month, with overall growth decreasing with age.

Growth slows markedly when brown shrimp reach approximately 100 mm. Beyond this size class, male growth slows in comparison to female growth.

Abundance:
P. aztecus can be abundant throughout its range and commercial catch rates for this species sometimes surpass those of other penaeids. The area around the Bay of Campeche, TX is one area where this species is highly abundant.

Locomotion:
P. aztecus is an active swimmer and burrower.

Reproduction:
P. aztecus becomes reproductive after reaching a size of 140 mm (Renfro 1964). This species is known to have an extended spawning season that is likely to vary in different geographic areas of its range. Gonads mature in August and become opaque white, yellow or tan in color when fully ripe (Broad 1950; Brown and Patlan 1974). Brown shrimp spawn offshore at depths that generally exceed 18 m (Larson et al. 1989)

Data from Burkenroad (1939) showed that sex ratios of P. aztecus change with water depth. In Louisiana waters, the sex ratio nearshore is balanced at approximately 1:1. However, in deeper waters ranging from 90 – 130 m in depth, the sex ratio changes to 1:2 in favor of females. Impregnated females in these waters occurred only beyond 18 m depth. This occurrence pattern is suggestive that spawning sites for P. aztecus are likely to be further offshore in deeper waters than occurs in other Penaeids such as P. dourarum (pink shrimp) and P. setiferus (white shrimp).

Embryology:
Eggs of P. aztecus are demersal and spherical, measuring approximately 0.26 – 0.28 mm in diameter (Pearson 1939 IN: Larson et al. 1989). Hatching occurs with 24 hours. Larvae develop offshore through 5 naupliar, 3 protozoeal, and 3 mysis stages before metamorphosing into postlarvae (Cook and Murphy 1969, 1971). Several postlarval stages occur prior to metamorphosis to the juvenile stage. Larval development takes approximately 11 days at a temperature of 32° C, and approximately 17 days at a temperature of 24° C (Cook and Murphy 1969).

In studies of wild Penaid populations, P. aztecus larvae are most commonly sampled below mid-depth. Protozoea of this species are likely to occur nearest the bottom, while postlarval stages occur at, or slightly above mid-depth. However, all stages ascend to surface waters with the onset of darkness.

Postlarvae return to coastlines on surface current during late winter and spring (Bearden 1961). Recruitment studies in the Carolinas revealed that postlarval P. aztecus began entering sounds in North Carolina from October through May, with peak recruitment in March and April (Zamora and Trent 1968). In South Carolina, P. aztecus postlarvae are found in all months of the year, with recruitment peaking during February and March (Bearden 1961). Juveniles are collected beginning in mid-April and continuing throughout the summer months (Williams 1955). In one Texas study, Copeland and Truitt (1966) observed that spring recruitment peaks were greatest during evening flood tides.

Young P. aztecus postlarvae enter estuaries on flood tides and migrate to shallow, low-salinity waters. Growth is rapid during the warmer summer months. As they grow, they begin migration into progressively more saline waters, eventually returning to offshore areas.

IV.  PHYSICAL TOLERANCES

Temperature:
Under laboratory conditions, larvae of P. aztecus did not complete development when temperatures were sustained below 24 ° C. Nauplii, protozoea and mysis stages survived at higher rates when held at temperatures above 24 ° C (Cook and Murphy 1969). The lethal maximum temperature for postlarval P. aztecus was 36.6 °C. Juveniles have been collected from waters at temperatures as high as 38 °C, though they experience physiological stress at temperatures as low as 10 °C, and at temperatures above 32 °C (Larson et al. 1989).

Burrowing has been studied as a behavioral response to low temperatures (Aldrich et al. 1968). In laboratory studies, P. aztecus was observed to burrow as temperatures fell below the 12 – 17 ° C range, and re-emerge from sediments when temperatures rose above 18 – 21.5° C. A closely related penaeid, P. setiferus (white shrimp) showed no such response to lower temperatures. It was hypothesized from this result that burrowing would have more survival value to P. aztecus than other congeners since P. aztecus is known to reach estuarine habitats earlier in the year than its congeners. P. setiferus, for example, arrives at coastal bays during the summer months.

Salinity:
At salinities below 27 ppt. or above 35 ppt. hatching rates and larval survival were diminished (Cook and Murphy 1969). In early laboratory studies of postlarval growth rates vs. salinity, growth did not differ significantly when salinity ranged between 5 – 40 ppt (Zein-Eldin 1963). However, when temperature was added as a factor, larvae held at these same salinities did show differences in growth rates. Postlarvae were able to survive for one month but did not grow when held at a temperature of 11°C at salinity of 15 ppt or above. Larvae grew at temperatures between 11 – 25 °C, with markedly improved growth rates at the higher end of that range. Later studies (i.e. Zein-Eldin and Griffith 1966) confirmed that optimal growth occurs at temperatures between 22.5 – 30 ° C. The osmoregulatory ability of P. aztecus is compromised in low temperatures, with larvae showing decreased tolerance to low salinity water at temperatures below 15°C.

At salinities under 2 ppt. Venkatamariah et al. (1972) observed disorientation and convulsive movements in brown shrimp, though other investigators have collected brown shrimp in waters where salinity was as low as 0.2 ppt. Williams (1960) observed that at the high end of the salinity scale, brown shrimp are able to continue osmoregulation even at salinity as high as 69 ppt.

Dissolved Oxygen:
Minello and Zimmerman (1989) found that brown shrimp can both detect and avoid hypoxic conditions. Juveniles ranging in length from 36 – 38 mm attempt avoidance when dissolved oxygen (D.O.) levels approach 1.5 – 2 ppm D.O. Lethal dissolved oxygen levels for juveniles occurs at approximately 0.8 ppm D.O.

Parasitic Infestations and Diseases:
Bacterial infections of brown shrimp are predominantly Vibrio, Aeromonas, and Pseudomonas. Ciliates and sporozoan protists, as well as larval nematodes and cestodes may also infest P. aztecus (Larson et al. 1989). Minello and Zimmerman (1989) suggested that bacterial infection and parasites do not cause direct mortality, but rather, reduce vigor and increase predation rates on brown shrimp.

V.  COMMUNITY ECOLOGY

Trophic Mode:
P. aztecus, like other penaeids, is an opportunistic omnivore that consumes some algal species (i.e., filamentous green algae, benthic diatoms, plant detritus, etc.) as well as small invertebrates such as copepods, mollusks, and annelids (Dall 1968; Odum and Heald 1972).

Jones (1973), in a Louisiana study, found a dietary shift in P. aztecus as body size increased. Small shrimp measuring 22 – 44 mm in length were observed to feed nonselectively on top-layer sediments. Shrimp in a larger size class, from 45 – 64 mm in length, became selective in their feeding, preferring only organic materials from top-layer sediments. Larger shrimp, from 65 – 100 mm in length became predatory, feeding on annelid worms, amphipods, zooplankton larvae, and nematodes.

Competitors:
P. aztecus is likely to compete with other crustacean species, as well as with some fishes for access to invertebrate prey.

Habitats:
Brown shrimp are commonly found in estuaries and littoral zones along coasts. Primary habitats for these shrimp are muddy bottom areas from the intertidal zone to approximately 110 m. The greatest density of P. aztecus occurs at depths between 27 – 55 m. This species is rarely observed at depths exceeding 165 m (Williams 1984).

In laboratory studies of substrate preference (Williams 1958), P. aztecus favored sandy mud and loose peat substrates over other offerings such as loose sand, and shell sand.

Activity Time:
P. aztecus is a burrowing shrimp that is considerably more active at night in open waters than it is during the daylight hours (Williams 1984).

VI.  SPECIAL STATUS

Special Status:
Commercially important

Fisheries Importance:
The brown shrimp is a highly valued commercial fishery species both within Florida and nationally. The statewide commercial catch of brown shrimp, Penaeus aztecus, between the years 1987 - 2001 was 31.5 million pounds, with a dollar value of over $71.1 million.  Within the 5 county area encompassing the IRL (Volusia, Brevard, Indian River, St. Lucie and Martin Counties) the commercial catch of brown shrimp accounts for approximately 16% of the statewide total, with a harvest of 4.6 million pounds, and a value of over $11.6 million.   This ranks the brown shrimp tenth in commercial value within the IRL, and sixteenth in pounds harvested.   

Figure 1 and Table 1 below show the dollar value of the brown shrimp fishery to IRL counties by year.  As shown, commercial catch ranged from a low of $65,978 in 1988 to a high of over $1.9 million in 1996.  Brevard County accounted for the bulk of the commercial catch in most years, with 58% of the total brown shrimp harvest in the IRL area.  Volusia County ranked second, with 32% of the harvest.   St. Lucie County reported commercial harvests for brown shrimp in only 5 years, from 1993 - 1998, yet accounted for 10% of the total catch.  Commercial catches for brown shrimp were not reported from either Indian River or Martin Counties. 




Figure 1.   Annual dollar value of the commercial catch of brown shrimp to the 5-county area
     of the Indian River Lagoon. 




Figure 2.  Total  brown shrimp dollar value and percentage by county for the years
     1987 - 2001.


 

Benefit in IRL:
Brown shrimp are commercially important in Florida. Fisheries data from 1996 shows brown shrimp catches in Brevard County, Indian River County and Martin County of approximately 906,900 pounds of the state-wide catch (data provided by Florida Department of Environmental Protection). Two other counties, St. Lucie and Volusia did not report commercial harvests of brown shrimp. In these counties, rock shrimp made up the bulk of commercial harvesting efforts.

Statistics from the National Marine Fisheries Service show that state-wide in Florida during 1998, 250 metric tons of brown shrimp were harvested, with a value of $1,475,059. In 1999, state-wide captures of brown shrimp were 395 metric tons, at a value of $2,163,466.

Economic Importance:
P. aztecus is extensively targeted as a commercial species throughout much of its range. Annual catch statistics for this species often exceeds those for other penaeid species especially in the Gulf of Mexico near the Bay of Campeche, TX.

The National Marine Fisheries Service reported commercial catches nation-wide for brown shrimp in1998 were 59,118.8 metric tons, with a commercial value of $222,957,225 dollars. For 1999, the commercial catch was 62,197.7 metric tons, with a value of $250,733,691 dollars.

VII.  Bibliography

Aldrich, D.V., C.E. Wood, and K.N. Baxter. 1968. An ecological interpretation
     of low temperature responses in Penaeus aztecus and P. setiferus
     postlarvae.  Bull. Mar. Sci. 18(1):61-71.

Bearden, C.M. 1961. Notes on postlarvae of commercial shrimp (Penaeus) in
     South Carolina. Contributions From Bears Bluff Laboratories, 33, 8 pp.

Burkenroad, M.D. 1939. Further observations on Penaeidae of the northern Gulf
     of Mexico. Bulletin of the Bingham Oceanographic Collection. 6(6):1-62.

Cook, H.L. and M.A Murphy. 1969. The culture of larval penaeid shrimp. Trans.
     Am. Fish. Soc. 98:751-754.

Cook, H.L. and M.A Murphy. 1971. Early developmental stages of the brown
     shrimp, Penaeus aztecus Ives, reared in the laboratory. Fish. Bull.
     69(1):223-239.

Copeland, B.J. and V. Truitt. 1966. Fauna of the Aransas Pass Inlet, Texas.
     Texas Journal of Science, 18(1):65-74.

Dall, W. 1968. Food and feeding of some Australian penaeid shrimp. Proc.
     World Sci. Conf. on Bio. and Cult. of Shrimps and Prawns. FAO Fisheries
     Reports, 57(2):251-258.

Jones, R.R. 1973. Utilization of Louisiana estuarine sediments as a source of
     nutrition for the brown shrimp, Penaeus aztecus. Ph.D. Dissertation,
     Louisiana State University, Baton Rouge.

Larson, S.C., M.J. Van Den Avyle, and E.L. Bozeman, Jr. 1989. Species
     profiles: Life histories and environmental requirements of coastal fishes and
     invertebrates (South Atlantic): brown shrimp. U.S. Fish and Wildlife Service
     Biological Reports 82(11.90). U.S. Army Corps. Of Engineers TR EL-82-4.
     14 pp.

Minello, T.J. and R.J. Zimmerman. 1989. The role of estuarine habitats in
     regulating the growth and survival of juvenile penaeid shrimp. In: Frontiers of
     shrimp research. Elsevier Press, New York, NY.

Odum, W.E. and E. Heald. 1972. Trophic analyses of an estuarine mangrove
     community. Bull. Mar. Sci. 22(3):671-738.

Renfro, W.C. 1964. Life history stages of a Gulf of Mexico brown shrimp. United
     States Fish and Wildlife Service Circular 183, pages 94-98.

Venkataramaiah, A., G.J. Lakshmi, and G. Gunter. 1972. The effects of salinity,
     temperature, and feeding levels on the food conversion, growth and survival
     rates of the shrimp Penaeus aztecus. Proc. Mar. Tech. Soc., Food-Drugs
     from the Sea.

Williams, A.B. 1955. A contribution to the life histories of commercial shrimps
     (Penaeidae) in North Carolina. Bulletin of Marine Science of the Gulf and
     Caribbean 5(2):116-146.

Williams, A.B. 1958. Substrates as a factor in shrimp distribution. Limnol. and
     Oceanogr. 3(3):283-290.

Williams, A.B. 1960. The influence of temperature on osmotic regulation in two
     species of estuarine shrimps (Penaeus). Bio. Bull. 119(3):560-571.

Williams, A.B. 1984. Shrimps, lobsters and crabs of the Atlantic coast of the
     eastern United States, Maine to Florida. Smithsonian Institution Press.
     Washington, D.C. 550 pp.

Wilson, B. Ecological survey of penaeid shrimp of the central Louisiana Gulf
     Coast and estuarine waters. Report to the Louisiana State Science
     Foundation.  Baton Rouge, Louisiana, 140 pp.

Zamora, G. and L. Trent. 1968. Use of dorsal carinal spines to differentiate
     postlarvae of brown shrimp, Penaeus aztecus Ives, and white shrimp, P.
     setiferus (Linnaeus). Contrib. Mar. Sci. 13:17-19.

Zein-Eldin, Z.P. 1963. Effect of salinity on growth of postlarval penaeid shrimp.
     Biol. Bull. 125(1): 188-195.

Zein-Eldin, Z.P. and G.W. Griffith. 1966. The effect of temperature upon the
     growth of laboratory-held postlarval Penaeus aztecus. Biol. Bull.
     131(1):186-196.

Report by:  K. Hill, Smithsonian Marine Station
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