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The Common Snook. Illustration by Diana Rome Peebles 1998.  Courtesy of Florida Fish and Wildlife Conservation Commission, Division of Marine Fisheries.

Species Name: 
Centropomus undecimalus (Bloch, 1792)

Common Name:
Common Snook, Robalo, Thin Snook

Species Description:
The common snook, Centropomus undecimalis, is a subtropical marine/estuarine species (Gilmore, et al 1978; Shafland and Foote 1983) which is by far the most abundant and wide-ranging of all snook species (Marshall 1958). It is easily distinguished by its sloping forehead, protruding lower jaw, prominent lateral line, elongate body form and the position of the tips of its pelvic fins in relation to the anus (Seaman and Collins 1983). In C. undecimalis these do not  overlap or reach the anus.  

The dorsal surface is a dull gray color with a yellow to green tint, and is separated from the ventral surface by a well defined black lateral line. The pectoral fins, pelvic fins, second dorsal fin, and the dorsal lobe of the caudal fin are all a bright canary yellow; however, some river specimens may be considerably darker in color than those from coastal waters.

Other identifying characteristics of this species: Second anal spine does not reach vertical from the caudal base; nor do the pectoral fins reach vertical from the tip of the pelvic fins. The maxillary reaches to or beyond the vertical as measured from the center of the eye.

Meristic Counts:
     Lateral Scales: 70-77
     Gill Rakers: 7-9
     Anal rays: 6
     Pectoral Rays: 15 or 16

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Centropomus undecimalis ranges from the coastal mid-Atlantic United States to Brazil, including the Gulf of Mexico and the Caribbean, with the center of abundance being coastal Florida. The species is restricted by water temperature to tropical and subtropical North, Central and South America. This species was formerly thought to be temperature limited north of Volusia County, Florida; however, C. undecimalis has been identified as far north as Pamlico Sound North Carolina. In Florida, the range of the common snook is sympatric with other snook species, though when C. undecimalis does occur with other related species, it is generally the more common.

IRL Distribution:
Occurs throughout the Indian River Lagoon.

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Individuals may reach up to 140 cm fork length (55 inches) and weigh up to 22 kg (49.5 lbs). Female specimens are generally larger than males of the same age.

Abundance:
Abundance of C. undecimalis in Florida centers on mangrove fringed coastal waters (Gilmore 1986). Numbers of snook have declined over the last 50 years due to commercial and recreational overharvesting and habitat degradation/destruction. A bill passed in the Florida legislature in 1957 prohibited commercial capture and sale of snook. Passage of this bill helped ameliorate fishing pressure on snook populations; however habitat loss and water quality degradation may have had more far reaching effects on snook than did commercial fishing pressures (Gilmore, Donohoe and Cooke 1983).

Reproduction:
Though Centropomus undecimalis adults may utilize fresh water habitats, they are unable to spawn in fresh water, as sperm become activated only in saline waters. Snook species in Florida have been observed to congregate for spawning at the mouths of rivers, inlets and canals. Gilmore et al. (1983) found that spawning activity is positively correlated with monthly rainfall patterns, but not necessarily with either temperature or salinity. However, terrestrial runoffs during rain events directly affect phytoplanktonic primary production via an increase in dissolved nutrients; thus spawning activity in this species is likely related to food availability.

When reproductively active, snook spawn in the evening over the course of several days. In Florida, two spawning peaks are observed: the first in June/July, the second in August/October.

Embryology:
Maturation of oocytes in follicles of C. undecimalis is typical of many teleost fishes, with rapid water uptake (measuring approximately 200% of egg volume) associated with cytoplasmic clarification (Wallace et al. 1993). Yolk averages 91% of egg mass (Seaman and Collins 1983; Lau and Schafland 1982).

In laboratory experiments, egg size was between approximately 0.6 - 0.8 mm in diameter (0.27 mm³ volume).

Larval size at hatching was approximately 1.4 - 1.5 mm SL. Larvae absorbed their yolk sacs in 4 days, upon reaching 2.2 mm SL. Melanophores become visible at 4 mm, and adult pigmentation begins to show by 7 mm. Ossification begins by the time 5 mm is achieved, with jaws becoming completely ossified and lined with teeth by 8.6 mm SL.

IV.  PHYSICAL TOLERANCES

Temperature:
Centropomus undecimalis is eurythermal, but sensitive to cold, with lethal minimum temperatures between 6-13 °C. In laboratory experiments, Schafland and Foote (1983) demonstrated that snook stop feeding at 14.2 °C, lose equilibrium at 12.7 °C, and die at 12,5 °C.

Lethal maximum temperatures for snook were shown by Chung and Mendez (1993) to be influenced by acclimation temperature. In laboratory experiments, lethal maximum temperatures for snook were between 38.7°C and 40.7 °C.

Salinity:
Snook are a euryhaline species with a preference for mangrove-fringed estuarine habitats. However, their wide salinity tolerance accommodates the various habitat choices made by snook as they transition from freshwater to estuarine and marine habitat areas.

Other Physical Tolerances:
Juvenile snook have been shown to survive dissolved oxygen levels of 0.4 ppm. This is partially due to the fact that survival of juveniles in low oxygen is weight dependent: larger snook (>160 mm SL) die faster under hypoxic conditions than do smaller specimens. They are, however, better able to migrate to more tolerable conditions. Conversely, small snook are not as effective at migration, yet they compensate for this deficiency by having higher ventilation rates than larger snook.

Under hypoxic conditions at depth, juvenile snook have been observed to move toward more oxygenated surface waters where they decrease their activity levels.

V.  COMMUNITY ECOLOGY

Trophic Mode:
C. undecimalis and other snook species can be considered opportunistic carnivores, feeding primarily on other pelagic fish species. The specific diet of C. undecimalis changes with habitat type. Juveniles in freshwater habitat areas in Florida prey primarily on palaemonid shrimp, microcrustaceans and Gambusia affinis (Mosquitofish). In salt marshes, snook maintain a similar diet, but may add other species such as Cyprinodon variegatus (Sheepshead Minnow). Snook in seagrass habitats feed primarily on fish and crustaceans, with Anchoa mitchelli (Bay Anchovy), Lagodon rhomboides (Pinfish), and penaeid shrimp composing the bulk of the diet.

Larval and juvenile growth patterns follow the classic pattern of slow initial growth in the month following hatching. However, nearly tripled growth rates occur thereafter until the end of the first year. Snook larvae may grow as much as 1 mm per day. This rate slows to approximately 0.15 mm per day after larvae reach 2.4 mm SL.

Snook are pelagic feeders with 2 daily feeding peaks: one approximately 2 hours before sunrise, and the second approximately 2-3 hours following sunset. The tidal cycle has been shown to affect feeding behavior, with increases in feeding activity noted with an increase in water flow following standing flood or ebb tides.

Competitors:
Snook are considered to be top predators with the exception of humans.

Habitat:
Juvenile snook utilize 3 distinct habitat areas in their first year: freshwater tributaries, salt marshes and seagrass beds. The smallest snook, those averaging 27 mm SL, primarily inhabit fresh water. When these small fish reach 40 -60 mm SL they migrate to salt marsh habitat areas, where they remain approximately 60 -90 days. Juveniles will next migrate to seagrass beds once they attain approximately 100 mm SL, and will remain in this habitat for 4 - 5 months. Seagrass beds 5 - 15 km from ocean inlets are the preferred habitat areas for Florida snook over 150 mm SL. Maturation begins when juveniles reach approximately 30cm. At this time many juvenile snook then disperse to various fresh water, brackish and marine habitat areas, and will remain generally non-migratory as adults except for congregating for spawning in high salinity areas.

Associated Species:
Eucinostomus argeneus (Spotfin Mojarra), E. gula (Silver Jenny), Mugil cephalus (Striped Mullet), Archosargus probatocephalus (Sheepshead), Elops saurus (Ladyfish) and Megalops atlanticus (Tarpon) are often closely associated with snook.

VI. SPECIAL STATUS

Special Status:
Fisheries

Fisheries Importance:
Though commercial fishing for snook is illegal in Florida, the species is still vitally important economically.  In 1986, Florida's sport fisheries for snook, tarpon, and other game fish were estimated to be worth $5 - 7 billion annually when all adjunct enterprises associated with sport fishing are taken into account. 

Muller et al (2001) assessed snook stocks in Florida and estimated that the highest overall abundance of snook occurred in the southern portion of the Indian River Lagoon.  Angler survey information shows that approximately 90% of the snook captured are released, with an average of 35,000 snook harvested annually from the 5-county area that encompasses the Indian River Lagoon.  Most (48.3%) are captured from the shoreline to 3 miles offshore, while harvests within the Indian River Lagoon account for 36% of the total, or an average of approximately 13,000 fish per year.  Inland waters other than the Indian River Lagoon accounted for 14.4% of the total, while offshore captures to 200 miles offshore accounted for only 1.3% of the total. 

  Figure 1.  Survey data for the common snook recreational fishery showing the number of
               fishes harvested in East Florida waters from 1997 - 2004.
 

  Figure 2.  Summary of the common snook recreational harvest and percentage of total by
               area from 1997 - 2004. 

 

              Table 1.  Summary data for the common snook, Centropomus undecimalis,
                        recreational fishery in Eastern Florida waters from 1997 - 2004.   Data provided
                        by National Marine Fisheries Service, Fisheries Statistics Division, NOAA.

 

                       Table 2.  By-county annual and cumulative percentages of the common snook
                                  harvest for the years 1997 - 2001.  Data provided by National
                                  Marine Fisheries Service, Fisheries Statistics Division, NOAA.

                        Table 3.  Summary of the common snook recreational harvest and 
                                     percentage of total fish captured in each area from 1997 - 2004.  Data
                                     provided by National Marine Fisheries Service, Fisheries Statistics
                                     Division, NOAA.

Report by:  K. Hill, Smithsonian Marine Station
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Page last updated: July 5,  2005