Biological Characteristics
This excerpt from "The 2000 Stock Assessment Update of Common Snook, Centropomus undecimalis" details snook biological characteristics.
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THE 2000 STOCK ASSESSMENT UPDATE OF COMMON SNOOK, Centropomus undecimalis
Robert G. Muller
July 12, 2000
Fish and Wildlife Conservation Commission
Florida Marine Research Institute
100 Eighth Avenue Southeast
St. Petersburg, Florida 33701-5095
DATA SOURCES
The biology and population dynamics of common snook in Florida has been investigated for populations statewide (Tringali and Bert 1996), in southwest Florida (Fore and Schmidt 1973; Bruger unpublished manuscript.; Thue et al. 1982), the Indian River Lagoon and Southeast Florida (Gilmore et al. 1983; Tucker and Campbell 1988; Taylor et al 1993, 1998a, 2000a), and Tampa Bay (Taylor et al. 1993, 1998a, 2000a; McMichael et al. 1989). Much unpublished data on snook population dynamics were made available by the Fish and Wildlife Conservation Commission’s Florida Marine Research Institute (FWC-FMRI); R. Taylor, personal communication and by the National Park Service’s South Florida Natural Resources Center in Everglades National Park (ENP); T. Schmidt, personal communication.
STOCK DISTRIBUTION
Common snook, Centropomus undecimalis, are found in estuaries and adjacent rivers and in nearshore waters of the tropical and sub-tropical western Atlantic (Gilmore et al. 1983, Rivas 1986). Fishes in the genus Centropomus are found in coastal waters bounded by intercepts of the 14° C water isotherm in the North Atlantic during February to the coastal intercepts of the 14° C water isotherm in the South Atlantic during August (Rivas 1986). Common snook are the most widely distributed species within the genus. Snook have been reported as far north as New York (Schaefer 1972) along the Atlantic seaboard and throughout the Gulf of Mexico. Common snook occur in abundance on the Atlantic coast of Florida from about Cape Canaveral south through the Keys and Dry Tortugas, and north to about Cedar Key on the gulf coast. Further west from there, they occur infrequently until about Galveston, Texas and then more or less continuously south to about Rio de Janeiro, Brazil (Gilmore et al. 1983, Rivas 1986).
Differences among the genetic diversity and genotype frequencies of common snook from the gulf coast of Florida, the Atlantic coast of Florida, and islands in the Caribbean are evidence for reproductive isolation among these populations (Tringali and Bert 1996). In Florida, snook from the Atlantic and gulf coasts are sufficiently isolated and have sufficiently different life histories that each group could be managed separately (Tringali and Bert 1996; Taylor et al.1993).
HABITAT REQUIREMENTS AND DISTRIBUTION PATTERN
Eggs and larval snook are found in polyhaline and euryhaline waters near estuarine passes or adjacent river mouths (Peters et al., 1998a; Tolley et al. 1987; R. Taylor, personal communication). Newly hatched larvae spend about 2.5 weeks in nearshore waters prior to their arrival in shallow-water nursery sites (Peters et al. 1998). Larvae have been found in euryhaline and oligohaline waters and apparently settle out of the plankton near mangrove prop-roots. According to reports, few larval snook have been collected. Extensive estuarine sampling of Naples Bay, Florida, produced 14 larval snook (Tolley et al. 1987). The lengths and distribution of larvae suggest that they hatch near the mouth of that estuary and remain near the bottom where flood tides transport them into the bay. Larvae have been successfully reared between 25° and 32° C, with maximum yolk utilization efficiency at 26° C (Limouzy 1993). Below 25° C and above 33° C larval snook did not survive for long after hatching. Small juvenile snook prefer low-energy, shallow waters, often near overhanging shoreline vegetation or seagrass meadows. Primary nursery habitat has been described as brackish, shallow warm water streams or dredged canals with slow currents, unvegetated bottoms and overhanging or submerged mangrove prop-roots (Fore and Schmidt 1973). Three distinct nursery habitats for snook on the Florida Atlantic coast are freshwater tributaries, estuaries, salt marshes, and sea grass beds (Gilmore et al. 1983). Snook move from shallow riparian habitats to sea grass meadows, mangrove fringe, and other deeper estuarine waters as they grow. Juvenile snook survive in waters with lower oxygen levels than can adults, correlating well with typical oxygen stress faced by snook during ontogenic shifts in habitat (Peterson and Gilmore 1991).
The distribution of adult common snook closely approximates the distribution of mangroves, which are their principal habitat, (Marshall 1958; Gilmore et al. 1983) however, they also occur along beaches, river mouths, nearshore reefs, salt marshes, sea grass meadows, and lakes. Adult snook appear to be less sensitive to cold water temperatures than larvae or small juveniles (Shafland and Foote 1983). The lower lethal limit to water temperature is 9°–14° C for juveniles and 6°–12° C for adults.
FOOD HABITS
Larval snook probably eat copepods although the few collected have had empty stomachs (Tolley et al. 1987). Small juvenile snook feed mainly on copepods and mysids (McMichael et al. 1989). Gilmore et al. (1983) also found that small snook found in marshes fed mainly on microcrustaceans until they were large enough to eat paleomonid shrimp and fish. Larger juveniles feed on paleomonid shrimp and cyprinodontid and poeciliid fishes (McMichael et al. 1989; Gilmore et al. 1983). A transition from intake of crustaceans to an intake of fishes apparently occurs at 45 mm (1.8 in) long. Large juveniles feed mainly on the abundant fishes found in sea grass beds and most organisms reported as food for adults are species associated with the water column.
REPRODUCTIVE LIFE HISTORY
Histological evidence shows that common snook are protandric hermaphrodites; i.e. snook begin life as males and then after maturation the fish become females (Taylor et al. 2000a). Observed transitional fish ranged from 240–824 mm FL (9.4–2.4 in) and from 1–7 years old. Gonads of sex-reversing snook simultaneously contain sex cells of both sexes: ovigerous amellae and dorsal remnants of ducts containing sperm. Protandry and differences in growth and mortality between sexes result in significantly skewed sex ratios. The majority of small snook are male and most large snook are female. Sex ratios reported from southwest Florida snook showed similarly skewed sex ratios (Volpe 1959, Thue et al. 1982).
Males reach sexual maturity during their first year at 150–200 mm (5.9–7.9 in) FL, which is much smaller and younger than previously suspected (Taylor et al. 2000a). In addition, current research indicates that mature females arise directly from mature males when the transitional gonadal tissue is reorganized shortly after spawning. If all males have the capacity to become female at some point in their life, then the maturation schedule for females would simply reflect the schedule of transition from males to females (see sex ratio relations below). However, if some portion of males do not have the capacity to change into females, then the expected proportion of mature females at a given length or age would be greater than that predicted by the sex ratio method.
The probability that a snook of a particular size will be a female increases with length or age and is closely approximated by coast-specific logistic regressions. We therefore calculated coast-specific models for the probability of a snook being a female. These were updated using all sex-length (n = 5116 Atlantic coast and n = 4,720 gulf coast) or sex-age (n = 4,549 Atlantic coast and n = 4,401 gulf coast) data available through 1999:
Prob(female) = (e -7.005 + 0.2187 Total length in inches) / ( 1 + e -7.005 + 0.2187 Total length in inches)
Prob(female) = (e -7.005 + 0.2187 Total length in inches) / ( 1 + e -2.725 + 03480 Age in years)
on the Atlantic coast and
Prob(female) = (e -4.557 + 01728 Total length in inches) / ( 1 + e -4.557 + 01728 Total length in inches)
Prob(female) = (e -1.129 + 0.1620 Age in years) / ( 1 + e -1.129 + 0.1620 Age in years)
on the gulf coast. When these models were used as a proxy for maturity in calculating spawning potential ratios, we called this the sex ratio method. For snook at 26" TL, the sex ratio is significantly skewed toward males on the Atlantic coast with about 3.7 males for each female and approximately even on the gulf coast (1.1 males for each female). For 34" TL snook, the sex ratio is skewed in favor of females, with 1.5 females for each male on the Atlantic coast and 3.7 females for each male on the gulf coast.
The reproductive season for common snook on both coasts of Florida extends over at least six months: on the gulf coast, from April through September and on the Atlantic coast from April through October (Taylor et al. 1998). Marshall (1958) found common snook from southwest Florida spawning from May to November with peak activity during May and June. Volpe (1959) determined that the spawning season for snook from Lee and Collier counties was during June and July based on specimens with flowing gametes. In September, he found no snook extruding gametes and concluded that peak spawning had passed. Based on observations of running-ripe male snook, Fore and Schmidt (1973) reported the spawning season in the Ten Thousand Island area of southwest Florida to be from the first of May to about the middle of November. Gilmore et al. (1983) suggested a protracted spawning season of April to December for snook along the east-central coast of Florida based on back-calculated hatching dates of recruiting juveniles. They noted two peaks in spawning activity, one during June and July, and another during the period of August to October. Using oocyte diameters and GSIs, Tucker and Campbell (1988) showed that spawning along the east-central coast of Florida occurred from early May until early October. The spawning season in Tampa Bay was determined by McMichael et al. (1989) to be from April until December based on back- calculated dates from juvenile otoliths. These temporal variations are to be expected, because physical parameters, especially temperature, which appears to mediate spawning (Gilmore et al. 1983, Bye 1984), vary from one location to the next.
Spawning occurs only after the water temperature warms from winter minimums to 25° C. Taylor et al. (1998) found that out of the variables moon phase, tide, amplitude, velocity, and time of day, the only significant physical factor that correlated to maximum oocyte diameters was time of the day. Spawning occurred between 14:00 and 20:00 hours. Spawning frequency of snook was inferred from the proportion of females found that contained either post-ovulatory follicles or hydrated oocytes. Using the proportion of the samples with post-ovulatory follicles, the estimated interval between spawning events ranged between 1.1 and 2.5 days. Using the hydrated oocyte method, the average spawning frequency for common snook in Tampa Bay was about once every 1.4 days (Taylor et al. 1993).
Because initial assessments of snook used the snook maturity schedule, we will continue to present spawning potential ratios (the ratio of mature female biomass with fishing to that without fishing mortality) calculated with the maturation schedule, and call this the maturity schedule method. We present estimates of SPR based on both the maturity schedule methods and the sex ratio method. The probability that a female snook is mature as she grows older was modeled by:
Prob(mature) = (e -5.8868 + 1.6853 age) / ( 1 + e -5.8868 + 1.6853 age)
on the Atlantic coast and
Prob(mature) = (e -5.5328 + 1.9878 age ) / ( 1 + e -5.5328 + 1.9878 age )
on the gulf coast (Taylor et al. 1993).
These equations predict 50% maturity at younger ages (3.5 years old on the Atlantic coast and 2.8 years old on the gulf coast) than the sex-ratio method. This difference causes more mature females to be estimated in the population and therefore higher SPR estimates result.
GROWTH PATTERNS
Snook on the Atlantic coast of Florida attain larger sizes than do snook on the gulf coast of Florida. The average observed sizes at age one for Atlantic and gulf coast male snook was 231 and 264 mm (9.1 and 10.4 in) FL, respectively (Taylor et al. 2000a). At age two, Atlantic coast males were also smaller than gulf coast males but Atlantic coast males continue to grow at a relatively rapid rate and surpass the average gulf coast male size-at-age by age four. Predicted sizes at age for females are larger for all ages on the Atlantic coast than on the gulf coast.
Snook can reach about 20 years old and over 1,100 mm FL (43.3 in). On the Atlantic coast, the oldest sexed common snook sampled by Taylor et al. (2000a) was an eighteen-year-old female and the largest was also a female (1,105 mm, 43.5 in) but she was only age sixteen. The oldest male was 15 years old and 865 mm (34.0 in) and the largest male was 908 mm (35.7 in) at age eleven. On the gulf coast, the oldest common snook was a 15 year old female and the largest snook was also a female (1,032 mm, 40.6 in) but she was only age 10. The oldest male was 12 years old and 810 mm (31.9 in) and the largest male on the gulf coast was 925 mm (36.4 in) but only age four.
Volpe (1959) reported a maximum age for combined sexes to be seven years, while Thue et al. (1982) reported eight years for southwest Florida snook. The maximum age for snook in Taylor et al. (2000a) was 21 years for both coasts. The differences in age among these studies are probably attributable to the different age determination methodology—scale ages by Thue et al. (1982), whole otoliths by Volpe (1959), and otolith sections by Taylor et al. (2000a)—and to the different methods of capture—hook and line was the primary gear in the older studies whereas seines were used by Taylor et al. (1998). Maximum observed sizes of 1,105 and 1,032 mm (43.5 in and 40.6 in) for Atlantic and gulf coast females (Taylor et al. 2000a) are larger than those reported previously by Volpe (1959, .970 mm or 38.2 in) or Thue et al. (1982, .950 mm or 37.4 in). These differences may also be the result of different sampling gear or the effects of having less stringent harvest regulations on the fishery during the time of the earlier research.
Coast-specific predictive equations for fork length at age (Taylor et al. 2000a) are:
Fork length (mm) = 989 ( 1 - e -0.235(Age - 0.0976) )
on the Atlantic coast and
Fork length (mm) = 947 ( 1 - e -0.175(Age + 1.352) )
on the gulf coast.
MORPHOMETRICS
Coast-specific length-length and weight-length relationships were developed by Taylor et al. (2000a) using snook collected during the period 1986–1991. The estimated length-length and length-weight relationships for Atlantic coast snook were:
Fork length (mm) = 0.953 * Total length (mm) - 10.85,
Fork length (mm) = 1.111 * Standard length (mm) + 7.12, and
Weight (g) = 8.28*10 -6 * Fork length 3.04(mm).
The estimated length-length and length-weight relationships for gulf coast snook were:
Fork length (mm) = 0.9512 * Total length (mm) - 14.86,
Fork length (mm) = 1.0630 * Standard length (mm) + 20.09, and
Weight (g) = 4.40 10 -6 * Fork length 3.11(mm).
These relations predict that a 26" total length snook on the Atlantic coast would weigh 7.0 lb and that the same size snook on the gulf coast would weigh about 5.8 lb.
Prior to July 1, 2004, the Fish and Wildlife Research Institute was known as the Florida Marine Research Institute. The institute name has not been changed in historical articles and articles that directly reference work done by the Florida Marine Research Institute. 
