LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
The spotted seatrout grows to a maximum size of approximately 100
cm (3.3 feet) total length (TL) and may weigh as much as 7.9 kg
(17.4 pounds). The lifespan is approximately 18 years (Hoese and
Moore 1977; Johnson and Seaman 1986).
Throughout Florida, young males outnumber
young females, but females tend to live longer than males (Klima
1959; Moffett 1961; Rutherford et al. 1982). Based on growth ring
analysis of scales, females also tend to grow somewhat faster than
males (Moffett 1961), with growth averaging 0.7 - 13 cm (0.3 - 5.1
inches) per year, depending on age. Rutherford et al. (1982) reported
growth is fastest during the first year, with a growth rate of approximately
21.2 cm (8.3 inches). This rate slows to approximately 4-5 cm (1.6
- 2 inches) per year for fish Ages II - IV. In the oldest fishes
studied, growth slowed to approximately 2.2 cm (0.8 inches) per
Growth rates in Cynoscion nebulosus
are temperature dependent, generally slowing or stopping completely
in the winter months due to slowing metabolism and reduced feeding
(Guest and Gunter 1958; Tabb 1961). Growth rates are highest in
July and August (Pearson 1929; Moffett 1961).
Murphy and Taylor (1994) estimated length-weight
relationships among spotted seatrout in different regions of Florida.
They reported that in all areas, males are heavier than females
of the same length. Further, there was some difference between estuaries
in terms of weight at any given length, with fishes from the Indian
River Lagoon and Apalachicola Bay tending to be heavier than seatrout
collected from Charlotte Harbor.
Spotted seatrout mature at 1-4 years of age (Lorio and Perret 1980).
Tabb (1961) reported that in the IRL, females mature at 3-4 years
of age, while males mature at 2-3 years of age. Females tend to
be larger at maturity than are males (Moody 1950; Moffett 1961).
Length at maturity varies by region; however, most spotted seatrout
spawn at 24-25 cm (9.4 - 9.8 inches) standard length (Moody 1950).
Cynoscion nebulosus spawn in nearshore
and estuarine waters (Mercer 1984; Johnson and Seaman 1986). They
are multiple spawners, as evidenced by the presence of vitellogenic
and fully yolked eggs in ovaries throughout the spawning season
(Brown-Peterson et al. 1988). The average time between successive
spawns was reported by Brown-Peterson et al. (1988) as 3.6 days,
though data from an experimental aquaculture population yielded
a period between spawns of 21 days (Tucker and Faulkner 1987). This
indicates that a female may spawn 9 - 60 times in a spawning season,
and release 3 - 20 million eggs annually (Murphy et al. 1999).
Spawning occurs at somewhat different times
depending on location. In Florida, spotted seatrout in the Ft. Myers
area spawn from late March through September, with a peak in June
(Moody 1950, Moffett 1961). In the Indian River Lagoon, spawning
occurs from mid-April through July, peaking from April - June (Tabb
1961), though some data indicates year-round spawning within the
IRL (Murphy and Taylor, FMRI, unpubl in Murphy et al. 1999). In
the Everglades, evidence indicates year-round spawning that peaks
both in spring and from late summer to early fall. (Jannke 1971;
Rutherford et al. 1982). Time of spawning is controlled by temperature
and salinity, with spotted seatrout in most areas spawning in waters
where temperatures are 25 - 28ºC, and salinity is 30 - 35 ppt
(Johnson and Seaman 1986).
Fecundity increases with size in Cynoscion
nebulosus. In Florida, Moody (1957) reported a 39.7 cm (15.6
inches) female collected at Cedar Key contained 464,000 eggs. Tabb
(1961) reported 15,000 - 1.1 million eggs per female from a collection
of females, aged at 4 - 8 years, and ranging in size from 32.5 -
62.5 cm (12.8 - 24.6 inches) standard length (SL). In a Louisiana
study, Sundararaj and Suttkus (1962) reported that Age III females
contribute approximately 41% of all eggs spawned, and thus have
the greatest reproductive potential of all age classes. Age IV females
contributed 27% of the eggs spawned, while Age II females contributed
24%. In an Everglades study, Rutherford et al. (1982) found that
most male spawners were Ages II, III, and IV.
Eggs are spherical, generally with a single oil droplet. Sometimes,
however, more than one droplet is observed (Tabb 1966). Eggs can
be both demersal and pelagic, depending on salinity conditions (Perret
et al. 1980). When salinity is high, eggs are buoyant. When salinity
drops below 25 parts per thousand (ppt), eggs tend to sink. Optimum
survival salinity for eggs and larvae is approximately 28 ppt (Taniguchi
Larvae hatch approximately 18 hours after
fertilization and measure 1.3 - 1.6 mm (0.05 - 0.06 inches) SL under
laboratory conditions (Fable et al. 1978). At 10 - 15 mm (0.4 -
0.6 inches) in length, larvae are most commonly found in shallow
seagrass beds in bays and lagoons. At water temperatures averaging
28 ºC (82.4 ºF), larvae persist for 20 days before metamorphosis.
Juveniles 6-8 weeks of age measure 2.5 - 5.0 cm (1.0 - 2.0 inches)
in length and begin to form schools consisting of similarly sized
individuals. Schooling behavior remains strong until approximately
Age VI - VII, when both males and females become semi-solitary (Tabb
In laboratory studies, spotted seatrout larvae
held at 24-26ºC (75.2 - 78.8 ºF) grew from 1.5 mm (0.05
inches) at hatching to 4.5 mm (0.18 inches) SL in 15days (Fable
et al. 1980).
Cynoscion nebulosus is among the top predators in many
estuaries, and sometimes is the only large carnivore in an estuary
(Tabb 1966). Small seatrout 20-50mm (0.8 - 2 inches) SL are planktivorous
and feed primarily on copepods (Moody 1950; Tabb 1961; Adams et
al. 1973; Rutherford et al. 1982; McMichael and Peters 1989). Larger
seatrout, to 70 mm (2.8 inches) in length feed primarily on benthic
invertebrates, especially mysids (Moody 1950, Darnell 1958; McMichael
and Peters 1989). A dietary shift occurs once seatrout reach approximately
40 - 150 mm (1.6 - 5.9 inches) and begin to feed more on penaeid
shrimps and small fishes (Moody 1950; McMichael and Peters 1989).
Indian River Lagoon seatrout were reported to consume shrimp in
the summer and early winter, the most abundant period for shrimp;
but switched to fish in late winter through spring (Tabb 1961).
Cynoscion nebulosus feed in midwater
or near the surface, with major food species including anchovies,
pinfish, silversides, mullet, croakers, menhaden, snapper, gobies,
mojarras and silver trout (Moody 1950; Darnell 1958; Adams et al.
1973; Rutherford et al. 1982).
Probable competitors for both food and space, particularly in the
early life history stages include other sciaenids, pinfish, menhaden,
catfishes, anchovies and silversides. Various types of invertebrate
larvae may also compete with larval seatrout for copepods and other
zooplankton (Tabb 1961; Rutherford et al. 1982).
Adult seatrout are likely to compete with
other shrimp-feeding and fish-feeding species such as sand trout,
Cynoscion arenarius; redfish, Sciaenops ocellatus;
flounders, Paralicthyes spp., catfishes, snook, Centropomus
spp., snapper, jacks, and tarpon, Megalops atlanitcus (Johnson
and Seaman 1986).
Larger fishes are the major predators on Cynoscion nebulosus.
Predatory birds, especially osprey, also impact seatrout populations.
Cynoscion nebulosus are commonly found in shallow, vegetated,
brackish and marine waters to a depth of approximately 10 m (32
feet). They are often associated with seagrasses and saltmarshes
adjacent to deeper channels that are used as refuges from temperature
extremes (Moody 1950; Tabb 1958).
Larvae are most common in shallow seagrass
beds in bays and lagoons during the summer months. Juveniles and
pre-recruits measuring less than 355 mm (13.9 inches) TL associate
with seagrasses, though some are found in channels and backwater
areas as well (Tabb 1966; McMichael and Peters 1989). Adults occur
in a wide variety of estuarine habitats including shallow seagrass
beds, oyster reefs, over sand bottoms, deep holes, in mangrove creeks,
and in areas having manmade structures such as docks and piers.
In Florida, spotted seatrout tend to spawn
and live in particular estuaries, never migrating more than short
distances (Johnson and Seaman 1986). Spawning habitat includes the
non-tidal areas of estuaries and bays, deeper channels adjacent
to seagrass beds, near tidal passes and also nearshore waters outside
of estuaries (Pearson 1929; Tabb 1961; Tabb et al. 1962; Jannke
Spawning typically occurs at night with individuals in the school
lightly bumping one another along the sides as males in the school
produce croaking and grunting sounds by vibration of their swim
bladders. Feeding occurs most actively in the early morning hours
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