Gulf menhaden, continued 



areas and utilize such areas as nursery grounds 

 (Simoneaux 1979). As juveniles grow, they begin to 

 move into deeper, higher salinity areas of the estuary 

 (Suttkus 1956, Dugas 1970, Fore 1970, Holcomb 

 1970, Fore and Baxter 1972, Tagatz and Wilkens 

 1973, Dunham 1975, Hinchee 1977, Perry and Boyes 

 1978, Allshouse 1983, Guillory et al. 1983, Marotz 

 1984, Deegan 1985, Shaw et al. 1985a, Shaw et al. 

 1985b, Deegan 1990). This migration appears to be 

 size related, but may also be influenced by environ- 

 mental parameters (Marotz 1984, Deegan 1985, 

 Deegan 1990). Larvae show a diel pattern in vertical 

 distribution, in which they concentrate at the water 

 surface by day, but are more vertically dispersed at 

 night (Sogard et al. 1 987). This is thought to be due to 

 a slow sinking in the water column as a result of passive 

 depth maintenance during the night time nonfeeding 

 period. During daylight hours, larvae are actively 

 swimming, and maintain their position close to the 

 surface. 



The gulf menhaden does not exhibit an extensive 

 migratory pattern (Ahrenholz 1 991 ). Adults and matur- 

 ing juveniles (80-1 05 mm SL) migrate from estuaries to 

 open Gulf waters to overwinter or spawn from late 

 summer to winter, with peak movement occurring from 

 Octoberto January (Roithmayrand Waller 1963, Dugas 

 1970, Holcomb 1970, Tagatz and Wilkens 1973, 

 Deegan 1985, Ahrenholz 1991). Some emigration of 

 larger individuals occurs throughout the year (Marotz 

 1984, Marotz et al. 1990). In Louisiana, most move- 

 ment of older fish is inshore/offshore with little east- 

 west movement noted (Shaw et al. 1 985a, Shaw et al. 

 1 985b). Tagging studies by Kroger and Pristas (1 974) 

 indicate localized populations with little movement 

 occurring between fishing grounds east and west of the 

 Mississippi River Delta. However, there is evidence 

 from other tagging studies that gulf menhaden which 

 leave estuaries and enter the Gulf of Mexico in the 

 edges of their range (e.g. Florida) tend to disperse or 

 "drift" towards the center of the range (e.g. Louisiana) 

 as they age (Ahrenholz 1 981 , Ahrenholz pers. comm.). 



The gulf menhaden has been reported to begin migra- 

 tion from Tampa Bay, Florida in June and July (Springer 

 and Woodburn 1960). Migration from Pensacola Bay, 

 Florida has been reported to occur by September 

 (Tagatz and Wilkens 1973). One study reports large 

 schools in Louisiana migrating offshore in June (Wagner 

 1973). Adults in the Gulf begin an apparent offshore 

 movement in October from the shallow waters inshore. 

 Movement back into estuaries after overwintering and/ 

 or spawning in the open Gulf occurs from March to April 

 (Christmas 1 981 , Lewis and Roithmayr 1 981 ). Christ- 

 mas (1981) speculates that this inshore movement is 

 "by random movement, probably in search of high food 

 concentrations." This leads the menhaden back into 



the food rich estuarine waters. Some studies indicate 

 that the lipid content of the menhaden is related to the 

 time of movement. Lipid and energy content increase 

 as fish metamorphose from larvae to subadults. Fish 

 with high lipid content are the first to migrate offshore 

 in response to small changes in temperature, and 

 those with lower lipid content migrate later or not at all 

 (Wagner 1973, Deegan 1985, Deegan 1986). 



Reproduction 



Mode : Reproduction is sexual, with separate male and 

 female sexes (gonochoristic). Milt and roe are broad- 

 cast, and fertilization is external. 



Spawning : Actual spawning in the wild has not been 

 observed (Guillory et al. 1 983). Information is based on 

 capture of eggs, larvae, spent adults, and laboratory 

 fertilizations. Most spawning probably occurs off the 

 Mississippi and Atchafalaya River deltas from nearshore 

 to about 97 km offshore, in waters from 2 to 1 28 m deep 

 (Roithmayr and Waller 1963, Etzold and Christmas 

 1 979, Lewis and Roithmayr 1 981 , Shaw et al. 1 985a, 

 Shaw et al. 1985b, Sogard et al. 1987), with most 

 spawning in waters less than 18 m deep (Christmas 

 and Waller 1975, Christmas et al 1988). Adults are 

 intermittent spawners, having as many as five peaks 

 during a season in different parts of the Gulf. A 

 spawning season usually runs from October through 

 March, but can begin as early as August and last as late 

 as May. Separate peaks can be observed during the 

 season from November to April (Miller 1965, Tagatz 

 and Wilkens 1 973, Sabins and Truesdale 1 974, Etzold 

 and Christmas 1979, Lewis and Roithmayr 1981, 

 Allshouse 1983, Guillory et al. 1983, Marotz 1984, 

 Shaw et al. 1985a, Christmas 1988, Warlen 1988, 

 Marotz etal. 1990). 



Fecundity : Actual fecundity for menhaden is difficult to 

 determine as they are intermittent, fractional spawners 

 (Lewis and Roithmayr 1 981 ). Studies have shown that 

 fecundity increases significantly with age and length 

 (Suttkus and Sundararaj 1961, Lewis and Roithmayr 

 1 981 ). Mean number of eggs per fish are: 21 ,960 in 

 age classes I; 68,655 in age class II; and 122,062 in 

 age class III (Suttkus and Sundararaj 1961). Lewis and 

 Roithmayr (1981) have developed equations to de- 

 scribe fecundity based on age, length, and weight. 



Growth and Development 



Egg Size and Embryonic Development : Eggs are plank- 

 tonic and pelagic. They are spherical with unsculptured 

 chorion, a faintly segmented yolk, and a single oil 

 droplet. Observed mean total diameters of eggs have 

 ranged from 1 .22 ± 0.04 to 1 .30 mm ± 0.05. Hatch rate 

 can vary from 1 to 3 days depending on the ambient 

 water temperature. In one study, eggs incubated at 1 9° 

 to 20°C and 30%<= salinity hatched in 40 to 42 hours. 



137 



