Gulf menhaden, continued 



Hatching of menhaden eggs occurs mostly from Octo- 

 ber to March (Hettler 1 984, Shaw et al. 1 985a, Christ- 

 mas et al. 1988, Powell 1993). 



Age and Size of Larvae : Larvae are 2.6 to 3.1 mm SL 

 immediately after hatching. Growth rate at 20° ± 2°C 

 averaged 0.30 ± 0.03 mm/day through 90 days of 

 rearing, but growth rate can vary with age and tempera- 

 ture (Chen et al. 1992, Powell 1993). Transformation 

 from the larval to juvenile form began at approximately 

 1 9 mm and was completed at approximately 25 mm SL 

 (Hettler 1984). One field study of larvae showed 

 metamorphosis beginning at 20-21 mm SL and being 

 completed at 30-35 mm SL. Other studies have 

 reported metamorphosis taking place when larvae 

 reach a total length (TL) of 30-40 mm TL and 30-33 mm 

 TL (Tagatz and Wilkens 1973, Guillory et al. 1983, 

 Deegan 1985, 1986). By May, most larvae have 

 metamorphosed into juveniles (Tagatz and Wilkens 

 1973). Size-selective mortality may be significant for 

 larval gulf menhaden, with the smaller larvae more 

 vulnerable to predation (Grimes and Isely 1 996). This 

 may result in overestimation of larval growth, as smaller 

 larvae are removed from the population. Growth of 

 larval fish proceeds through a series of ontogenetic 

 intervals, with periods of rapid growth followed by 

 periods in which structures form (Raynie and Shaw 

 1994). Raynie and Shaw (1994) reported that the 

 growth rate of larval gulf menhaden was lower in 

 estuaries than in coastal waters, as they approached 

 metamorphosis to the juvenile stage. 



Juvenile Size Range : Juveniles may grow as much as 

 20-30 mm/month and become sub-adults at SL's greater 

 than 85 mm. 



Age and Size of Adults : Menhaden mature after two 

 seasons of growth and have a maximum life span of 

 five years (Nelson and Ahrenholz 1981). Nicholson 

 (1978) developed the following year class size infor- 

 mation based on fork length (FL) data from ports 

 throughout the Gulf of Mexico: 

 Age-0: 1 02-1 23 mm FL range with 1 1 5 mm mean FL, 

 22-47 g range with 32 g mean weight (W). 

 Age-I: 147-165 mm FL range with 155 mm mean FL, 

 65-101 g range with 78 g mean W. 

 Age II: 181-188 mm FL range with 184 mm mean FL, 

 122-148 g range with 133 g mean W. 

 Age III: 201-214 mm FL range with 207 mm mean FL, 

 170-217 g range with 190 g mean W. 

 Nicholson (1978) also presents a length-weight equa- 

 tion for gulf menhaden based on these data. 



Aging of gulf menhaden based on scale analysis is 

 problematic, and length-frequency data are not reli- 

 able forassigning age classes. However, otolith analy- 

 sis suggests that age IV fish do exist in the population 



(Vaughan et al. 1996). The bulk of the population is 

 composed of fish from age classes I and II, with few 

 class III and even fewer class IV fish present (Christ- 

 mas et al. 1 988, NOAA 1 992). Sizes at maturity range 

 from 147-165 mm FL (Nicholson 1978). Lewis and 

 Roithmayr (1981) found no maturing ova in fish less 

 than 100 mm FL. Growth information has been com- 

 pared from Florida and Louisiana by Springer and 

 Woodburn (1 960); they found that Florida's menhaden 

 seemed to grow at a slower rate that those in Louisi- 

 ana, and that both groups experienced "a sudden burst 

 of growth after May." Maximum lengths up to 250 mm, 

 and weights up to 300 g have been recorded. Slight 

 sexual dimorphism has been reported for menhaden, 

 but it is insufficient to readily distinguish the sexes 

 (McHugh et al. 1959, Turner 1969, Hoese and Moore 

 1977, NOAA 1992). 



Food and Feeding 



Trophic Mode : Larvae are selective carnivores feeding 

 on zooplankters. Metamorphosis of larvae into juve- 

 niles is accompanied by loss of teeth. Juveniles and 

 adults then become omnivorous filter feeders at the 

 first and second trophic level of the food web utilizing 

 phytoplankton, zooplankton, and detritus (Guillory et 

 al. 1983, Govoni et al. 1983, Deegan 1985, Deegan 

 1986, Deegan et al. 1990, Ahrenholz 1991). Food 

 availability affects swimming speeds, with increased 

 swimming speeds associated with increased food avail- 

 ability in the water column (Durbin et al. 1981). Gulf 

 menhaden are unique in that much of their stored 

 energy is lipid which results in the highest energy 

 content per gram weight found among estuarine spe- 

 cies. As predators, gulf menhaden ingest large num- 

 bers of planktonic larvae of other species, but the 

 effects of this predation have not been quantified. Its 

 role as an important forage species is also in need of 

 more research (Christmas et al. 1988). 



Food Items : Small larvae feed on larger phytoplankton 

 and some zooplankton (Ahrenholz 1991). As larvae 

 grow, phytoplankton is replaced in importance by larger 

 zooplankton, such as copepods, tintinnids, pteropods, 

 and invertebrate eggs (Ahrenholz 1991, Chen et al. 

 1 992). The diet of the remaining developmental stages 

 of this species consists of phytoplankton, zooplankton, 

 and detritus (Deegan 1985, Deegan 1986). 



Biological Interactions 



Predation : Gulf menhaden are potential prey fora large 

 variety of predators throughout their life cycle (Ahrenholz 

 1991). Many invertebrate predators (e.g. chaetog- 

 naths), especially in oceanic waters, probably prey on 

 this species (Ahrenholz 1 991 ). Other potential inverte- 

 brate predators include squids, ctenophores, and jelly- 

 fishes. Predation of larval gulf menhaden may be size- 

 selective, with predation highest for smaller larvae 



138 



