Gizzard shad, continued 



during the fall and winter. Adults in salt water migrate 

 upstream to spawn during spring months (Gunter 

 1938, Gunter 1945, Pineda 1975, Jones et al. 1978). 

 The increased abundance in inshore waters during 

 winter months (November-February) may be due to 

 this upstream spawning movement (Chambers and 

 Sparks 1959). 



Reproduction 



Mode : Reproduction is sexual, with separate male and 

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

 cast, and fertilization is external. 



Spawning : Spawning takes place in freshwater sloughs, 

 ponds, lakes, and rivers, from mid-March to late Au- 

 gust, with a peak from April to June in temperate 

 waters. A second spawn may occur in late summer in 

 some areas. This spawning period is generally later 

 and more prolonged than that of Alabama shad l/\losa 

 alabamae) or American shad (Alosa sapidissima) (Swift 

 et al. 1 977, Lippson et al. 1 979). Eggs are scattered in 

 open water or along the shoreline. Several individuals 

 of each sex are often involved at the time of gamete 

 release, which usually takes place at midday with rising 

 temperatures that range from 10 to 28.9°C. They are 

 reported to be most active around 18°C (Miller 1960, 

 Bodola 1 966, Kelley 1 965, Jones et al. 1 978, Manooch 

 1984). 



Fecundity : Reported fecundity ranges from 3,000 to 

 543,900, but can change with age, averaging 59,480 at 

 Age 1, 378,990 at Age II and declining to21 5,330 at Age 

 VI (Bodola 1966, Manooch 1984). 



Growth and Development 



Egg Size and Embryonic Development Eggs are de- 

 mersal and adhesive, sticking to the substrate (rocks, 

 sticks, roots, etc.) if it is not covered with sediment. 

 Fertilized eggs are creamy yellow, nearly transparent, 

 and 0.75 mm in size. When eggs are first extruded they 

 are hard and irregularly shaped, but become spherical 

 after contact with water. The incubation period is 

 temperature dependent and lasts from 36 hours to 1 

 week. Egg hatching occurs after 95 hours at 1 7°C and 

 36 hours at 27°C (Lippson and Moran 1974, Jones et 

 al. 1978). 



Age and Size of Larvae : At hatching larvae are around 

 3.25 mm TL. This stage lasts for a few weeks, during 

 which the alimentary canal develops into the form 

 necessary for omnivorous filter-feeding (Miller 1960). 



Juvenile Size Range : The juvenile stage is reached at 

 about 20 mm TL. Juveniles mature in about 2 or 3 

 years, with some females maturing as soon as 1 year. 

 Average length at maturity is 178-279 mm TL. 



Age and Size of Adults : In Florida, gizzard shad aver- 

 aged about 254 mm after the first year, 31 7.5 mm after 

 the second and 345.4 mm after the third with none 

 surviving to the fourth year. In other areas, particularly 

 temperate freshwater locations, growth is much slower 

 with a life span extending to almost 10 years (Miller 

 1960), but most fish die before they are 7 years old 

 (Manooch 1984). This species has attained lengths up 

 to 520.7 mm TL, but does not commonly grow larger 

 than 254 to 355.6 mm TL (Miller 1960). 



Food and Feeding 



Trophic Mode : Gizzard shad are primarily filter-feeders 

 (Miller 1 963). For a short period after hatching, larvae 

 are carnivorous. Juveniles and adults become filter- 

 feeders. They may feed both on the bottom and in the 

 water column, and may or may not be selective (Baker 

 and Schmitz 1971). 



Food Items : During the first few weeks as larvae, the 

 primary food items are small animals, such as proto- 

 zoa, waterfleas (Cladocera), copepods and ostracods 

 (Miller 1 960). After this initial phase when the intestine 

 has had a chance to develop, there is a switch to algae, 

 zooplankton, detritus, and bottom sediments contain- 

 ing benthic infauna (Miller 1963, Baker and Schmitz 

 1971, Lippson etal. 1979). 



Biological Interactions 



Predation : Although this species provides a forage 

 base for predator fish, the rapid first year growth of the 

 gizzard shad often makes it nearly invulnerable to 

 predation by the fall of its first year (Jenkins 1 970, Lee 

 et al. 1 980). Known estuarine predators of this species 

 include spotted gar and longnose gar (Bonham 1 940, 

 Darnell 1 958), and freshwater predators include large- 

 mouth bass (Micropterus salmoides) (Houser and 

 Netsch 1971) and white bass (Morone chrysops) 

 (Netschetal. 1971). 



Factors Influencing Populations Gizzard shad popula- 

 tions usually grow rapidly when introduced into new 

 systems (e.g., reservoirs), possibly due to abundant 

 detritus and other food sources. Where gizzard shad 

 are abundant, they affect the populations, growth and 

 habitat of game fish such as largemouth bass 

 {Micropterus salmoides) and crappie (Pomoxis spe- 

 cies) (Jenkins 1 970, Guest et al. 1 990). Where they co- 

 occur with threadfin shad (Dorosoma petenense), it is 

 possible that the two species compete for available 

 food sources (Baker and Schmitz 1971). Winter kills 

 occasionally occur in the lower Great Lakes, and when 

 they do, gizzard shad provide a source of food for birds 

 (Miller 1 960). Extensive die-offs may also occur in late 

 summer (Mettee et al. 1996). 



150 



