Alosa sapidissima (Wilson, 1811)

4 | AMERICAN SHAD View in CoL

4.1 | Life history

4.1.1 | Diet

On their migration back to sea, adult American shad prey on smaller freshwater fish, including shield darters ( Percina peltata ) and, at times, cannibalize juvenile American shad (Table 1; Chittenden Jr., 1976b). Beyond these nearshore data, there are only a few studies to rely on for characterizing American shad diet in the marine environment; however, adults are known to return to their planktonic diet at sea and primarily feed on copepods and mysids ( Walburg & Nichols, 1967). Stomach content analyses have also shown that adults prey on small fish (Table 1; Facey & Van Den Avyle, 1986; Walburg & Nichols, 1967). More information is needed to better understand American shad diet at sea during juvenile and adult life stages.

4.1.2 | Growth

In addition to freshwater environments, YOY American shad also use estuarine areas as nursery grounds ( Crecco et al., 1983; McCormick et al., 1996), typically in low-velocity waters where food availability is higher ( Limburg, 1996). Juveniles grow to 38 – 114 mm long before they emigrate into the marine environment in autumn ( Bigelow & Schroeder, 1953; Mitchell, 1925), suggesting that growth and factors associated with size influence the timing of seaward emigration (Chittenden Jr., 1969; Miller et al., 1973). Most growth occurs at sea, where prespawned fish spend 2 – 6 years until they reach sexual maturity (Table 1; Bigelow & Schroeder, 1953; Walburg & Nichols, 1967). Males reach maturity younger than females, at around 2 years or when they reach 290 mm (on average), whereas females mature in their third or fourth year when they are closer to 400 mm ( Walburg & Nichols, 1967). As such, American shad are sexually dimorphic; females are typically larger than males within their age class ( Du et al., 2023). Compared to alewife and blueback herring, American shad are the largest among these Alosidae , and adults can grow up to 760 mm and weigh up to 5500 g ( Bigelow & Schroeder, 1953). Their maximum size depends on latitude, and northern populations reach larger sizes than their southern contingents (Table 1; Gilligan-Lunda et al., 2021; Poulet et al., 2023; Robins et al., 1986). The VBGF parameters for American shad reinforce their size difference when compared to the parameters for alewife and blueback herring, with an average of 481 mm for L ∞, 0.44 for K, and 0.32 for t 0 ( Gilligan-Lunda et al., 2021).

4.1.3 | Spawning

American shad exhibit spawning site fidelity to their natal rivers (Table 1; Carscadden & Leggett, 1975; Hill, 1959; Nichols, 1966; Talbot, 1954), with low levels of straying (3%; Melvin et al., 1986) in some populations ( Mansueti & Kolb, 1953; Williams & Daborn, 1984). Spawning strategies also differ latitudinally where northern populations are iteroparous and can spawn five ( Grote et al., 2014; McBride et al., 2016) to seven times ( Provost, 1987), whereas southern populations are semelparous (reproduce once then die; Table 1; Leggett & Carscadden, 1978; Poulet et al., 2023). Semelparous populations include those that spawn in the St. Johns River, Florida ( Limburg et al., 2003) and the Ogeechee River, Georgia ( Sykes, 1956). This semelparity observed in southern populations appears to be an adaptation to the consistent weather in the south, resulting in a higher probability of successful recruitment and consequently higher fecundity, when defined as the rate of successful recruitment ( Carscadden & Leggett, 1975; Roff, 1992).

4.1.4 | Mortality

American shad have been reported to live up to 13 years, and there is no recorded evidence of sexual dimorphism regarding maximum age, like in blueback herring populations ( Altman & Dittmer, 1962). However, the natural mortality rate does differ based on latitude, where northern populations reach an older maximum age due to alternate reproductive styles, much like alewife ( Gilligan-Lunda et al., 2021; Poulet et al., 2023). Other barriers that limit survival include predation, target fisheries, and by-catch (Table 1; Bailey et al., 2004; Bethoney et al., 2013). At sea, American shad are preyed on by sharks, bluefin tuna ( Thunnus thynnus ), kingfish ( Scomberomorus cavalla ), and porpoises in the southern United States ( Walburg & Nichols, 1967). Passive tagging studies on American shad have also revealed that dogfish ( S. acanthias ) and Atlantic cod ( G. morhua ) also prey on American shad ( Dadswell, 1990). Additionally, there are records of seals preying on American shad as they begin their spawning runs into river mouths ( Dadswell, 1990). Although northern American shad are iteroparous, like gaspereau, it is not uncommon for adults to die on their spawning migration. For instance, in northern New Jersey, dead, egg-bound females were found during their migration upstream to spawning grounds, likely attributable to starvation (Chittenden Jr., 1976a). Therefore, attrition of American shad when they enter freshwater forms a key part of the species' marine demography.

4.2 | Behavior

4.2.1 | Migration and foraging

American shad exhibit a complex marine migration pattern influenced by both environmental cues and intrinsic factors. They undertake extensive seasonal migrations in surface waters ( Neves & Depres, 1979) along the northwest Atlantic, spanning from Newfoundland and Labrador ( Dempson et al., 1983; Limburg et al., 2003) to Florida (Table 1; Williams & Bruger, 1972). YOY juveniles either spend the first year in the lower estuary of their natal river ( Hoffman et al., 2008; Leggett & Whitney, 1972) or exhibit short residency, staying within fresh water for the summer, then migrating to sea in the fall ( Greene et al., 2009; Neves & Depres, 1979). Upon entering the marine environment, juveniles join large intraspecific schools of immature and postspawning adults for feeding and growth ( Bigelow & Schroeder, 1953; Dadswell et al., 1987). As water temperatures cool later into the fall, schools migrate in surface waters ( Bigelow & Schroeder, 1953) southward to overwintering sites, which include deeper waters, 40 – 175 km offshore Florida, the Mid-Atlantic Bight, and along the Scotian Shelf ( Collette & Klein-Macphee, 2002; Dadswell et al., 1987). The aggregations in these overwintering sites are heterogeneous mixtures of American shad populations from many rivers ( Dadswell et al., 1987).

Spawning can occur when water temperatures cool to 8 – 26 C ( Walburg & Nichols, 1967); however, peak spawning begins when water temperatures fall between 12 and 21 C ( Jessop, 1975; Jones et al., 1978). Therefore, spawning migrations are temporally influenced, and southern populations begin migrating toward natal rivers in January, with some reaching the southern limit of their range in the St. Johns River, Florida (Table 1; Limburg et al., 2003). Northern populations begin migrations to spawning rivers progressively later into the spring as latitude increases ( Limburg et al., 2003). During the summer, spawning continues upstream from the Delaware River to the St. Lawrence River, and northern American shad populations concentrate in the surface waters of foraging grounds ( Leim, 1924; Themelis, 1986) in the inner Bay of Fundy, the inner Gulf of St. Lawrence, and off of Newfoundland and Labrador (Table 1; Dadswell et al., 1987).

Foraging behavior varies based on life stage and habitat. Walter III and Olney (2003) demonstrated that adult stomach fullness index was highest in the marine environment (Table 1), followed by estuarine habitats, then freshwater, indicating the majority of feeding takes place at sea. There is little information about juvenile stomach fullness; however, juveniles are known to form feeding schools along the coast ( Greene et al., 2009; Neves & Depres, 1979) and demonstrate diel feeding patterns primarily foraging in the evening ( Johnson & Dropkin, 1996; Massmann, 1963). Additionally, planktivorous adults feed both passively by filtering prey as they swim and by actively ambushing ( Harris & McBride, 2009), suggesting that American shad mainly consume what they encounter and feed if suitable prey is available ( Atkinson, 1951; Leim, 1924; Walter III & Olney, 2003).