Alosa pseudoharengus (Wilson, 1811)

Brown, Caliyena R., Sergio, Ava J. A., Bate, Caitlin S., Koopman, Natalie, Roland, Joshua B., Notman-Grobler, Oscar D. P., Mastrodimitropoulos, Paris M. B., Piczak, Morgan L. & Lennox, Robert J., 2025, A review of migratory Alosidae marine ecology in the northwest Atlantic, Journal of Fish Biology 106 (3), pp. 677-695 : 678-682

2 | ALEWIFE

2.1 | Life history

2.1.1 | Diet

Alewife begin their lives in freshwater, feeding on small, planktonic crustacean nauplii before they grow and move toward the sea ( Bozeman Jr & Van Den Ayvle, 1989). Although the pelagic diet of invasive alewife in the Laurentian Great Lakes is well understood (e. g., Pothoven & Vanderploeg, 2004; Stewart et al., 2009; Strus & Hurley, 1992), thorough descriptions of the diet of anadromous alewife in the Atlantic are still limited. Some insights into their diet in the marine environment were, however, gathered from stomach content analyses from alewife caught along the continental shelf from New England to Cape Hatteras, North Carolina ( Bowman et al., 2000). In that study, non-decapod crustaceans represented the majority of the alewife's prey by weight, and adults were found to primarily filter-feed on copepods and krill. However, the stomach contents of larger individuals (> 20 cm) included mollusks, cnidarians, and juvenile fish, suggesting an ontogenic shift in alewife from primarily filter feeding to a mixed strategy that includes predation on a higher diversity of taxa (Table 1; Bowman et al., 2000).

2.1.2 | Growth

In the Chesapeake Bay estuary, juveniles were found to grow to about 114 – 127 mm long in their first year (Table 1; Hildebrand & Schroeder, 1927); however, once alewife enter the sea, there is a distinct lack of information regarding their rate of growth from the time they enter the ocean to when they return inshore. Several studies have shown that alewife are sexually dimorphic where females spend a longer time at sea and reach a larger average adult length of 284.3 mm (standard deviation = 15.77), whereas males reach an average length of 271.6 mm (standard deviation = 13.09; Table 1; Fay et al., 1983; Marjadi et al., 2019). The observed sexual dimorphism is also exhibited in the model parameters from the von Bertalanffy growth function (VBGF; Bertalannfy, von L., 1938) where L ∞ is the mean asymptotic length of fish, K is the Brody growth coefficient and represents how quickly fish approach L ∞, and t 0 is the hypothetical age at which fish length is equal to zero ( x -intercept; Gilligan-Lunda et al., 2021). For alewife, L ∞ ranged from 291.67- (males) to 310.48 mm (females), K ranged from 0.4 (females) to 0.441 (males), and t 0 ranged from 0.103 (females) to 0.142 (males; Messieh, 1977). Alewife take 3 – 6 years to mature at sea before returning to rivers to spawn (Table 1; Loesch, 1987). Individuals at lower latitudes are typically shorter lived and quick to mature, which is consistent with the latitudinal relationship across ectotherms (Table 1; Munch & Salinas, 2009).

2.1.3 | Spawning

Alewife preferentially return to their natal stream or pond, relying on olfactory mechanisms to detect the odor of the water from which they hatched (Table 1; Thunberg, 1971). The frequency of iteroparity in alewife populations is latitudinally influenced, with northern latitudes having a higher percentage of repeat spawners annually (Table 1; Fay et al., 1983). Iteroparity also varies at smaller spatial scales between sites, suggesting that river-specific drivers, like anthropogenic impacts (habitat degradation, overfishing, dams, and loss of river connectivity), are primary determinants of the survival and fidelity of anadromous alewife populations ( Hare et al., 2021; Spares et al., 2023). In the Gaspereau River in Nova Scotia, the alewife stock is heavily impacted by overfishing; of the previously spawned adults, only 11.1% of males and 7.4% of females returned the following years, compared to an estimated 50% return rate in rivers with minimal anthropogenic stressors ( Gibson, 2000). Unlike salmonids, for which males expend more energy spawning and females have higher rates of repeat spawning ( Jonsson et al., 1991), interannual returns are higher among male alewife than females (Table 1). However, this may be attributed to artificial selection, with the longer mean length of females at spawning age increasing their likelihood of being caught in gillnets, decreasing their survival during spawning season ( Spares et al., 2023).

2.1.4 | Mortality

Southerly alewife live only 3 – 4 years, compared to their northern counterparts, which can live up to 9 – 10 years (Table 1; Fay et al., 1983). Predation is a primary cause of natural mortality, and alewife contribute to the diet of piscivorous fishes, birds, and mammals ( Hare et al., 2021). Freshwater predation is a major bottleneck, and some predators may feed exclusively on alewife during their spawning migration ( Hare et al., 2021), whereas predation at sea is more diffuse. Anadromous alewife store substantial energy from the ocean in their fatty tissues, contributing to the nutrient input of freshwater systems following their annual spawning events. Similar subsidies to the marine environment must be relevant, although they are less commonly considered than the subsidies provided to freshwater systems for anadromous species (Table 1; Dias et al., 2019). Although there is ample research on marine isotopes carried by anadromous fishes into fresh water ( Durbin et al., 1979), there is little comparable work to find freshwater isotopic signatures in the marine environment that might suggest how alewife provide a reciprocal subsidy to the ocean via their anadromy. Nevertheless, alewife have shown to be significant to the diets of demersal groundfish, indicating a high degree of predation mortality at sea ( Link & Garrison, 2002). Young of year (YOY) alewife immigrating from estuaries to the northeastern coastal shelf have even been suggested to influence the movement behavior of predator species (i.e., co-migration; Hare et al., 2021). Historically, the pursuit of alewife by Atlantic cod ( Gadus morhua and other gadids) may have been a key driver of gadid movement into estuaries and river mouths where alewife aggregate to spawn ( Ames & Lichter, 2013). In estuaries where alewife were extirpated, inshore gadid populations also disappeared and did not return, suggesting a high predator – prey linkage between gadids and alewife in the northeastern Atlantic and a role of freshwater – marine nutrient subsidies (Table 1; Ames & Lichter, 2013). Further, modeling suggests that if alewife populations increase, their migrations to and from the marine environment could positively impact species of economic importance and conservation concern by acting as a stable food source during unpredictable changes in climate for key marine species ( Dias et al., 2019).

2.2 | Behavior

2.2.1 | Migration and foraging

Anadromous alewife range from North Carolina to Newfoundland (Table 1; Collette & Klein-Macphee, 2002; ASMFC, 2009). YOY alewife typically migrate to marine environments in June and July ( Schmidt et al., 1988). Once at sea, juveniles join large intraspecific feeding schools of similar-sized individuals, with smaller alewives preferring shallower regions compared to larger adults (Table 1; Stone and Jessop, 1981). These feeding schools are mixed-stock, meaning that several different alewife populations are moving and feeding together ( DFO, 1986; Rulifson & Dadswell, 2020). Alewife are also known to form mixed-species schools with other herrings like menhaden ( Brevoortia spp. ) or blueback herring ( A. aestivalis ; Bigelow & Schroeder, 1953; Rulifson & Dadswell, 2020).

Despite sustaining a critical fishery, there are limited investigations of the marine migratory movement of alewife ( Gibson et al., 2017; Huveneers et al., 2016; Neves, 1981; Stone & Jessop, 1992; Tsitrin et al., 2020; Tsitrin et al., 2022; Ogburn et al., 2024). However, a recent acoustic telemetry study demonstrated that alewife from Choptank River, Maryland, migrated northward into Georges Bank and the Gulf of Maine in the summer and moved southward in the fall and winter ( Ogburn et al., 2024). These findings validate the yearly migratory patterns inferred from catch and by-catch data, which suggest that alewife move northward and inshore in the spring, as many prepare to return to fresh water for spawning, and finally offshore and southward in the fall (Table 1; Neves, 1981; Stone & Jessop, 1992). This migration pattern was observed in populations from the Mid-Atlantic Bight, where summer and fall catches were concentrated in Nantucket Shoals, Georges Bank, and coastal Gulf of Maine regions, suggesting they moved northward from their natal stream ( Neves, 1981). Furthermore, winter catches indicated they return to the Mid-Atlantic coastline in the winter and spring (Table 1; Neves, 1981; Tsitrin et al., 2022). These migratory patterns were further validated by alewife catch data along the coast of Nova Scotia that indicated alewife distribution shifted inshore and northward in spring along the Scotian Shelf and offshore and southward in fall toward the Gulf of Maine and Bay of Fundy ( Stone & Jessop, 1992). The extent of offshore overwintering for alewife is still unknown; however, the theorized seasonal distribution of alewife closely resembles American shad migratory movements, which suggests that alewife may use a mixture of inshore and offshore foraging areas annually (Table 1; Neves, 1981).

Alewife movement at sea is presently thought to be regulated by biological factors as they follow zooplankton productivity ( DFO, 1986; Neves, 1981; Tsitrin et al., 2022). When alewife feed on plankton, they undertake diel vertical migrations following the vertical movement of zooplankton throughout the water column (Table 1; Neves, 1981; Stone & Daborn, 1987). Stomach content analyses suggest alewife particulate feed on macrozooplankton when water visibility is high during the day and filter feed on microzooplankton during low visibility at night (Table 1; Gilmurray, 1980; Stone & Jessop, 1992).

Recent studies have demonstrated that alewife marine movement is also influenced by water temperature, tidal currents, salinity, and depth ( Tsitrin et al., 2022; Turner et al., 2016), suggesting their distributions are constrained by suitable oceanographic conditions. For instance, seasonal spawning migrations are thought to be triggered by water temperatures around 5 – 10 C ( Jessop & Parker, 1988; Tsitrin et al., 2022). Therefore, mature alewife begin spawning migrations toward rivers in late March in the south and progressively later into July further north (Table 1; Cole et al., 1980; Mullen et al., 1986). A recent tagging study also suggests that alewife strongly avoid marine temperatures above 14 C ( Tsitrin et al., 2022). In this study, postspawned alewife emigrating from Gaspereau River, Nova Scotia, remained in the Minas basin and foraged in nearshore habitat for 20 days before water temperatures warmed, cueing offshore migration (Table 1; Tsitrin et al., 2022). Similar behavior was observed in American shad, which use cold tidal currents as migration corridors throughout at-sea movement ( Neves & Depres, 1979; Tsitrin et al., 2022).