Alosa aestivalis (Mitchill, 1814)

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 : 682-684

3 | BLUEBACK HERRING

3.1 | Life history

3.1.1 | Diet

Blueback herring consume a variety of prey, including pelagic and benthic species; however, their diet is more selective compared to alewife and primarily feed on zooplankton ( Mullen et al., 1986; Stone & Daborn, 1987). As juveniles grow and move from their natal rivers into estuaries, larger zooplankton, such as adult copepods (e. g., Eurytemora affinis and Cyclops vernalis ), become the primary diet (Table 1; Mullen et al., 1986). Adults in the marine environment are predominantly planktivorous and continue to consume zooplankton, although they also consume small fishes, eggs of other fish, and crustaceans such as pelagic shrimp (Table 1; Bigelow & Schroeder, 1953; Munroe, 2002).

3.1.2 | Growth

Adult blueback herring can reach lengths of 197 – 322 mm in fork length and weigh 93 – 468 g for ages 3 – 12 ( Jessop, 1993). Like alewife, adults are sexually dimorphic, where females are slightly larger and have an average total length of 289 mm, whereas males average 277 mm in length (Table 1; Bowlby & Gibson, 2016; Loesch & Lund, 1977). However, it is unclear whether there are further size similarities between alewife and blueback herring. Contradictory reports on size differences in Nova Scotia stated that blueback herring were substantially smaller than alewife on average ( Bowlby & Gibson, 2016), whereas a different study found blueback herring to be only marginally smaller ( Jessop, 1993). Parameters from the VBGF for blueback herring demonstrate the observed sexual dimorphism and the argument that blueback herring are smaller in size than alewife, with L ∞ ranging from 231.33- (males) to 259.85 mm (females), K ranging from 0.469 (females) to 0.590 (males), and t 0 ranging from 0.283 (females) to 0.338 (males; Messieh, 1977).

Most of the growth in blueback herring occurs during the first couple of years at sea and plateaus after they reach sexual maturity. Adults exhibit very little growth between spawning events, such that individuals ranging from 3 to 5 years old only grow an average of 13 mm between annual spawning (Table 1; Mullen et al., 1986; Bowlby & Gibson, 2016). Therefore, blueback herring that have spawned multiple times are smaller for their given age, particularly in weight, suggesting an energetic trade-off between frequent spawning and at-sea growth (Table 1; Bowlby & Gibson, 2016). Sex-specific variation exists within age at first reproduction, where maturation takes 4 – 5 years for females and 3 – 4 years for males. For instance, the highest proportion of first-time male spawners is at ages 3 and 4, accounting for 46.9% and 49.8% of the individuals in each age class, respectively ( Marcy, 1969), whereas most first-time female spawners are older, ages 4 and 5, and account for 74.6% and 16.4% of individuals observed at each age class during spawning events ( Marcy, 1969). The causes for this variation remain unclear ( Bowlby & Gibson, 2016).

3.1.3 | Spawning

Blueback herring are iteroparous and will spawn up to four times before death (Table 1; McBride et al., 2014; Bowlby & Gibson, 2016). They exhibit a degree of spawning site fidelity and will typically return to the same river to spawn (Table 1; Bozeman Jr & Van Den Ayvle, 1989); however, the frequency of return and the proportion that stray to novel spawning sites are unclear. It is speculated that blueback herring rely on olfactory mechanisms, similar to alewife; however, their migratory cues back to natal rivers are not well understood. Additionally, it is unknown whether ovary weight and fork length are related, but research suggests fecundity positively correlates with age and decreases with increasing latitude when measured gravimetrically (Table 1; Jessop, 1993). The length of the spawning season prolongs with increasing latitude and may be a function of cooler northern temperatures ( Loesch & Lund, 1977; Lynch et al., 2015), likely compensating for the decreased fecundity at these latitudes.

Although spawning typically occurs in freshwater ( Bigelow & Schroeder, 1953), blueback herring will also fertilize eggs in slightly brackish ( Kuntz & Radcliffe, 1918) and fully brackish waters ( Breder, 1948). Regardless of salinity, studies suggest that spawning occurs beyond the influence of the tide ( Bigelow & Schroeder, 1953; Hildebrand, 1963; Hildebrand & Schroeder, 1927). Despite the overwhelming similarities between alewife and blueback herring, there can be some spatial and temporal isolation between the two species during spawning events ( Loesch & Lund, 1977). Both species can differ spatially by spawning in different systems or within the same river system but in different areas depending on spawning habitat preferences ( Loesch & Lund, 1977). Temporal isolation occurred during the spawning migration to the Tusket River, Nova Scotia; alewife returned to spawn more than 4 weeks earlier than blueback herring (Table 1; Bowlby & Gibson, 2016).

3.1.4 | Mortality

In situ observations during field studies in Nova Scotia suggest that the maximum age of blueback herring is variable. In the Tusket River, the oldest individual observed was 8 years old ( Bowlby & Gibson, 2016); however, during sampling across the Tusket, Mactaquac, Gaspereau, and Margaree rivers, the maximum age observed was 12 (Table 1; Jessop, 1993). Females tend to live longer than males, and males are more abundant in younger age classes (3 – 5 years), whereas the abundance of older female blueback herring (7+ years) tends to be far greater ( Loesch & Lund, 1977).

As forage fish, blueback herring provide an important connection between marine and estuarine food webs, transferring nutrients from their oceanic zooplankton food source to coastal piscivore predators ( Ames & Lichter, 2013). Death in the marine environment is largely due to predation by a variety of predators, including spiny dogfish ( Squalus acanthias ), Atlantic cod ( Gadus morhua ), silver hake ( Merluccius bilinearis ), white hake ( Urophycis tenuis ), Atlantic halibut ( Hippoglossus hippoglossus ), bluefish ( Pomatomus saltatrix ), weakfish ( Cynoscion regalis ), striped bass ( Morone saxatillis ), seals, gulls, and terns ( Munroe, 2002).

3.2 | Behavior

3.2.1 | Migration and foraging

Blueback herring have a marine range from Florida ( Hildebrand & Schroeder, 1927) to Nova Scotia (Table 1; Bigelow & Schroeder, 1953; Rulifson & Dadswell, 2020), where prespawned individuals spend 2 – 5 years growing and feeding after leaving their natal rivers ( Loesch, 1987). Despite the extensive proportion of their lives spent at sea, relatively little is known about blueback herring marine ecology in comparison to their freshwater, riverine migrations (Bethoney, Stokesbury, & Cadrin, 2014; Neves, 1981; Rulifson & Dadswell, 2020; Stone & Jessop, 1992). However, recordings of long-distance feeding migrations show migratory routes from North Carolina to Nova Scotia, which align with the routes of American shad ( Rulifson & Dadswell, 2020). There is also evidence of the separation into migratory contingents, where a proportion of blueback herring are semi-resident in estuarine regions near their natal rivers and use inshore embayments for feeding instead of undertaking large offshore migrations (Table 1; Stone & Jessop, 1992; Rulifson & Dadswell, 2020). Due to the complex movement dynamics in each population, summer foraging aggregations consist of mixed stocks and therefore include multiple contingents from different spawning rivers (Table 1; Rulifson & Dadswell, 2020).

When foraging in the marine environment, blueback herring benefit from schooling behavior (i.e., optimizing feeding and avoiding predators, etc.) and group into single-species aggregations at sea or school with alewife (Table 1; Bethoney et al., 2013; Rulifson & Dadswell, 2020). During summer feeding migrations, blueback herring follow zooplankton throughout the marine environment. There is evidence that blueback herring undertake diel vertical migrations ( Jessop, 1990), which mimic the diel movements of zooplankton (Table 1; Neves, 1981). However, the availability of suitable water temperatures may limit their vertical feeding migrations ( Stone & Jessop, 1992). Similar to their vertical movements, the horizontal migrations of blueback herring are likely tied to zooplankton concentrations at different latitudes throughout the year, outside of the spawning migration ( Neves, 1981; Stone & Jessop, 1992). Researchers in the Minas basin in Nova Scotia have found evidence of filter-feeding among anadromous blueback herring, although they may also undertake active foraging in the marine environment (Table 1; Stone & Daborn, 1987).

After foraging at sea during summer, blueback herring move southward in the fall and aggregate in offshore overwintering sites in the Middle Atlantic Bight ( Neves, 1981) and along the Scotian Shelf from the Gulf of Maine to the Bay of Fundy ( Stone & Jessop, 1992). Neves (1981) speculated that temperature and availability of zooplankton drive the selection of specific overwintering sites. Although blueback herring are typically associated with inshore habitats and remain in shallower waters than alewife (Bethoney, Stokesbury, Schondelmeier, et al., 2014; Neves, 1981), during winter, they are found comparatively deeper, likely seeking warmer bottom water, preferring water>5 C offshore ( Stone & Jessop, 1992). This temperature preference suggests that blueblack migration strongly follows temperature contours in the northern limit of their range ( Canada; Table 1; Stone & Jessop, 1992).

In early spring, blueback herring start to occupy mid-depth, nearshore waters along the Atlantic coast as they move northward to reach spawning rivers in late spring ( Neves, 1981; Stone & Jessop, 1992). The timing of arrival for the spring spawning migration is, again, thought to be temperature-driven, timing their movements to coincide with an optimal water temperature range of 14 – 22 C for spawning (Table 1; Bi et al., 2021, Loesch & Lund, 1977, Ogburn et al., 2024). Blueback herring prefer warmer spawning temperatures than alewife, and so, they generally arrive later than alewife in their shared rivers ( Bigelow & Schroeder, 1953). For blueback herring, this leads to a late-April arrival in spawning rivers (although this varies), and they can remain at these sites for several months into the summer before they return to sea ( Jones et al., 1978).