Some Atlantic menhaden become sexually mature during their second year (late age-1), but most do not mature until their third year (late age-2). Spawning occurs primarily in late fall and winter. Thus, most Atlantic menhaden spawn for the first time at age-2 or 3, i.e., just before or after their third birthday (by convention, on March 1), and continue spawning every year until death. First-spawning age-3 fish have accounted for most of the stock's egg production since 1965. Atlantic menhaden mature at smaller sizes at the southern end of their range, 180 mm FL in the south Atlantic region versus 210 mm FL in the Chesapeake Bay area and 230 mm in the north and middle Atlantic regions, because of latitudinal differences in size-at- age and the fact that larger fish of a given age are distributed farther north than smaller fish of the same cohort.

The growing season begins in the spring and ends in the fall as water temperatures rise above and fall below 15°C. Atlantic menhaden reach lengths of about 500 mm TL and weights of over 1.5 kg. Fish as old as age-8 were fairly common in the spawning population during the 1950s and early 1960s, but fish older than age-6 have been rare in recent years. However, an exceptionally large (433 mm FL; 1,551 g) Atlantic menhaden from Chesapeake Bay taken in August 1996.

There is evidence for density-dependent growth in Atlantic menhaden, at least in young fish. Comparison of annual weights at age for age-1, -2, and -3 fish and age-1+ population size estimates for the 1955-84 period indicated an inverse relationship between the two parameters, suggesting that growth was accelerated during the late 1960s in response to low population size and reduced during the mid-1970s and early 1980s when population size was high. The reduction in mean weight at age 3 was particularly dramatic, declining 60% between 1976 and 1978 and remaining low through 1984. However, other studies demonstrated that the growth rates of fish after recruitment in their first year of life was not correlated with abundance, but did depend on size at recruitment, indicating that fish probably recruited at smaller sizes in years of high juvenile population size and vice-versa. Thus, density-dependent effects probably occur during the estuarine nursery period. Negative correlations between the mean lengths of age 0.5 and 0.75 fish and the number of recruits at age 0.5 support this hypothesis. The observed decline in sizes at age in the fishery is also due in part to a shift in fishing to the south where smaller fish at a given age are found.