Life History of Hickory Shad in the St. Johns River, Florida

Abstract

Hickory shad Alosa mediocris spawn in rivers from Maryland to Florida, but little research has been published regarding their life history. We collected hickory shad in the St. Johns River, Florida, during five consecutive spawning years from 2000 to 2005. Hickory shad migrated upstream by December and remained in the river and its tributaries until March. Females were larger than males and accounted for 17.7% of the adults collected. All but one female examined were mature and contained yolked oocytes. No females had hydrated oocytes, but four had fresh postovulatory follicles (POFs < 24 h old), indicating a batch spawning pattern. Females with fresh POFs were collected from Shad Alley and the Wekiva and Econlockhatchee rivers, but otherwise the spawning grounds could not be delineated. Fish constituted over 97% of the diet by weight and were found in 11.5% of the stomachs examined. Scale and otolith ages produced the same age range, from 1 to 7 years. A test of symmetry did not detect systematic age differences between the two methods, although exact agreement was only 57%. Spawning marks on scales indicated that mature hickory shad ranged in age from 2 to 7 years and that most spawning individuals were age 3 or 4. Hickory shad are iteroparous, and fewer than 50% showed evidence of previous spawning. Our estimates of sex ratio, age distribution, and age at maturity differed from values measured in the 1970s by Williams et al. (1975). We are uncertain whether the later age at maturity in the early 2000s relative to the 1970s was due to a biological change or a difference in sampling methods between the two studies. There is an apparent latitudinal pattern in the timing of the spawning migration for hickory shad; individuals in more northerly rivers spawned later in the season than those in southern systems.