Variability in juvenile growth, mortality, maturity, and abundance of American shad and blueback herring in Virginia

Abstract

Young-of-year (YOY) population dynamics of American shad and blueback herring in Virginia\u27s rivers were examined with an emphasis on variability in growth and mortality rates. In addition, an analysis was conducted to relate juvenile abundance indices of American shad to adult indices to establish a stock-recruitment relationship. to accomplish the stock recruitment relationship, an additional study that examined maturation schedules and inter-annual variability in maturation schedules among stocks was performed. Results of population dynamics studies found that growth and mortality rates of American shad and blueback herring varied by river and year and that conspecific abundance was an important factor in predicting size at the end of summer indicating density-dependent regulation of growth. For both species, instantaneous growth was highest at 40 d and decreased by 100 d as water temperatures decreased each year. American shad Mortality rates increased from 2005 to 2007 with highest rates observed in the James River, while mortality rates for blueback herring had no clear pattern. Large year-classes of Alosa spp. that are smaller in size prior to emigration into the coastal ocean may experience higher mortality rates resulting in recruitment stabilization. Despite the close proximity of four river systems studied, there were persistent differences in growth and mortality rates that support observed trends in year-class strength. High mortality rates observed in the James River support the observation of almost complete recruitment failure of wild American shad in this system. Furthermore, low mortality rates observed in the Rappahannock River are consistent with an increasing trend in recruitment of maturing females, while lower mortality rates and similar growth rates observed in the Mattaponi River compared with the Pamunkey River support long term observations of greater juvenile production of American shad in the Mattaponi River. Young-of-year blueback herring recruitment in Virginia rivers was an episodic process with multiple peaks observed throughout summer indicating that cohorts were not fully vulnerable to sampling and that there may be movement out of adjacent habitats. Maturation of American shad showed consistent patterns with most American shad mature by age 5. However, year-class specific estimates ranged from 50% to 85% mature in the James River, from 59% to 79% in the Rappahannock River, and from 60% to 87% in the York River. Based on current monitoring, the age of full recruitment to the staked gillnet fishery is age 5, corresponding to the age when the largest proportion of the population reaches maturity. Harvest of age 5 females in the historical fishery was probably a contributing factor to its collapse during the 1980s. Persistent differences in maturity schedules of American shad between the James and York rivers, and to a lesser extent with those from the Rappahannock River, show that neighboring stocks that experience similar climate regimes can have stock-specific maturity patterns that persist through time. Using juvenile abundance indices (or hatchery release in the case of the James River), the estimated maturity schedule, and estimated adult mortality rates, resulted in a predicted index that matched observed trends from 1998 to 2006 for three stocks of American shad. This study demonstrates that population dynamics of juvenile Alosa spp. is highly variable in a temporal and spatial context involving density-dependent processes that act to regulate year class production

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