POPULATION BIOLOGY OF BAY ANCHOVY ANCHOA-MITCHILLI IN THE MID-CHESAPEAKE BAY

The population biology of bay anchovy Anchoa mitchilli was studied from July 1986 to December 1987 to estimate relative abundance, age structure, growth and mortality. Trawl surveys indicated that mean abundance, indexed by catch-per-unit-effort (CPUE), was higher in the July-November period in 1986 than in 1987. Mean CPUE was highest in September of each year when catches were dominated by recruiting, young-of-the-year (YOY) anchovy. Annuli on otoliths indicated the presence of 4 age classes. Maximum age was 3+, when some individuals were 85 mm fork length (FL) and >5 g wet wt. The population consisted primarily of YOY and age 1+ individuals. Observed lengths-at-age were highly variable. Although most growth in length was completed by age 1+, significant increases in length and, especially, weight occurred in older fish. The parameters of a von Bertalanffy growth model, fitted to the means of back-calculated lengths at otolith annuli and fall marks, are: L(infinity) = 129.3 mm FL, K = 0.23, and t(0) = -1.23 yr. The best estimates of mean fork length-at-age were generated by a seasonally oscillating version of the von Bertalanffy model, with L(infinity) = 107.0 mm FL, K = 0.36 and t(0) = -0.81 yr. Fork lengths-at-age, based upon this model, are: age 1, 50.4 mm; age 2, 67.6 mm; and age 3, 79.5 mm. The instantaneous natural mortality rates of recruited anchovy are high. Annual mortality rates, estimated from catch curve analyses, ranged from 2.2 to 2.9 (89 to 95% yr(-1)). The fast growth and high mortality rates indicated that maximum cohort biomass is achieved during the late postlarval or early juvenile period.