Spatial and Temporal Patterns in Recruitment Process SCALE AND PATTERN IN RECRUITMENT PROCESSES OF BAY ANCHOVY IN CHESAPEAKE BAY

Recruitment of bay anchovy (Anchoa mitchilli) varies annually in Chesapeake Bay, and levels and patterns are related to variability in hydrological conditions and the spatial distribution of spawning stock biomass. Midwater-trawl surveys, conducted three times annually from 1995-1999, over the entire 320-km length of the Bay, provided information on annual and regional patterns of recruitment, and their relationships to variability in the estuarine environment. Adult biomass of anchovy within the Bay at the beginning of spawning seasons in 1995-1999 varied six-fold, but it alone was not directly related to the young-of-the-year (YOY) recruitment level. Levels of recruitment in October were low in 1995 and 1996 (6 to 7 X 10) but higher in 1997-1999 (19 to 52 X 10). An important feature of the recruitment process is an ontogenetic migration in which YOY bay anchovy tended to move upbay until they are approximately 45 mm TL, after which they begin to move downbay. The strong salinity gradient may act as a partial barrier to upbay or downbay migration, the effect being more pronounced for small (<60 mm TL) anchovy. However, seasonal water temperature was more important in determining the latitudinal distribution of spawning stock biomass. Late-summer recruitment of YOY anchovy was high when water temperature was low in April-May, inhibiting upbay migration of adults at the onset of the spawning season, and insuring that most spawning occurred in the lower and middle region of the Bay. A modified Ricker stock-recruitment model that included the latitudinal range of adult migration between April and July, explained 98% of recruitment variability.