Regional and temporal variability in distribution and abundance of bay anchovy (Anchoa mitchilli) eggs, larvae, and adult biomass in the Chesapeake Bay

Patterns and variability in reproductive output of pelagic fish are seldom determined at the ecosystem scale. We examined temporal and spatial variabi lity in spawning by bay anchovy (Anchoa mitchilli), and in distribution and abundances of its pelagic early-life stages, throughout Chesapeake Bay. On two cruises in June and July 1993, ichthyoplankton and zooplankton were co llected on 15 transects at 18.5-km (10 nautical mile) intervals over the 26 0-km length of the bay. Finer-scale sampling was carried out in a grid of s tations between two transects on each cruise. Regional abundance patterns o f bay anchovy eggs and larvae in the lower, mid, and upper Bay were compare d with zooplankton abundances, environmental variables, and biovolumes of t wo gelatinous predators-the scyphomedusa Chrysaora quinquecirrha and the lo bate ctenophore Mnemiopsis leidyi. Abundances of anchovy eggs, and, especia lly, larvae were higher in July than in June. Baywide daily egg production increased from 4.25 x 10(12) in June to 8.43 x 10(12) in July. Concentratio ns of zooplankton that are potential anchovy prey nearly doubled on a baywi de basis between June and July, while biovolumes of the ctenophore declined . Except for scyphomedusan biovolumes, all analyzed organisms differed regi onally in abundance and were patchily distributed at 1-km to 10-km sampling scales. Negative correlations between larval anchovy abundances and gelati nous predator biovolumes suggested that predation may have controlled abund ances of bay anchovy early-life stages. Biomasses of adult anchovy, estimat ed from daily egg productions, were higher in the lower Bay and remarkably similar-23,433 tons in June and 23,194 tons in July. Most spawning by bay a nchovy occurred during July in the seaward third of Chesapeake Bay, emphasi zing the importance of this region for recruitment potential of the Bay's m ost abundant fish.