The Baltic Macoma: abundance and distribution of an important winter food of diving ducks in Chesapeake Bay

Abstract

Poor water quality and widespread depletion of wild celery (Vallisneria americana) and other submerged aquatic plants important as waterfowl foods has resulted in the continued dependence of canvasbacks (Aythya valisineria) on Baltic clams (Macoma balthica) as their primary winter food. Despite this dependence, no information exists regarding the distribution and abundance of Baltic clams, and changes that occur in Baltic clam populations over time. We conducted benthic surveys to determine the distribution and abundance of Macoma balthica and M. mitchelli in major tributaries of the upper and middle Chesapeake Bay. Tributaries sampled included the Chester River south to Monie Bay on the Eastern Shore, and Middle River to the Potomac River on the western shore of Maryland. Data on depth, bottom type, shellfish, and submerged aquatic vegetation were recorded. The Baltic clam had the highest frequency of occurrence (50%), followed by M. mitchelli (36%). Other species of clams occurred in less than 14% of the 2995 sites sampled. In addition, every third month since June 1991, we have studied seasonal changes in Baltic clam abundance and recruitment at 13 high abundance sites. Recruitment was recorded at only one high density site (97% increase) between June and September transect surveys. Clam length histograms clearly indicated a new year class and depletion of older cohorts. Average summer decline in Baltic clam abundance at the other 12 sites was 59% and ranged from -11% to -97%. Based on clam length, younger cohorts were depleted at a higher rate. More than half of the Baltic clam population was depleted during the summer and spring recruitment was low.