A series of fall surveys and spring resurveys from 2005 to 2010 were examined to determine whether overwinter mortality was a common occurrence in oyster populations from Delaware Bay. Box counts tended to be lower in the spring more often than expected by chance. Occasionally, counts of live oysters also declined. A proportionately larger reduction in dredge efficiency during the winter for boxes compared with live oysters was judged the most likely reason, rather than disarticulation of boxes. Little evidence for overwinter mortality was found; however, reefs in the center of the salinity range exhibited a larger decrease in live oysters than boxes in the spring resurvey. This happenstance provided the only evidence for overwinter mortality that could be distinguished above the countervailing trend provided by the decline in catchability of boxes that frequently occurred. Before ascribing variations in population descriptors during the winter to biological processes, the alternative of an overwinter change in the efficiency of the sampling gear must be excluded.