Since 1978 the Town of Islip, NY has conducted annual surveys in Great South Bay, NY of hard clam (Mercenaria mercenaria) and macro-benthic predator abundance. This comprehensive data set was analyzed using an Empirical Orthogonal Functional Analysis (EOF) and a Virtual Population Analysis (VPA). The former provided insights into patterns and correspondences of hard clams and predators; the latter provided an analysis of changes in hard clam population structure over the past 25 years. The EOF analysis showed that fluctuations in predator abundance are: 1) mostly in phase over the survey region and 2) dominated by year-to-year fluctuations in abundance. Results for the hard clams show that abundance fluctuations are: 1) in phase over the survey region and 2) dominated by a decreasing trend in abundance over time. The primary EOF modes essentially were uncoupled which implies no strong predator-prey interactions between the predators and hard clams.

The VPA showed that hard clam age classes 1 through 3 were under-represented in the abundance measurements because these sizes did not contain enough hard clams to support the population at larger age classes. The mortality rate calculated from the VPA clustered into two primary groups. From the 1980s to the early 1990s, hard clam abundances remained at approximately 0.4 - 0.5 ind m^-2 and the population was characterized by a mortality rate of about -0.4 yr^-1. From the mid-1990s to present, Great South Bay hard clams have been characterized by lower population densities and by an apparent decline in 7-8 cm clams that is not observed in larger clams. The decrease in these clams results in unrealistic mortality rate estimates because insufficient clams occur at smaller lengths to support the observed abundances of the larger clams. Since the early 1990s, the Great South Bay hard clam population has continued to age, skewing the length-frequency distribution to larger lengths, and inputs of small hard clams are insufficient to maintain a normal length-frequency distribution. These results suggest that a fundamental change in hard clam population structure in Great South Bay occurred in the early to mid-1990s.