Age data play an important role in the Atlantic surfclam [Spisula solidissima (Dillwyn, 1817)] stock assessment and their accuracy depends on understanding the periodicity and timing of annulus formation. In the United States, the surfclam stock area spans approximately 5° latitude in the northwest Atlantic. In recent years, surfclams at the southern end of this area have undergone changes in abundance, distribution, and growth, all likely due to increasing ocean temperature. To determine whether the periodicity or timing of annulus formation has also changed, shells from live surfclams were collected during every month of the year from three regions spanning 4° latitude: southern New England (northernmost), New Jersey (middle), and the Delmarva Peninsula (southernmost). Edge analysis and marginal increment analysis indicated a single annulus was formed each year in each region and by all age groups, verifying surfclams can still be aged reliably throughout the managed area. Some regional or agespecific variations were also noted. The model-derived peak times of annulus formation were mid-December for southern New England, late November for New Jersey, and early December for Delmarva. The youngest clams in New Jersey and Delmarva formed annuli about a month earlier than the older age classes. Shell lengths at age, estimated from chondrophore heights and fit to a von Bertalanffy growth model, indicated a latitudinal gradient in growth and asymptotic size. Surfclams grew slowest and to the smallest size in Delmarva [k = 0.217 (0.002 SE), L∞ = 153.0 (0.45 SE)] and fastest and to the largest size in southern New England [k = 0.298 (0.002 SE), L∞ = 162.0 (0.35 SE)]. Historic von Bertalanffy growth parameters, some unpublished, are also reviewed. There is some evidence of a decrease in k and L∞ since 1980, particularly in the New Jersey and Delmarva regions where fishing has been historically concentrated.
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Vol. 35 • No. 4