MEPS prepress abstract  -  DOI:

Changes in the spatial distribution and anatomy of a range shift for Atlantic surfclams Spisula solidissima in the Mid-Atlantic Bight and on Georges Bank

Jeremy R. Timbs*, Eric N. Powell, Roger Mann


ABSTRACT: Atlantic surfclam Spisula solidissima support one of the largest fisheries on the US northeast coast. Using ~30 yr of data from surfclam stock surveys, variance-to-mean ratios (VtMR) were calculated both temporally and spatially for a range of surfclam size classes to determine the degree of patchiness. The VtMR declined from the 1980s to present in all regions (offshore Delmarva, New Jersey, Long Island, Southern New England, Georges Bank); however, VtMR rose with increasing clam size. Taylor’s power law (TPL) analysis corroborated the VtMR; surfclams are highly patchy across their range. The clam’s patchy proclivity varied significantly regionally. Regions supporting the bulk of the stock were characterized by significantly higher degrees of patchiness and exhibited a higher exponent for the TPL. A species distribution function model corroborated findings of declining patchiness over time, supporting the hypothesis that warming of Mid-Atlantic continental shelf bottom waters is both driving surfclams into new habitat and extirpating them from nearshore and southern areas. Size-dependent and temporal trends in VtMR and temporal relative stability in TPL suggest that range expansion is conduced by regional settlement of larvae, followed by biased mortality in suboptimal habitats, which ultimately re-establishes the increased patchiness characteristic of larger animals but which also predisposes the species to a rapid range shift. Declining VtMR over time may be a symptom of range expansion along the leading range boundary increasing the proportion of newly occupied habitat sans mature patch characteristics coupled with range recession removing the older mature patches along the range’s trailing edge.