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Marine Ecology Progress Series

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MEPS 620:77-97 (2019)  -  DOI: https://doi.org/10.3354/meps12964

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

Jeremy R. Timbs1,*, Eric N. Powell1, Roger Mann2

1Gulf Coast Research Laboratory, The University of Southern Mississippi, Ocean Springs, MS 39564, USA
2Virginia Institute of Marine Sciences, The College of William and Mary, Gloucester Point, VA 23062-1346, USA
*Corresponding author:

ABSTRACT: The Atlantic surfclam Spisula solidissima supports one of the largest fisheries on the US northeast coast. Using ~30 yr of data from surfclam stock surveys, variance-to-mean ratios (VtMRs) 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; the surfclam is highly patchy across its range. The surfclam’s proclivity for a patchy distribution varied 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 the surfclam into new habitat and extirpating it from nearshore and southern areas. Size-dependent and temporal trends in VtMRs and temporal relative stability in TPL suggest that range expansion is conduced by regional settlement of larvae, followed by biased mortality in suboptimal habitats. This biased mortality ultimately re-establishes the increased patchiness characteristic of larger animals but also predisposes the species to a rapid range shift. Declining VtMRs over time may be a symptom of range expansion along the leading range boundary that has increased the proportion of newly occupied habitat without mature patch characteristics while, at the same time, range recession has removed the older mature patches along the range’s trailing edge.


KEY WORDS: Patchy distribution · Larval transport · Ocean warming · Larval connectivity · Range shift


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Cite this article as: Timbs JR, Powell EN, Mann R (2019) Changes in the spatial distribution and anatomy of a range shift for the Atlantic surfclam Spisula solidissima in the Mid-Atlantic Bight and on Georges Bank. Mar Ecol Prog Ser 620:77-97. https://doi.org/10.3354/meps12964

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