Date of Award

Fall 12-2022

Document Type


Degree Name

Master of Science (MS)


Ocean & Earth Sciences


Ocean and Earth Sciences

Committee Director

John M. Klinck

Committee Member

Eileen E. Hofmann

Committee Member

Daphne Munroe


The Atlantic surfclam (Spisula solidissima) is a long-lived benthic biomass dominant organism that occurs on the Middle Atlantic Bight (MAB) continental shelf between 10 m and 50 m. Trends in Atlantic surfclam population specific growth and mortality rates were analyzed using four decades of age and length observations obtained from NOAA stock surveys from the 1980s to 2010s in six regions distributed along the MAB. Atlantic surfclam specific growth rates and asymptotic lengths were estimated from the age and length observations using the von Bertalanffy growth model. The analysis showed that the Atlantic surfclam median asymptotic length in the southern regions of the MAB was smaller, 88 mm, relative to asymptotic lengths of 110 mm to 141 mm estimated for the northern survey regions. The asymptotic lengths estimated from observations in the southernmost survey region declined by 35% over the four decades. Constant and age-dependent specific mortality rates were estimated from the age and length observations with the relationship given in Hoenig (1985) and a hyperbolic tangent relationship based on the change in number with age, respectively. The decadal averaged specific mortality rate overall survey regions of 0.20 y−1 is consistent with mortality rate estimates from literature, but considerable variability was obtained within and between survey regions. The highest specific mortality rates of 0.18 yr−1 to 0.58 yr−1 were associated with the southern survey regions, where the Atlantic surfclam population had age distributions skewed to younger ages. The estimated specific growth and mortality rates were input to a numerical model that simulates the population dynamics of Atlantic surfclams to assess controls on the alongshelf gradient in population density. A comparison of simulated distributions with observed distributions showed that age-dependent mortality is the primary determinant of the Atlantic surfclam population density gradient. The trends in population growth and mortality rates reflect changes in Atlantic surfclams imposed by warming bottom water temperatures. These results have implications for projecting Atlantic surfclam responses to a warming habitat.


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Copyright, 2022, by Mauricio González Díaz, All Rights Reserved.