Date Awarded

2022

Document Type

Thesis

Degree Name

Master of Science (M.Sc.)

Department

Virginia Institute of Marine Science

Advisor

Roger L Mann

Committee Member

Mary C Fabrizio

Committee Member

Donglai Gong

Committee Member

Eric N Powell

Abstract

Climate change is one of the greatest challenges the natural world currently faces, particularly in marine ecosystems, as many marine organisms are sensitive to warming water temperatures and other aspects of climate change. The Mid Atlantic Bight region is warming rapidly in comparison to the rest of the world’s oceans. In the face of climate change, organisms must either adapt to altered environmental conditions or shift their distribution to avoid extinction. One such example of the latter is the Atlantic Surfclam (Spisula solidissima), whose range has exhibited a shift over the past 4 decades, recorded in both stock assessments and fishing industry activity. This organism supports a substantial fishery, worth approximately $30 million annually. The surfclam’s optimal temperature range is relatively narrow, with thermal stress occurring around 20oC and death occurring with sustained temperatures beyond 24oC. This organism’s sensitivity and exposure to climate change make it likely that this species will exhibit a climate-driven range shift, which may be incorrectly interpreted as a response to fishing pressure in an otherwise well managed stock. This study used growth data stored in archived Atlantic Surfclam shell hinges to investigate the relationship between climate drivers and the historic range shift of this species. Shells used in this analysis came from federal survey collections performed by the Northeast Fisheries Science Center in the years of 1986, 2008, and 2011-2016. A random 10% sample was imaged and aged to fit von Bertalanffy growth curves to individual clams. Parameters from the von Bertalanffy growth model (maximum size and the growth coefficient, k, used as a proxy for growth rate) were used to summarize clam growth. Linear models were constructed to use individual and environmental variables (location, station depth, temperature, time, and age at collection) to predict indices of clam growth. Results found that depth had a negative effect on maximum size; clams grew smaller in deeper water and larger in shallower water. The most useful temperature metric was number of extreme warm days (days above 20oC). Temperature had a negative effect on maximum size, but a positive effect on growth rate; clams grew faster but to smaller maximum sizes with higher occurrence of extreme warm events, likely reflective of respiratory and physiological demands not being met by feeding ability at high temperatures in large clams. Over time, clams in the south decreased in size while clams in the north increased in size, suggesting a northward movement of optimal range, potentially explained by increased food abundance in northern areas of this region. Over time, growth rate in deeper depths increased, but did not match the earlier reversed pattern in magnitude of higher growth rates in shallower waters, suggesting that an offshore movement is occurring but may be limited by decreased light penetration, benthic primary production, and food availability at increasing depths. These results describe a relationship between environmental variables and surfclam growth, supporting the occurrence of a historical range shift driven by climate change. These results may be useful in future studies predicting continued effects of climate change on this species growth and distribution.

DOI

https://dx.doi.org/10.25773/v5-jm09-k444

Rights

© The Author

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