Date Awarded

Summer 2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Richard W Brill

Committee Member

Rob J Latour

Committee Member

Marjorie A M Friedrichs

Committee Member

Alistair J Hobday

Abstract

A changing climate has been identified as a major driver of changes in marine species’ distribution, phenology, and habitat selection in recent decades and is expected to continue to influence these traits. These changes are not only happening in our oceans, but within coastal habitats as well, where waters are susceptible to sudden changes in temperature and oxygen levels are influenced by nutrient inputs. These changes which will likely impact fish species that utilize these areas as nurseries, spawning habitat, or foraging grounds. In this dissertation I consider climate impacts on two important predators, the sandbar shark (Carcharhinus plumbeus) and cobia (Rachycentron canadum), both of which rely on coastal habitats like Chesapeake Bay for their survival. I used a series of physiological, survey, tagging, and modeling studies to estimate the current and future impacts of climate change on these two species. Sandbar sharks are unable to handle temperatures as warm as 32°C physiology, but in the wild prefer temperatures between 22-26°C. As a result, I estimate bottom habitat losses in Chesapeake Bay by end-of-century for juvenile sandbar sharks. Although they are relatively intolerant of hypoxia (critical oxygen concentration = 3.5 mg l-1), juvenile sandbar shark appear to prefer areas on the fringes of hypoxic zones to avoid larger sharks and find more abundant prey. Therefore, the continued reduction in oxygen levels throughout the entire water column actually improves juvenile sandbar shark suitable habitat. Being a bottom dwelling species, sandbar shark in Chesapeake Bay may be forced to remain in non-preferred bottom habitat, move up in the water column, or shift to shallower habitats. Cobia are tolerant of high temperatures (32°C) and low oxygen (1.7-2.4 mg/l) which should allow them to withstand the detrimental effects of climate change in Chesapeake Bay, at least through mid-century. Hypoxia and elevated temperatures reduce survival of cobia that are exercised to exhaustion. Although the physiology experiments and habitat models suggest cobia will withstand climate change through mid-century, declines in their suitable habitat in Chesapeake Bay are expected by end-of-century. I project arrival time to occur earlier and departure time to occur later when temperatures are warmer and that by mid- and end-of-century cobia may spend on average up to 30 and 65 more days, respectively, in Chesapeake Bay. As conditions worsen in more southern estuaries, cobia may shift spawning habitat in estuaries and bays further north, such as Delaware Bay, New York/New Jersey Bight, and Long Island Sound, where conditions are more thermally suitable. Over the next 60-80 years, suitable cobia habitat is projected to shift northward from spring to fall and to decrease over the U.S. continental shelf. As cobia shift into new areas, the development of regulations in more northern states will become necessary to promote a sustainable cobia fishery. As species shift their distributions as a result of climate change, it is imperative that we understand why and how these shifts are occurring so that both managers and fishers can ensure important resources continue to be fished sustainably.

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