Predicting the Impacts of Climate Change on the Sandbar Shark and Cobia

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

Seasonal shifts in the movement and distribution of green sea turtles Chelonia mydas in response to anthropogenically altered water temperatures

Anthropogenically altered water temperatures (AAWT) have the potential to affect the movement and distribution of marine ectothermic species. Green sea turtles (GSTs) Chelonia mydas are an ectothermic species observed inhabiting 2 sites with AAWT at the northern point of their geographical range in the eastern Pacific. An acoustic receiver array was deployed with temperature loggers at the San Gabriel River, Long Beach, CA, where 2 power plants discharge warm water into the river, and at the 7th St. Basin, Seal Beach, CA, a dredged shallow basin with warmer water compared to surrounding coastal habitats during the summer months. Juvenile GSTs (n = 22, straight carapace length = 45.2 to 96.8 cm) were tagged with acoustic transmitters. Turtles in the basin migrated into the river during winter months when temperatures dropped below 15°C. During the winter, turtles were most frequently detected at the river receiver stations adjacent to and downstream of the power plants. This suggests that GSTs use the warm effluent as a thermal refuge, avoiding colder areas upstream of the power plants and near the river mouth. In the summer, turtles were most frequently detected at receiver stations upstream of the power plants, potentially exploiting areas of the river with higher primary productivity. AAWT sustain the northernmost aggregation of GSTs in the eastern Pacific year round; however, based on GST thermal tolerance, this population is expected to change their movement patterns when the power plants discontinue discharging warm water by 2029

In the face of climate change and exhaustive exercise: the physiological response of an important recreational fish species

Cobia (Rachycentron canadum) support recreational fisheries along the US mid- and south-Atlantic states and have been recently subjected to increased fishing effort, primarily during their spawning season in coastal habitats where increasing temperatures and expanding hypoxic zones are occurring due to climate change. We therefore undertook a study to quantify the physiological abilities of cobia to withstand increases in temperature and hypoxia, including their ability to recover from exhaustive exercise. Respirometry was conducted on cobia from Chesapeake Bay to determine aerobic scope, critical oxygen saturation, ventilation volume and the time to recover from exhaustive exercise under temperature and oxygen conditions projected to be more common in inshore areas by the middle and end of this century. Cobia physiologically tolerated predicted mid- and end-of-century temperatures (28–32°C) and oxygen concentrations as low as 1.7–2.4 mg l−1. Our results indicated cobia can withstand environmental fluctuations that occur in coastal habitats and the broad environmental conditions their prey items can tolerate. However, at these high temperatures, some cobia did suffer post-exercise mortality. It appears cobia will be able to withstand near-future climate impacts in coastal habitats like Chesapeake Bay, but as conditions worsen, catch-and-release fishing may result in higher mortality than under present conditions

Habitat use and behavior of the east Pacific green turtle, Chelonia mydas in an urbanized system

Green sea turtles, Chelonia mydas, are known to inhabit populated and often urbanized areas. To understand turtle habitat use and behavior within these unique habitats, seven juvenile green turtles were fitted with acoustic transmitters (September 2012 – August 2014), of which two transmitters included an accelerometer (AP transmitter). One individual fitted with an AP transmitter was tracked using a passive acoustic array in an urbanized river, the San Gabriel River, Long Beach, CA (33°45’ N, 118°05’ W). Three additional turtles in this river and three turtles (one with AP transmitter) in a restored estuary (33°44’ N, 118°03’ W) in southern California were actively tracked for two non-consecutive 24-h periods. Those fitted with AP transmitters indicated that turtles were less active at night (0.58 ± 0.56 m/s2 and 0.50 ± 0.63 m/s2) than during the day (0.86 ± 0.63 m/s2 and 0.78 ± 0.60 m/s2) at both sites. Activity data and corresponding movements of the actively tracked turtle fitted with the AP transmitter were used to infer resting periods for other tracked individuals. Turtles rested near bridge pilings and runoff outflows in the river to potentially shelter from tidal flow. Turtles used significantly larger daily areas in the urbanized river (0.046 ± 0.023 km2) where resources may be patchier and less abundant, compared to turtles in the estuary (0.024 ± 0.012 km2) where large, dense eelgrass beds are present. Based on the habitat use and behaviors of green sea turtles, it appears that some green sea turtles are able to make use of both highly developed and restored habitats and likely benefit from certain aspects of development

Estimating Shifts in Phenology and Habitat Use of Cobia in Chesapeake Bay Under Climate Change

Cobia (Rachycentron canadum) is a large coastal pelagic fish species that represents an important fishery in many coastal Atlantic states of the U.S. They are heavily fished in Virginia when they migrate into Chesapeake Bay during the summer to spawn and feed. These coastal habitats have been subjected to warming and increased hypoxia which in turn could impact the timing of migration and the habitat suitability of Chesapeake Bay. With conditions expected to worsen, we project current and future habitat suitability of Chesapeake Bay for cobia and predict changes in their arrival and departure times as conditions shift. To do this we developed a depth integrated habitat model from archival tagging and physiology data from cobia that used Chesapeake Bay, and applied the model to contemporary and future temperature and oxygen output from a coupled hydrodynamic-biogeochemical model of Chesapeake Bay. We found that estimated arrival occurs earlier and estimated departure time occurs 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. By mid-century we do not expect habitat suitability to change substantially for cobia, but by end-of-century we project it will significantly decline and shift closer to the mouth of Chesapeake Bay. Our study provides evidence that cobia will have the capacity to withstand near term impacts of climate change, but that their migration phenology varies from year to year with changing temperatures. These findings emphasize the need to incorporate the relationship between fishes and their environment into how fisheries are managed. This information can also help guide managers when deciding the timing and allocation of a fishery

The impacts of warming and hypoxia on the performance of an obligate ram ventilator

Climate change is causing the warming and deoxygenation of coastal habitats like Chesapeake Bay that serve as important nursery habitats for many marine fish species. As conditions continue to change, it is important to understand how these changes impact individual species\u27 behavioral and metabolic performance. The sandbar shark (Carcharhinus plumbeus) is an obligate ram-ventilating apex predator whose juveniles use Chesapeake Bay as a nursery ground up to 10 years of age. The objective of this study was to measure juvenile sandbar shark metabolic and behavioral performance as a proxy for overall performance (i.e. fitness or success) when exposed to warm and hypoxic water. Juvenile sandbar sharks (79.5-113.5 cm total length) were collected from an estuary along the eastern shore of Virginia and returned to lab where they were fitted with an accelerometer, placed in a respirometer and exposed to varying temperatures and oxygen levels. Juvenile sandbar shark overall performance declined substantially at 32 degrees C or when dissolved oxygen concentration was reduced below 3.5 mg l(-1) (51% oxygen saturation between 24-32 degrees C). As the extent of warm hypoxic water increases in Chesapeake Bay, we expect that the available sandbar shark nursery habitat will be reduced, which may negatively impact the population of sandbar sharks in the western Atlantic as well as the overall health of the ecosystem within Chesapeake Bay

Combined Effects of Acute Temperature Change and Elevated pCO2 on the Metabolic Rates and Hypoxia Tolerances of Clearnose Skate (Rostaraja eglanteria), Summer Flounder (Paralichthys dentatus), and Thorny Skate (Amblyraja radiata)

Understanding how rising temperatures, ocean acidification, and hypoxia affect the performance of coastal fishes is essential to predicting species-specific responses to climate change. Although a population’s habitat influences physiological performance, little work has explicitly examined the multi-stressor responses of species from habitats differing in natural variability. Here, clearnose skate (Rostaraja eglanteria) and summer flounder (Paralichthys dentatus) from mid-Atlantic estuaries, and thorny skate (Amblyraja radiata) from the Gulf of Maine, were acutely exposed to current and projected temperatures (20, 24, or 28 °C; 22 or 30 °C; and 9, 13, or 15 °C, respectively) and acidification conditions (pH 7.8 or 7.4). We tested metabolic rates and hypoxia tolerance using intermittent-flow respirometry. All three species exhibited increases in standard metabolic rate under an 8 °C temperature increase (Q10 of 1.71, 1.07, and 2.56, respectively), although this was most pronounced in the thorny skate. At the lowest test temperature and under the low pH treatment, all three species exhibited significant increases in standard metabolic rate (44–105%; p \u3c 0.05) and decreases in hypoxia tolerance (60–84% increases in critical oxygen pressure; p \u3c 0.05). This study demonstrates the interactive effects of increasing temperature and changing ocean carbonate chemistry are species-specific, the implications of which should be considered within the context of habitat. Associated dataset: Gail D. Schweiterman, Daniel P. Crear et al. 2019. Metabolic Rates and Hypoxia Tolerences of clearnose skate (Rostaraja eglanteria), summer flounder (Paralichthys dentatus), and thorny skate (Amblyraja radiata) https://doi.org/10.25773/qmew-c18

Metabolic Rates and Hypoxia Tolerences of clearnose skate (Rostaraja eglanteria), summer flounder (Paralichthys dentatus), and thorny skate (Amblyraja radiata)

These data were collected following methods described in the associated publication: LINK “Combined Effects of Acute Temperature Change and Elevated pCO2 on the Metabolic Rates and Hypoxia Tolerances of Clearnose Skate (Rostaraja eglanteria), Summer Flounder (Paralichthys dentatus), and Thorny Skate (Amblyraja radiata)”. Schweiterman, G.D. et al. 2019 Biology, 8(3), 56

Seawater carbonate chemistry and metabolic rates and hypoxia tolerances of clearnose skate (Rostaraja eglanteria), summer flounder (Paralichthys dentatus), and thorny skate (Amblyraja radiata)

Understanding how rising temperatures, ocean acidification, and hypoxia affect the performance of coastal fishes is essential to predicting species-specific responses to climate change. Although a population's habitat influences physiological performance, little work has explicitly examined the multi-stressor responses of species from habitats differing in natural variability. Here, clearnose skate (Rostaraja eglanteria) and summer flounder (Paralichthys dentatus) from mid-Atlantic estuaries, and thorny skate (Amblyraja radiata) from the Gulf of Maine, were acutely exposed to current and projected temperatures (20, 24, or 28 °C; 22 or 30 °C; and 9, 13, or 15 °C, respectively) and acidification conditions (pH 7.8 or 7.4). We tested metabolic rates and hypoxia tolerance using intermittent-flow respirometry. All three species exhibited increases in standard metabolic rate under an 8 °C temperature increase (Q10 of 1.71, 1.07, and 2.56, respectively), although this was most pronounced in the thorny skate. At the lowest test temperature and under the low pH treatment, all three species exhibited significant increases in standard metabolic rate (44–105%; p < 0.05) and decreases in hypoxia tolerance (60–84% increases in critical oxygen pressure; p < 0.05). This study demonstrates the interactive effects of increasing temperature and changing ocean carbonate chemistry are species-specific, the implications of which should be considered within the context of habitat

Building use‐inspired species distribution models: using multiple data types to examine and improve model performance

Species distribution models (SDMs) are becoming an important tool for marine conservation and management. Yet while there is an increasing diversity and volume of marine biodiversity data for training SDMs, little practical guidance is available on how to leverage distinct data types to build robust models. We explored the effect of different data types on the fit, performance and predictive ability of SDMs by comparing models trained with four data types for a heavily exploited pelagic fish, the blue shark (Prionace glauca), in the Northwest Atlantic: two fishery dependent (conventional mark-recapture tags, fisheries observer records) and two fishery independent (satellite-linked electronic tags, pop-up archival tags). We found that all four data types can result in robust models, but differences among spatial predictions highlighted the need to consider ecological realism in model selection and interpretation regardless of data type. Differences among models were primarily attributed to biases in how each data type, and the associated representation of absences, sampled the environment and summarized the resulting species distributions. Outputs from model ensembles and a model trained on all pooled data both proved effective for combining inferences across data types and provided more ecologically realistic predictions than individual models. Our results provide valuable guidance for practitioners developing SDMs. With increasing access to diverse data sources, future work should further develop truly integrative modeling approaches that can explicitly leverage the strengths of individual data types while statistically accounting for limitations, such as sampling biases.</p