The Chesapeake Bay is the largest estuary in the United States and is highly productive making it a key habitat for species like striped bass and summer flounder. Striped bass and summer flounder have exhibited changes in abundance over recent years within the Chesapeake Bay. There is a lack of understanding of how environmental factors may be impacting striped bass and summer flounder abundance and how other species may be responding to these environmental drivers. To improve our understanding of striped bass, summer flounder and the aquatic community we set out to identify the environmental drives of their abundance changes from local to global scales. The fish community in the Bay has previously shifted due to environmental perturbations, so we also investigate if the community has changed more recently. We used a hurdle model to standardize interannual abundance of fish species caught in the Chesapeake Bay from two sources of fishery-independent trawl survey data. This standardization process led to relative abundance indices for 58 late juvenile to adult species from 2002-2018, and relative abundance indices for 26 juvenile species from 1995-2019. Species with similar and contrary abundance trends with striped bass and summer flounder were identified through a correlation analysis, and life history traits were assessed between species to determine mechanisms of change. We then used nonmetric multidimensional scaling (nMDS) to see if the community structure had drastically changed, the applied a principal response curve (PRC) to investigate the spatial and functional group change of the community. Among the factors examined, sea surface temperature (SST) in the Bay has increased since 2006 based on change point analysis while the mean Bay SST range and gradient have both decreased. Striped bass have had variable abundance in the Bay but exhibited a positive correlation with increasing SST. Summer flounder have declined in abundance since 2006 in the Bay and exhibited a negative correlation with increasing SST. Striped bass and summer flounder had relationships with global climate oscillations such as the winter North Atlantic Oscillation, Atlantic Multidecadal Oscillation, and the Gulf Stream North Wall oscillation. From a community perspective, our results showed that the late juvenile to adult fish community shifted after 2011, which coincided with the freshwater surge due Hurricane Irene and Tropical Storm Lee. The increase in abundance of striped bass, Bay anchovy, and Atlantic menhaden, and decrease in abundance of summer flounder, weakfish, spot, and Atlantic croaker were the main species that contributed to the difference in community structure after 2011. The change in functional group structure after 2011 was defined by a decline in opportunistic demersal fish, and this change was significantly different in the portion of the Bay north of the Potomac River. We did not identify any significant correlation between functional groups and environmental factors but did discover the important relationship that top predators like striped bass and summer flounder have with their key fish prey species, both in presence and population trends. Long term monitoring and further research in to how the community changed over smaller periods and the distribution changes of species could improve our understanding of what is impacting the Chesapeake Bay community to inform better management strategies. / Master of Science / The Chesapeake Bay is an important habitat for many animals, including fish. Of the fish species that inhabit the Bay, striped bass and summer flounder are two highly sought after commercial and recreational species, and are top predators, which means they are economically and ecologically important. Striped bass and summer flounder have exhibited changes in abundance over recent years within the Chesapeake Bay, and there is a lack of understanding what environmental factors may be driving their abundance trends and if other species are responding similarly. To conserve striped bass and summer flounder, management strategies should consider the relationship these species have with their environment and other species. Through this research, we set out to understand what environmental factors are impacting their population trends, and the trends of the aquatic community which can be used to inform effective management decisions for the future. To accomplish our research goals, we first used a hurdle model to determine the interannual abundance of 58 adult and 26 juvenile fish species from 2002-2018 and 1995-2018 respectively. We then investigated whether any environmental factors exhibited relationships with striped bass and summer flounder. Again, through correlation analysis, we identified species with similar and contrary abundance trends with striped bass and summer flounder and compared the traits of these species to try to identify the mechanisms of their abundance trends. To visualize if and how much the fish community composition had changed over time we used nonmetric multidimensional scaling (nMDS), then applied a principal response curve (PRC) to investigate the spatial change and life history trait change in the fish community. The results showed us that striped bass and summer flounder do exhibit relationships with environmental factors. Sea surface temperature (SST) in the Bay has increased, while the range and gradient has decreased, and an increase in the mean SST occurred in 2006. We found that striped bass had variable abundance in the Bay but correlated positively with increasing SST, however summer flounder abundance has decreased since 2006 in the Bay and has a negative correlation with SST. Striped bass and summer flounder exhibited relationships with global climate-oscillations that impact the Chesapeake Bay, making those factors important to consider for their management. The results of the community assessment showed that the late juvenile to adult fish community shifted after 2011. This change in community structure coincided with a decrease in water quality and a freshwater surge in 2011 that was caused by Hurricane Irene and Tropical Storm Lee. After the community shift, striped bass, Bay anchovy, and Atlantic menhaden increased in abundance, while summer flounder, weakfish, spot and Atlantic croaker decreased in abundance. These species were the top species that contributed to the change in community structure. The change in community structure was greatest, and significant in the northern portion of the Bay. From these results we identified the important relationship between predators and prey both in species presence and abundance trends. Striped bass abundance increased alongside an increase of their key prey species, Bay Anchovy and Atlantic menhaden, while summer flounder abundance decreased along with two of their key prey species spot and Atlantic croaker. This research identified environmental factors that contribute to abundance trends of striped bass and summer flounder and highlighted the importance of multispecies interactions within the aquatic Chesapeake Bay community. Long term monitoring and more research into finer scale spatial and temporal changes of fish in the Bay could further improve management recommendations.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/118619 |
| Date | 27 March 2024 |
| Creators | Oakley, Josephine Marie |
| Contributors | Fish and Wildlife Conservation, Jiao, Yan, Smith, Eric P., Kindsvater, Holly K. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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