Climate-driven shifts in abundance, distribution, and composition of the pelagic fish community in a rapidly changing Pacific Arctic

Abstract

The Chukchi Sea is experiencing rapid environmental change due to warming temperatures, reductions in sea ice, and increases in transport of warm southern-origin waters into the region. Continued changes in the physical environment may further alter the marine ecosystem, resulting in changes in the community structure and composition. This dissertation examines the role of the environment in structuring the distribution and composition of the pelagic fish community in the eastern Chukchi Sea. Using a multi-platform approach centered around the use of fisheries acoustics, I investigate the impact of changing climate on the distribution of age-0 gadids that have historically dominated the pelagic fish community in the Chukchi Sea in summer. Chapter 2 uses repeat acoustic surveys conducted using autonomous surface vehicles and particle tracking simulations to examine potential movement of the age-0 gadid population in summer. During summer, the size of fishes increased during a period of wind-driven retention on the shelf, indicating that the Chukchi shelf may serve as an important nursery region for these age-0 fishes. Chapter 3 investigates the changes in the abundance and distribution of pelagic fishes observed in summer acoustic-trawl surveys in 2012, 2013, 2017, and 2019. While age-0 Arctic cod (Boreogadus saida) remain the dominant pelagic fish, walleye pollock (Gadus chalcogrammus), a boreal species, have become increasingly abundant in recent years, driven by increases in temperature and transport on the Chukchi Sea shelf. This suggests that environmental conditions now allow pollock to extend their northern range into the southern and central Chukchi Sea, at least on a seasonal basis. In Chapter 4, two years of seafloor-mounted moored echosounder observations are used to identify the seasonal abundance and movement of the age-0 fishes observed in summer. The moorings identify strong seasonal trends in fish abundance and pelagic community composition driven by the advective transport of fishes from the south. The cumulative flux of fishes to the northeast suggests that these age-0 fishes are likely exported off the shelf in late fall and winter. Chapter 5 presents a classroom activity on underwater acoustics for use in elementary to undergraduate education, emphasizing the importance of hands-on experiential learning and ocean technology. Together, these findings indicate that the composition of the pelagic fish community in the Chukchi Sea is primarily structured by transport. Changes in the transport of Pacific-origin water is increasing the abundance of boreal species in the Chukchi Sea, with the potential to further alter the composition of the pelagic community in the future. This dissertation provides insight into how the integration of observations across a wide range of spatial and temporal scales can help identify the mechanisms that drive the distribution and composition of pelagic fishes. These findings further our understanding of the mechanisms that drive fish abundance and distribution in the Chukchi Sea, and the identification of the role of transport in shaping fish distributions in the region provides us with information that will help to predict the future state of Arctic fish communities.