Assessing Predator Risk to Diadromous Fish Conservation in the Penobscot River Estuary

• Main Author: Leach, Lauri
• Format: Text
• Language: unknown
• Published: DigitalCommons@UMaine 2020
• Subjects: pinniped; predator-prey interactions; river restoration; Atlantic Salmon; prey buffering; photo-identification; Marine Biology; Terrestrial and Aquatic Ecology; Atlantic salmon; Northwest Atlantic; Phoca vitulina; Salmo salar
• Online Access: https://digitalcommons.library.umaine.edu/etd/3324; https://digitalcommons.library.umaine.edu/cgi/viewcontent.cgi?article=4400&context=etd

Successful conservation of pinnipeds in the northwest Atlantic has led to increasing populations of harbor (Phoca vitulina) and gray seals (Halichoerus grypus) in the Gulf of Maine. Seals are often perceived as predators and competitors for fish, and as a result, come into conflict with fisheries and fish conservation. Increasing numbers of seals have become a recent concern in the Penobscot River Estuary in Maine, as habitat restoration and diadromous fish conservation have been top priorities in this region for the past decade. To understand how pinnipeds are responding to these efforts, as well as the risks they pose to diadromous fish, we evaluated spatial and temporal overlaps in the presence of seals and diadromous fish from 2012 to 2020. Utilizing data from a survey in the estuary, counts of seals on haul-outs, as well as presence of swimming seals, were compared across seasons and years, and related to fish biomass estimates. Seal presence in the estuary peaks in the spring, and we did not detect significant differences in counts of hauled out seals in recent years. We detected a non-significant, negative relationship between seal count and fish biomass, presumably due to relative presence of each peaking in different seasons. Seal predation of endangered Atlantic Salmon (Salmo salar) is a concern, so we assessed the risk seals pose to salmon. Salmon demographic data, including the presence of seal-induced injury, have been collected from salmon passing upriver at the southernmost dams. A comparison of seal-induced injury rate from 2012 to 2019 revealed that seal-induced injury has been declining. Rather than being most influenced by local seal populations, increasing river herring returns were strongly related to declining seal-induced injury rates, highlighting the importance of prey buffering in this system. We used photo-identification to understand how individual seals are using the estuary. Photographs were taken of individual seals between 2019 and 2020. Overall, 27 harbor seals and 88 gray seals ...