Date of Award

Fall 12-2020

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)


Marine Biology


Kristina Cammen

Second Committee Member

Gayle Zydlewski

Third Committee Member

Stephanie Wood


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 were identified. There were 62 harbor seal and 29 gray seal resighting events. Continued efforts could allow for eventual abundance estimates of gray seals using the lower estuary. Comparison to other photo-identification catalogs in the region would also allow a better understanding of how these animals utilize regional sites and an improved understanding of the role pinnipeds play in marine and coastal ecosystems.

It appears that seal presence in the Penobscot is based more on regional trends in seal abundance and distribution rather than local prey biomass, however, it is possible that pinnipeds may be slower to respond to restoration efforts than the fish species in this system. Better understanding pinniped response to restoration and changing predator-prey interactions in this system could help reconcile the competing objectives of marine mammal protection and fish restoration. A multi-species approach is essential to successful conservation. A healthier ecosystem can benefit both predators and prey, as improved water quality, increased access to historic habitat, and rebounding fish populations are already proving to have significant benefits to multiple species that use the river.