Steingass S, Horning M, Bishop AM. Space use of Pacific harbor seals (Phoca vitulina richardii) from two haulout locations along the Oregon coast. PloS one. 2019 Jul 31;14(7):e0219484. Click Here for Open Access Paper
A quickly changing coastline
The Oregon coast is a dynamic and vibrant 363-mile stretch of the eastern Pacific. While a few thousand miles separate the coasts of Oregon and Alaska, the coastal ocean in both places hosts many of the same species, and in some cases, animals that travel thousands of miles annually between winter feeding grounds and spring breeding, pupping and calving habitat.
The Oregon coast is part of the Northern California Current Large Marine Ecosystem; this is one of four ‘eastern boundary current systems’ – eastern coastlines around the world that are highly productive and important for fisheries and biodiversity. It is home to at least 29 species of marine mammals, and each year around $740,000,000 of commercial fisheries landings are brought into Oregon ports, making this environment important for animals as well as people.
Visit one of more than 90 locations on the Oregon coast, and you will see one of Oregon’s most ubiquitous coastal wildlife species – the Pacific harbor seal. There are an estimated 10,000-12,000 of these animals in the state today; however this has not always been the case. Prior to the enactment of the 1972 Marine Mammal Protection Act, numbers of harbor seals in the state had decreased to around 500 total. This was largely due to state-issued bounties on these animals as they were seen as a nuisance to fisheries.
Radio tracking studies, visual surveys, and dietary analysis have all been conducted on seals in Oregon throughout the years. However, recent studies have not kept up with the increase of animals that has taken place since the 1970’s. Since populations of these animals stabilized around a decade ago, little has been done in terms of studying the ecology and behavior of these animals in Oregon.
As part of my doctoral dissertation, I attached Wildlife Computers SPOT5 tags to 24 adult harbor seals in 2014-15. We caught animals on the beach, primarily using large ‘hoop nets’ – essentially large durable nets with flexible hoops that are used to catch animals individually – weighed them, and attached satellite tags to the back of the head using glue or epoxy.
These efforts were made possible by a team of AMAZING people who dedicated their time and effort to catch and monitor seals. This includes team members from UC Santa Cruz, San Jose State University, Moss Landing Marine Laboratories, Alaska SeaLife Center, CICESE Baja, Oregon State University, Portland State University, and the Marine Mammal Center.
After the tags sensed that they had entered the water via two electrical current sensors that complete a circuit in salt water, they began transmitting to a network of satellites each time the animal reached the surface. Since the tags were able to sense whether they were ‘wet’ or ‘dry’, we could also tell when animals were likely on land, or in the water. The SPOT tags transmitted anywhere from 20-324 days and provided an unprecedented dataset of harbor seal movement on the Oregon coast, painting a picture of the ecology of a common marine mammal in our coastal ocean.
The seals literally ‘painted’ the continental shelf – they used most aspects of it north of the tumultuous and swift Cape Blanco area, where the continental shelf narrows creating strong currents. I recorded both the ‘home range’ of animals – where 95% of logged locations fell – and the ‘core area’ – an area representing high use, meaning they were most important to individuals.
What types of habitat did animals use?
All in all, nearly 50% (okay, 46.70%) of habitat use was within bays, rivers and estuaries. Part of this is because pinnipeds have to spend time on land between foraging trips. However, they also spent a fair amount of times in these locations in the water, meaning they were doing something other than just sleeping. Each trip the animals took between resting on land took an average of 26.96 hours; matching pretty closely to Oregon’s 22 hour tidal cycle. This matches up pretty well with what we know about seals – that they tend to spend time on land when tides are lowest. When they were on land, seals spent on average 9.43 hours, but this was also pretty variable between individuals.
Seals’ home ranges included a wide area of Oregon’s continental shelf, including sections just outside of bays and estuaries, as well as Heceta Bank. Heceta Bank is a section of the Oregon continental shelf that spans nearly 70km westward from the shore, and is highly productive. The sea mounts at the edge of the bank also proved to be important areas.
Animals’ core areas were primarily regions very close to a number of bays. This included the Siuslaw River, Alsea Bay and Netarts Bay (the two tagging locations), Tillamook Bay, Siletz Bay, and Yaquina Bay. What this means is that many animals spent a lot of time near where they were tagged. This shows that seals showed what is termed ‘site fidelity’ – being faithful to certain haulouts where animals rest and return to after a day’s foraging. However, some animals did travel quite a bit seasonally, visiting certain areas likely based on prey availability. For instance, one animal traveled from Alsea Bay and overwintered at Desdemona Sands in the Columbia River.
As much as some animals traveled, some didn’t at all. One animal, who was tagged for nearly two months, never seemed to leave a 3km area in Netarts Bay. This must mean that they had found a reliable food source very nearby, making it unnecessary to travel very far! To contrast this, the average core area of all animals was 364 km2.
What did we learn?
All in all, a lot of valuable information was gained from this study for a common marine mammal that we don’t know too much about. First, while animals were pretty faithful to the sites where they were originally captured, they tended to travel a lot. Which means, the seals you see one day at a particular site may not be the same seals day-to-day. Individuals were also very variable and unique in how they used their habitat – each seal had different movement patterns, pointing to the fact that they each have their own strategies for finding food. This relates to other data that suggests that while populations of seals are very general in their ecological role, individual animals tend to specialize in their behaviors, relying on predictable and seasonal food resources throughout the year.
There is still a lot to be learned – this study focused on adult seals, and primarily males (23 of the 24 animals we caught and tagged were males). We also don’t know what particular food sources these animals were relying on during their daily activities. A different tag in the future – perhaps more precise GPS tags with time-depth recorders could let us know where in the water column these seals are spending their time.
But this data, even though it is just the beginning, is very important to understanding how our coastal ocean works. Marine mammals are sentinels for the health of our oceans, and can highlight areas of high productivity or seasonal importance for other species. Furthermore, with our oceans quickly changing, it is important to know today how things are, so that we can understand how these things can change tomorrow. If we have no past data to compare, how do we know how or if anything is ‘normal’? I hope this dataset is helpful for the future of coastal management and conservation, and that more information like this can continue to be gathered in the future
Written by: Dr. Sheanna Steingass, Marine Mammal Program Leader, Oregon Department of Fish and Wildlife
All activities conducted under NMFS MMPA Permit #16991