For probably a thousand years, harbor seals have raised their pups in the shelter of Disenchantment Bay. For nearly as long, the Yakutat Tlingit Tribe has relied on the seals as an important part of their diet and culture. In recent decades, increasing numbers of cruise ships began touring this glacial fjord, where passengers might witness a four-story iceberg calving from Alaska’s largest tidewater glacier.
A team of scientists is working to make sure seals and humans can continue to coexist in this unique and essential habitat.
The team put Argos satellite tags on 45 seal pups in June 2016 as part of an ongoing study to find out how cruise ship traffic affects their behavior, which could have implications for the growth and survival of harbor seals in this glacial fjord. The tagged pups then transmitted detailed information about their movements and environment. That was a game changer.
“In the past we’ve done aerial surveys of the whole population, getting the big picture. Now we are able to get detailed behavioral information on individual seals. That means we can begin to close the information gap on whether disruption by cruise ships affects seal populations,” said marine mammal biologist John Jansen of NOAA’s Alaska Fisheries Science Center. “We know that seal behavior is disrupted if ships get too close, and we know what distance is too close, but we don’t know if that disruption could have a negative impact on the harbor seal population.”
At the same time they track pups, the team monitors and maps vessel traffic. With this information, they figure out how vessel traffic distance and frequency corresponds with pup behavior. The results help resource managers determine whether further action needs to be taken to minimize disturbance by vessels.
Jansen is co-leader of this collaborative research with other scientists from NOAA Fisheries, the National Park Service, University of Alaska Fairbanks, and The Marine Mammal Center. The town of Yakutat has provided support including landing craft for deploying the ship tracking system, and the local Tlingit tribe has collaborated and contributed Traditional Knowledge.
“They hunt seals for subsistence from childhood and know a lot about seal behavior and where they typically haul out. That knowledge helped shorten our learning curve in the field,” Jansen says.
Harbor Seals in a Jam?
For a harbor seal pup in a glacial fjord, resting on ice is a necessary part of growing up.
When a pup rests on an ice floe it is saving energy. Because it stays dry, the pup uses little energy to stay warm, and can instead use the energy from mom’s milk to grow and gain the blubber needed to survive the winter.
A pup in the cold glacial water spends energy to stay warm. If it spends too much time in the water, it can fail to grow and fatten up, and that has serious consequences.
“Glacial seals have unique challenges. The water in glacial fjords is much colder than in other environments where harbor seals aggregate. When you spend time in this water, you lose more energy,” Jansen explained. “Pups in other environments might normally spend 40-70% of their time in the water. But if glacial pups spend more than 50% of their time in the water, they might divert more energy to keeping warm and end up weighing less than other pups at the end of summer. If pups go into their first winter at a lower weight, they are less likely to survive.”
That’s how disruption by vessel traffic could affect seal populations. When a ship gets too close, a harbor seal pup dives for safety into the water. If ships get too close too often, the pup spends too much time in the water, and not enough time resting on ice, growing and gaining blubber before winter.
An advantage of glacial sites is the floating ice that allows pups to spend more time out of the water compared to those using tidally-flushed rocks or sand bars. This may explain why Alaskan glacial areas have some of the largest aggregations of harbor seals in the world and may be akin to nurseries: ideal habitat.
In the late 1980s fewer than 12 cruise ships a year visited Disenchantment Bay. By the late 1990s, 130-140 ships were touring the fjord each summer. Around that time the local Tlingit tribe expressed concern that the seal population in the bay was declining.
“We began this work in response to their concern,” said Jansen. “Our previous work allowed us to document short-term reactions of seals to ships, but the missing piece is whether those reactions affect the population’s long-term stability. The work we are doing this year and next will begin to answer that question.”
Pups Making Tracks
The pups that were tagged in June 2016 transmitted their locations via satellite, and the initial results expanded our understanding of harbor seal behavior.
“Harbor seals, especially juveniles, are known for making some pretty lengthy migrations. But we are surprised at how far very young pups are venturing out—hundreds of miles,” Jansen said.
Twenty pups ventured outside of the shelter of Disenchantment Bay. Some headed north out of the bay, others south, with no obvious pattern other than staying close to shore. Some visited Icy and Columbia bays and “got a taste of life in another glacial bay.” One pup left Disenchantment Bay, out to sea, then came back--“maybe a training trip with mom,” said Jansen.
These 2016 results were a preliminary look at pup movement and behavior. The main focus of the study will be the period before pups wean and leave Disenchantment Bay. The team will find out how much time each pup spends resting on ice versus swimming in water. They will compare this data with the level of ship traffic that each pup is exposed to. Using estimates of how much energy pups use in and out of the water, they will determine whether disturbance by ship traffic is likely to affect pup growth, therefore jeopardizing pup survival.
“We’re also interested in the possible emigration of these pups and whether they come back to Disenchantment Bay next spring” said Jansen. “With any luck (and enough battery life) these satellite tags will answer that question, transmitting the pups’ locations periodically for their whole first year of life.”
What if cruise ship traffic does play a role in the decline of harbor seals in Disenchantment Bay? The solution is simple, according to Jansen: give the seals space. In response to concerns about disruption, NOAA implemented vessel approach guidelines, suggesting that cruise ships keep a voluntary 500 meter distance from seals resting on ice, as well as avoiding particular areas and times when seals typically haul out on ice. Jansen reports anecdotal evidence that cruise ships are respecting that distance. “We saw ships entering and leaving the bay on the far side of Haenke Island, away from the area where seals usually haul out.”
Maybe the solution is already underway.