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Alaska Department of Fish and Game

Harbor Seal Research
Vessel Interactions

Background Information

Two harbor seals on the ice with a cruise ship in the background

Glacial fjords in Alaska are popular tourism destinations. Harbor seals are plentiful in some of these fjords, and many may have traveled from elsewhere to pup and breed amid the icebergs. Numerous studies have noted that approaching vessels disturb seals, causing them to leave their resting platforms. There is concern that cumulative effects associated with the additional energetic costs of such disturbance could decrease a seal’s ability to reproduce and, ultimately, to survive.

We monitored behavior and heart rates of radio-tagged seals before, during, and after vessel disturbance (see harbor seal heart-rate study) and are analyzing the data to estimate the energetic costs of a single vessel disturbance to harbor seals. Currently, no data are available to determine how often individual harbor seals are disturbed by vessels. A few disturbances may be inconsequential; however, if an individual is repeatedly disturbed, the impact may be more serious. Pups may be especially at risk; they are small and thin at birth and have approximately 3 weeks to fatten up before they are abruptly weaned and abandoned, and then must learn to forage on their own. Repeated disturbance by vessels could decrease a pup’s nursing and resting time and increase their energetic costs of staying warm if vessels force them to spend more time in the icy water. Continued disturbances after weaning could deplete a pup’s energy reserves during their early, inefficient foraging, which could reduce their probability of survival. Repeated disturbance of reproductive females may result in energetic costs that will deplete their body stores, resulting in termination of pregnancy or inadequate lactation performance, potentially compromising their fitness and the survival of their offspring. Increased stress levels, which may be associated with vessel disturbance that occurs over an extended period of time, can also negatively impact animal health and reproductive success. Any one of these factors, or a combination of them, could result in population declines.

Assessing the impacts of vessel disturbance on harbor seal populations presents a major challenge, because (1) the extent of disturbance caused by vessels needs to be determined, and subsequently, (2) the complex sequence of negative energy balance, increased stress, and weight reduction that could result in decreased survival and reproduction would need to be documented. Vessel disturbance does not result in immediate mortality (except in the case of lethal collisions, not generally reported in harbor seals), nor does it result in immediate failed reproduction. Rather, the cumulative effects of multiple disturbances have the potential to negatively impact survival or reproduction of harbor seals.

Because multiple additional factors (e.g., nutrition, disease, and predation) can also affect survival and reproductive success, no single research project can definitively make a direct link between vessel disturbances and population processes in harbor seals. Nonetheless, documentation of the frequency of vessel disturbance can provide information about whether vessel disturbance occurs frequently enough to have the potential to negatively affect harbor seals; information that is needed to make informed management decisions. Specifically, if the frequency of disturbance is found to be low, the potential for negative impacts at the individual and population level are likely inconsequential. In contrast, the cumulative effects of a high frequency of disturbance would indicate such negative impacts are possible.

Study Objectives


1. Estimate the frequency of disturbance for individual seals in a constricted glacial inlet with high vessel traffic. Disturbance will be determined using data from radio tags temporarily attached to harbor seals. Sensors on the tag detect a change from dry to wet when a seal leaves a haulout and enters the water. The number of times that a seal enters the water when vessels are in the proximity will be quantified.

2. Determine activity budgets (e.g., frequency and duration of haul-out bouts, and intervals between haul-out bouts) relative to numbers of vessels present (e.g., present vs. absent, few vs. many vessels), and vessel proximity.

  • When a change in behavior from dry to wet is recorded when vessels are in the proximity, evaluate whether a seal will resume hauling out shortly thereafter, or remains in the water for extended periods of time, thereby increasing the duration of the effects of a disturbance.
  • Assess whether the average amount of time an individual spends hauled out each day changes (e.g., decreases) as the amount of vessel traffic increases, or the proximity of vessels decreases.
  • Compare interval between haul-out bouts and haul-out durations before and after a change from dry to wet is noted for a seal, between instances when disturbances are detected (when vessels are nearby) and times when no vessels are in the proximity.

At a later date pending funding and whether the data are sufficient to do so, we will explore the feasibility of achieving the following objectives:


3. If sufficient disturbances are recorded, estimate disturbance rates by vessel type.

4. Determine the relationship between the probability that a seal enters the water, and the distance to vessels, vessel type, vessel speed, and vessel direction.

Application to Management and Conservation

In 2013, the National Marine Fisheries Service (NMFS) requested public comments regarding whether guidelines for vessels while observing harbor seals in glacial areas needed revision, or whether regulations needed to be implemented to protect harbor seals from vessel disturbance. After reviewing those comments, NMFS made plans to adopt new voluntary guidelines for vessel operators in the 2015 season to minimize disturbance of harbor seals in glacial habitat. NMFS proposes to monitor vessel behavior after those guidelines are implemented to determine whether they are working, or whether vessel regulations will be necessary. Pending successful completion of our studies, we anticipate being able to evaluate the frequency of disturbance (possibly by specific vessel types) and whether that level of disturbance has the potential to result in negative consequences to individual seals and the population. Other agencies are planning similar research in different glacial habitats. These results should contribute information that will allow NMFS to make a more informed decision on voluntary guidelines and possible future vessel regulations.

For additional information about this project contact Gail Blundell at 907-465-4345 or email her at