Combined physiological and behavioral observations to assess the influence of vessel encounters on harbor seals in glacial fjords of southeast Alaska

Most studies examining disturbance of seals define disturbance as entry into the water. However, behavior alone may not be an accurate indicator of the timing, magnitude, or physiological cost of disturbances. This study examines changes in harbor seal heart rates in response to two levels of vessel disturbances; 1) 'incidental traffic' defined as presence of vessels in the area while seals were hauled out; and 2) 'experimental disturbance' defined as direct vessel approaches to seals until the seal entered the water. Incidental traffic resulted in a 4 bpm vessel− 1 increase in heart rate while seals were hauled out. Mean incidental traffic during haulouts was 0.26 (range 0 to 8.95) vessels, and small vessels caused the largest increase in heart rate. Experimental disturbances resulted in a 5 bpm increase in heart rate upon initiation of vigilance, defined as the head-lift behavior. In-water heart rate was significantly lower after an experimental disturbance compared to other water entries, suggesting that seals shift to an energetically conservative mode in response to disturbances. During the haulout following an experimental disturbance, seal heart rate was significantly higher than other haulouts, suggesting that there is an added energetic cost of disturbance. Also, sex, mass, current and previous haul-out duration, in-water duration, day of year, hour of day, ambient temperature, and light level were found to have significant influence on harbor seal heart rates; demonstrating that a complex assortment of factors affect heart rate and careful consideration of these factors must be included in disturbance studies. Whereas previous findings have shown that vessel encounters alter seal behavior, this study presents evidence that encounters have energetic and physiological consequences while the seals are hauled out and these consequences persist for some time after the water entry behavior. Accordingly, exposure of harbor seals to increased vessel traffic may result in altered behavior, increased energetic expenditures, and increased exposure to stress, negatively affecting the health, condition, and reproductive success of harbor seal populations that reside in glacial fjords.