Pacific harbor seal (Phoca vitulina richardii) abundance in Johns Hopkins Inlet (JHI), a glacial fjord in Glacier Bay National Park (GBNP), Alaska, has been decreasing at a concerning rate. Disease, increased predation, emigration, depletion of prey resources, and disturbance by vessels all have been implicated as causal factors of the abundance decrease. This study involved evaluating the role of disturbance by vessels during 2007 and 2008 as a causal factor in the harbor seal abundance decrease by (1) quantifying the vessel disturbance regime experienced by seals in JHI; and (2) quantifying behavioral and bioenergetic impacts of vessels on seals in JHI.

The presence of vessels altered the haulout patterns and behavioral activity budgets of seals by increasing the rate of flushing (vacating an iceberg and entering the water) and increasing vigilance behavior. According to the bioenergetic model used during this study, all seals flushed by vessels incurred an energetic cost, though that cost was disproportionately greater for pups than non-pups.

The overall proportion of the JHI seal aggregation that was impacted by vessel disturbance was relatively low; however, repeated disturbance may be inducing the relocation of seals to other areas, and direct energetic impacts may be decreasing the individual fitness levels of pups. Vessel disturbance, therefore, may be playing direct and indirect roles in the harbor seal abundance decrease in JHI.