Factors Affecting Haul-Out Behavior of Harbor Seals (<i>Phoca vitulina</i>) in Tidewater Glacier Inlets in Alaska: Can Tourism Vessels and Seals Coexist?

<div><p>Large numbers of harbor seals (<i>Phoca vitulina</i>) use habitat in tidewater glaciers in Alaska for pupping, breeding, and molting. Glacial fjords are also popular tourist destinations; however, visitation by numerous vessels can result in disturbance of seals during critical life-history phases. We explored factors affecting haul-out behavior of harbor seals at a glacial site frequented by tourism vessels. In 2008-10, we deployed VHF transmitters on 107 seals in Endicott Arm, Alaska. We remotely monitored presence and haul-out behavior of tagged seals and documented vessel presence with time-lapse cameras. We evaluated the influence of environmental and physical factors on the probability of being hauled out, duration of haul-out bouts, and as factors associated with the start and end of a haulout. Location, season, hour, and interactions of location by year, season, hour, and sex significantly influenced haul-out probability, as did ice, weather, and vessels. Seals were more likely to be hauled out with greater ice availability during the middle of the day, and less likely to be hauled out if vessels were present. Cruise ships had the strongest negative effect; however, most vessel types negatively affected haul-out probability. Haul-out duration was longest in association with starting on incoming tides, clear skies, no precipitation, occurring in the middle of the day, and ending in the late afternoon or evening. End of haulouts was associated with increasing cloud cover, low ice availability, and vessel presence; large-sized tourism vessels or all-vessel-types combined were significant predictors of ending a haul-out bout. Probability of being hauled out was highest in June, during pupping season. Potential disturbances of harbor seals could be reduced, enabling longer resting times for seals and fewer interruptions for nursing pups, if vessels focused the majority of visits to glacial habitat to before or after the hours of 08:00-17:00 or, less optimally, 09:00-16:00.</p></div

Dates of operation for time-lapse cameras.

<p>Cameras were used to detect vessel presence and assess environmental variables such as weather conditions and ice coverage. Horizontal bars indicate the range of dates (color-coded by camera site) that each time-lapse camera was functional at each remote monitoring station throughout the years of the study.</p

Proportion of time that radio-tagged harbor seals spent in glacial habitat.

<p>Proportion of time seals (n = 104; tagged in Endicott Arm) spent in Tracy or Endicott Arms by year and season. Vertical bars are 95% Confidence Intervals, by year, season, and location (on left), and (on right) by season (all years combined). Season numbers and their respective range of dates are in the legend.</p

Haul-out duration for radio-tagged harbor seals by season and inlet.

<p>Geometric mean duration (in hours) of haulouts for seals in Endicott and Tracy Arms during each ~2 week season.</p

Summary of Vessel Effects on Seal Behavior.

<p>Estimates of haul-out probability, haul-out start, and haul-out end are odds ratios, which estimate the effects of vessel presence for each size category, relative to when there were no vessels of that size present. Estimates for durations are the geometric mean duration (dur.) of haulouts when a vessel was present relative to when there were no vessels. The observed effect of longer duration of haulouts in the presence of vessels is likely spurious, as a result of the confounding effects of the presence of numerous vessels during midday, at a time when the majority of seals haul out (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125486#pone.0125486.g008" target="_blank">Fig 8</a>). Significant effects (i.e., factors where the CI does not cross 1 or no effect) have solid symbols.</p

Haul-out probability as a function of predictor variables.

<p>Odds ratios are the relative odds of a seal being hauled out for one value of the covariates vs. another value [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125486#pone.0125486.ref047" target="_blank">47</a>]. Marginal means (i.e., SAS least-squares means; [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125486#pone.0125486.ref041" target="_blank">41</a>]) are the estimated probability of a seal being hauled out for each level of the predictor variable, adjusted for other predictors in the model. Analyses with respect to ice, vessels, cloud cover, precip, and sky, were based on data from 2009–2010 and only for the daylight hours >0500 to <2200.</p><p>¹Models containing these predictors omitted the season*sex interaction to facilitate convergence; these factors (i.e., ice, precipitation, sky condition, and cloud cover) were included in models singly, rather than simultaneously.</p><p>²Results for binary vessel predictors (i.e., absence/presence); results for the binary vessel variables are easier to interpret, however models using continuous vessel predictors showed similar patterns.</p><p>Haul-out probability as a function of predictor variables.</p

Percentage of observations when vessels were observed in the study area.

<p>Percent of 15 minute observation periods in which zero, one, or more vessels (all vessel-types combined) were present in inner Endicott Arm and in Tracy Arm in 2009 and 2010. Outer Endicott is omitted because vessels generally moved through the area to reach the inner Arm.</p

Percent ice coverage.

<p>The percent of all 15 minute observation periods in which three categories of ice coverage were observed in 2009 and 2010 for all three remote monitoring sites in the study area. The percentage of the area covered by icebergs within each time-lapse photograph was estimated and categorized in thirds, as low (<33% ice coverage), medium (33–66%), and high ice coverage (>66%).</p

Extent of time that vessels were observed in inner Endicott Arm.

<p>Number of 15 minute observation periods in inner Endicott Arm when zero, one, or more vessels were present for the following size categories: Large (>50m; tourism vessels, private vessels, and sailboats), Medium (≥4.5m <50m; private vessels, and sailboats), and Small (<4.5m; skiffs and inflatables). Only inner Endicott is shown because the majority of seal telemetry-detections occurred there.</p

Predictors related to the probability of an observation being an ‘end-of-haulout’ observation.

<p>Predictors of the probability of an observation being an ‘end-of-haulout’ observation were assessed compared to predictors consistent with ‘middle-of-haulout’ observations. Odds ratios are the relative odds of a seal being hauled out for one value of the covariate vs. another value [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125486#pone.0125486.ref047" target="_blank">47</a>]. Numbers in parentheses for vessel predictors are numbers of vessels.</p><p>¹Odds ratios for ‘Low’ and ‘High’ ice are relative to ‘Medium’ ice.</p><p>²Results for binary vessel predictors (i.e., absence/presence); results for the binary vessel variables are easier to interpret, but show similar patterns to the continuous vessel predictor models. Odds ratios for vessel predictors are the odds of an ‘end’ observation when a vessel is present, relative to when no vessel is present.</p><p>Predictors related to the probability of an observation being an ‘end-of-haulout’ observation.</p