Behavior of ringed seals diving under shore-fast sea ice
Abstract
Behavioral studies of pinnipeds have been concentrated in the temperate zone, while the majority of phocid seals breed in high latitude, ice-covered waters. The under-ice movements and behavior of ringed seals, the most ice-adapted of northern pinnipeds, were investigated in 1989-1992. Fifteen ringed seals were captured 1 or more times in nets designed to line seal breathing holes in sea ice. The behavior of seals, diving under shore-fast sea ice, was monitored acoustically during the spring breeding season. Frequent dives with extended periods at depth by subadult and adult seals, including lactating females, were interpreted to be foraging dives. Median dive durations were less than 10.0 min; the maximal observed duration was 26.4 min. The maximal observed dive depth, 222 m, was limited by water depth in the study area. Modal dive depths were between 10 and 45 m for breeding-aged males and between 100 and 145 m for subadult males and post-parturient females. Median dive durations were 4.0 min for adult males and 7.5 min for adult females. Body mass was a better predictor of maximal dive duration (r$\sp2$ = 0.94) than was age, but maximal durations were shorter than were predicted using measures of oxygen stores and presumed metabolic rates. There was no consistent relationship between light level and the frequency or depth of dives. The allocation of time to descent, foraging, ascent, and surface phases of the diving cycle was examined for 9,778 under-ice dives. Adult males and yearlings of both sexes concentrated their activity within 25 m of the surface, while subadults and adult females spent long periods foraging at depths greater than 100 m. The proportion of the dive cycle spent at the surface decreased and the proportion spent in foraging patches increased with increasing depth contrary to the predictions of a model based on the marginal value theorem. Ringed seals may sacrifice efficient use of oxygen in favor of a risk adverse strategy. When surfacing at breathing holes in the ice, ringed seals are highly vulnerable to predation. They may minimize that threat by returning to the surface with large oxygen reserves.
Degree
Ph.D.
Advisors
Waser, Purdue University.
Subject Area
Ecology|Zoology
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