Satellite-linked depth recorders (SDRs) were attached to 47 harbor seals in
Prince William Sound, Alaska, during 1992-1996. Parameters describing divi
ng effort, diving focus, and focal depth (depth bin to which diving was foc
used) were calculated from binned data on maximum dive depth and time spent
at depth, and analyzed using repeated-measures mixed models. This analysis
method accounted for individual variability, temporal autocorrelation, and
the binned nature of SDR data, which are often ignored using standard stat
istical techniques. Results indicated that diving effort remained steady fr
om September to April, when seals spent 68%-75% of their overall time in th
e water. Time spent in the water declined to 60% in May and to about 40% in
July. Seals spent the most time in the water at night and the least in the
morning. The diving of all seals in all months was highly focused, Overall
, diving was focused to one depth bin approximately 75% of the time. Diving
was more focused for females than for males and subadults. Focal dive dept
h was deepest in winter and shallowest during May-July. Focal depth and div
ing focus varied by region. Collinearity between month and region in the fo
cal depth model suggests that seals move in winter to regions where prey ar
e found deeper in the water column. Variations in diving behavior presumabl
y result from combinations of regional bathymetry, seasonal cycles in type
or depth distribution of prey, and seal life-cycle events such as reproduct
ion and molting.