Comparative Diving Patterns of Pinnipeds and Seabirds

Abstract

General ecological information resulting from modern dive studies has been limited because analyses and conclusions are study- and species-specific. In this work, a series of unrelated divers was studied and compared using the same analytical procedures. More than 230 000 dives from 12 species were analyzed, and ∼140 000 of these dives were classified according to dive shape. The species included one cormorant, three penguins, two eared seals, five true seals, and a walrus. Dive profiles could generally be characterized as one of four shapes: square, V, skewed right, or skewed left. In light of this, a universal shape classification protocol was developed that also offers potential solutions for “on board” memory limitations and transmission constraints for archival time–depth recorders and satellite-linked time–depth recorders. Comparisons of dive data recorded with different sample intervals indicated the need for a standardization relative to mean dive duration (i.e., an equal number of data points per dive). Comparative analyses across these dive types and the different species revealed that square dives were always, and by far, the most abundant dive type, usually followed by V dives, and then the skewed dives. The percentage of time that the animals spent at the bottom of square dives (∼50%), as well as the variation in depth during this bottom time (∼15%) were also quite uniform across species, indicating that similar foraging patterns were being used, at least relative to the shape of dives. Observed differences across species revealed that larger divers generally dived deeper and longer than did smaller ones, although fur seals and walrus were exceptions, with more limited diving performance than expected based on body size. Also, smaller divers had a tighter coupling between dive depth and duration than did larger ones, indicating that they may be more duration limited. Few other dive variables (e.g., the rate at which dive duration increases with depth, the percentage of dives within each dive type, the percentage of bottom time, the coefficient of variation of depth during bottom time, and the mean wiggle distance per depth during square dives) were affected by body size, but instead physical (water depth) and ecological (type of prey) constraints appeared to play major roles. Analyses using calculated aerobic dive limit (cADL) indicated that generic calculations are problematic and that estimates of diving metabolic rate can drastically influence cADL and resultant findings. However, even using crude estimators, comparisons of cADL across dive types indicated that square dives and V dives most often exceeded the cADL for large and small divers, respectively. This indicates that square dives and V dives may be the predominant foraging dive types for larger and smaller divers, respectively, as animals would be expected to push their limits most during this activity. However, the abundance of square dives within the small divers (>60%) indicates that these dives may have a foraging role as well. Functional analyses of the determined dive types were in general agreement with those from previous work indicating that the various dive types have foraging (benthic and pelagic), traveling, exploring, resting, and processing functions. However, for most species, except Weddell seal and southern elephant seal (rare but likely important), skewed dives were rare and are likely to be of little importance to these animals' diving regimes. Overall similarities in the dive patterns of the various species suggest that these animals exploit the aquatic environment in a similar way.