Depth of dives

Eurasian otters foraged as deep as 15 metres (see Chapter 4), but the vast majority of dives occurred in shallow water. For instance, in Shetland, 54% of 3558 dives were in water that was less than 2 metres deep and 98% were less than 8 metres (Nolet et al. 1993). When we compared dives at different depths in the strip of water within 100 metres from the shore, there was a highly significant preference for shallow places (Fig. 9.7).

The reason for the shallow water preference was not immediately obvious. We considered several hypotheses to explain it (Kruuk etal. 1985; Nolet etal. 1993), such as that otters used shallow waters more because: (1) they could catch more fish there; (2) it might be energetically more efficient, either because more underwater time could be spent on searching rather than on travelling to and from the bottom, or to and from areas further off shore; or (3) perhaps breathing was more efficient during shallow dives.

In fish-trapping observations (see Chapter 8) we did indeed find more fish in shallow water, but they were smaller, and as prey biomass (fish weight per area) there were no significant differences with depth. The otters' fishing success (weight of prey caught per time spent diving) was not significantly greater in shallow water (Kruuk etal. 1985). In deeper water prey was larger, but the handling time of prey (time spent in transporting and eating) increased proportionally with the size of the prey (Nolet et al. 1993). We rejected the first hypothesis.

Hypothesis 2 suggested that shallow diving migh involve less effort, because of the shorter distance to the surface and/or the shore. There was a clear relationship between dive depths and the duration of dives (Fig. 9.8). Otters' fishing bouts lasted longer if they included fewer deep dives, and the subsequent recovery on land from the hunt was shorter (Nolet and Kruuk 1989; Nolet et al. 1993). This indicated that fishing at greater depths is energetically more demanding.

A further variable to be considered is breathing efficiency. For several diving animals, the longer they have been under water, the greater the proportion of total time they have to spend on recovery, on 'catching their breath' (Kramer 1988). Somehow, a diving animal has to establish an optimal balance between underwater time and recovery effort. Houston and McNamara (1994) argued that our otter data supported such an 'optimal breathing hypothesis': the shallower and shorter the dives, the more efficiently an otter can divide its time and effort between searching and recovering. In Eurasian otters the recovery time on the surface, after a dive, is directly proportional to the diving time—it is about half (Nolet etal. 1993) (Fig. 9.9)—so this in itself could not explain a preference for short dives, but possibly a greater breathing effort is involved for deep dives. In other words, shallow dives are energetically less

Figure 9.7 Preferring the shallows: numbers of dives by Eurasian otters in Shetland, in sites of different depths. Proportion of 1008 dives, divided by proportion of 100-metre wide strip along coast of the given depth (r2 = 0.70).

Figure 9.8 Deeper waters involve longer dives: mean times underwater in dives by Eurasian otters in Shetland, at different depths. Values are means and standard errors of 1181 dives, when no prey was caught.

Mean dive time (s)

Figure 9.9 After a long dive, a long recovery is needed (Eurasian otters in Shetland). Means of dives at different depths; 1039 observations (r2 = 0.98; surface time = dive time X [0.53 - 2.2]).

Mean dive time (s)

Figure 9.9 After a long dive, a long recovery is needed (Eurasian otters in Shetland). Means of dives at different depths; 1039 observations (r2 = 0.98; surface time = dive time X [0.53 - 2.2]).

demanding than deep ones, because of greater effort involved in deep dives and subsequent recovery, for a similar return of prey.

Incidental observations for various other otter species suggest that their choice of depths for foraging is similiar to that of the Eurasian, with similar energetic consequences. As usual, the exception is the sea otter, which behaves differently and goes much deeper. It is also the only otter that takes more than one prey to the surface, as described earlier. Ralls etal. (1995) found by radio-tracking that young male sea otters foraged in deeper waters, at an average of 30 m, whereas, for instance, adult females dived to mean depths of 22 m, with underwater times of well over 1 minute.

In the Aleutian Islands, Kenyon (1969) noticed that, when sufficient food is available, sea otters prefer to forage in (what he called) shallow waters 2-30 m deep, and no deeper than 50 m, with males going deeper than females. Jim Estes (1980), observing sea otters in the same area, commonly found animals feeding at depths of 40 m or more.

More recently, Bodkin etal. (2004) observed more than 180,000 dives of 14 sea otters with radio-transmitters in Alaska. Most of the foraging dives were between 2 and 30 m deep, 85% of dives by females were less than 20 m deep, but half of all dives by males were at 45 m or deeper, with less than 2% of all dives going further down than 55 m. The deepest dive recorded was 100 m.

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