Deep Diving in Wild, Free-Ranging Beluga Whales, Delphinapterus leucas

A. W. Martin Sea Mammal Research Unit, c/o British Antarctic Survey, High Cross, Madingley Woad Cambridge CB3 OET, U.K.

and T. G. Smith' Department of Fisheries and Oceans, Arctic Biological Station, 555 BsuEevard St-Pierre, Ste-Anne-de-Bellevue (Qugbec) H9X 3W4, Canada

Martin, A. R., and T. 6. Smith. 1992. Deep diving in wild, free-ranging beluga whales, Delphinapterers leucas. Can. 1. Fish. Aquat. Sci. 49: 462-466.

Three adult female beluga whales, Delphinapterus leucas, were captured during summer in the Canadian high Arctic and equipped with satellite-linked radio transmitters that measured and recorded ambient pressure every 20 s. Diving behaviour was divisible into three distinct categories: (1) near-surface, (2) "spike" dives to 20- 150 rn depth, and (3) prolonged, flat-bottomed dives to a maximum depth of 350 m. The duration of these prolonged dives was 9.3-1 3.7 min, but the upper bound is known to be underestimated. Average descent rates varied between dives in the range 1.43-2.20 mls and ascent rates between 1.23 and 1.84 rnls. Within periods of 20 s duration, maximum descent and ascent rates were 2.55 and 2.35 mls, respectively. After most prolonged dives the rate of ascent declined progressively as the surface was approached, particularly in the final 108 m. Such dives were usually to, or near to, the seabed and were probably for foraging. Up to 42% of an animal's time could be spent at depths of 8 rn or more.

be cornportement de plong6e de trois femelles belugas adultes, Delphinapterus leueas, a et$$tudi& durant It&$. Nous les avows captur6es dans le haut arctique canadien et avons place, sur leur dos, des 6metteurs satellites capables de mesure la pression ambiante i une Br6quence de 20 s. be comportement de plong6e se divise en trois categories distinctes : (1) plong6es A proximit6 de la surface, (2) plong6es "verticales aller-retour" de 20- 150 m et (3) plong6es de plus longkse dur$e, 2 profil plat - avec d6placement horizontal - allant jarsqul% une profondeur rnaximale de 350 m. La duke calcul6e des plong6e longue duke est de 9,3 ti plus de 13,7 min. bes taux rnoyens de descente variaient entre 1,43 et 2,263 mls et ceux d'ascension entre 1,23 et 1,84 mls. Durant des p6riodes de dur6e de 20 s, les taux maximums de descente et d'ascension 6taient de 2,55 et 2,35 mls respectivement. Apres des plongks de longue duree, le taksx d'ascension ralentissait en approchant la surface, particulierement dans les derniers 100 m. Les belugas en plong6e se rendaient normalement pr$s ou jusqu'au fond, vraisernblablement pour des fins d'alimentation. Jusqu'A 42 % du budget horaire d'un beluga peut &re pass6 2 des profondeurs de 8 m ou plus.

Received luly 3, 199 7 Accepted October 4, 199 1 (JB441)

v ery little is known about the diving behaviour of ceta- ceans. For logistical, financial, and technological rea- sons it has proven very difficult to gain information on

the underwater activity of a group of animals which are totally aquatic, large in body size, and fast-swimming. Ridgway (1986) reviewed the published accounts of the depth and dura- tion of dives of cetaceans and found idomation on 10 species. Of these, five were known only from studies of captive animals, two from indirect behavioural evidence, one from measure- ments on a harpooned whale, and only two from studies of wild, unrestrained animals. These two are the sperm whale (Physeter maeroeephw&us) (Nods and H m e y 1972; Gordon 1987; Mullins et al. 1988; Papastavrou et al. 1989) and the common dolphin (Delphinus delphis) (Evans 197 1).

The published evidence for diving behaviour of the beluga, Dekphinapterus keueas, suggests contradictions. Field studies report dives to 28 rn or less (Tomilin 1957; Kleinenberg et al.

'Current address: Department of Fishekes and Oceans, Pacific Bio- logical Station, 3 190 Hammond Bay Road, Nanaimo, B .C. V9R 5K6, Caunada.

19@), yet studies of trained belugas in the open sea have dem- onstrated a physiological capability of diving to about 650 m and for up to 16 min (Ridgway et al. 1984). The stomachs o f freshly dead belugas have often contained organisms which could only have been obtained from the seabed (Klumov and Dorofeev 1936; Vladikov 1947; Kleinenberg et al. 1969), but whether this food was gathered in shallow or deep water could not be detemined.

To examine the diving behaviour of wild, free-ranging belu- gas, we equipped thee animals with small electronic devices which would allow accurate determination of their geographical location and provide detailed dive profile data.

Materials and Methods

Belugas were captured by driving them into shallow water along the shore of Cunningham Inlet, N. W.T., Canada (74"05?4, 93"45 'W) . The selected animals were restrained by first placing a hoop net over their heads and around their pec- toral fins. Once captured they were further secured by tying a

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rubber covered rope around the caudal peduncle and anchoring it on shore. The animal could then be restrained with just the dorsal surface above water by one person sitting ow its back.

Three female beluga whales were equipped with back- mounted electronic packages comprising a UHF radio trans- mitter ((Toyocom Ltd., Tokyo, Japan), a micrsprocessor, and pressure and emergence sensors. The units were powered by lithium thionyl chloride batteries and housed in tubes of mo- dised aluminium, capable of withstanding pressures equivalent to a water depth in excess of 2000 m. They were attached to the dorsal ridge of the animals by means of a "saddle" of pli- able material, and three 4.5-mm-diameter nylon pins which migrate out of the tissue after a period of weeks, releasing the package. One transmitter was deployed at Cunningham Inlet in the summer of 1989 and the sther two in 1990.

Signds from the transmitters were received by polar-orbit NOAA satellites and relayed to the laboratory via System k g o s , based in Toulouse, France. m e n certain operational conditions are met, System k g o s permits the geographical location of transmitters to be determined to an accuracy of bet- ter than 1 km during a satellite "pass" (Fancy et al. 1988; Keating et d* B 99 1). Such passes occur more thm 28 times per day at the high latitudes at which this study was carried out. In addition to locational data, infomation on ambient pressure was collected and relayed by satellite. Pressure was measured every 20 s continuously and, at each transmission, the Iast 32 pressure measurements were relayed, giving the depth of the animal during the 10.3 min immediately prior to surfacing. With this protocol, depth data were collected for approximately 23% of the period during which a transmitter was functioning, spread throughout the 24-h period but with a greater coverage during daylight hours. Accuracy of these measurements, checked by repeated 66cycling" to maximum depth in a baaf.0- metric pressure chamber, was to approximately & 2.5 m of the true depth down to 650 m. Water depths at known locations of the study animals were determined from the appropriate chart issued by the Canadian Hydrographic Service.

Results

All three transmitters were attached to adult female belugas, measuring 29 1-4 16 em in body length. Transmitter No. 5802 was deployed at 17333 local t h e on 27 July 1989. Signals were received locally for about 30 min while the animal swam out of Cunninghm Inkt, but only one useful dive profile was sub- sequently received by the satellite. This dive was complekd at 17121 local time on 3 August at an unknown location, but in deep water, as the maximum dive depth was 321 m.

Transmitter No. 580% was placed on an animal at 13:07 local time ow 17 July 1990 md functioned correctly for 8 d. The whale moved out of Cunningham Inlet after release and spent the entire period of transmitter operation in deep water in Bmow Strait, in depths sf 150-350 m.

Transmitter No. 5803 was deployed at 15:36 local time on 18 July 1990 and functioned comctly for 1 1 d. The animal remained within Cunninghm Inlet, in water depths of 48 m or less, for the first 5 d after capture md then headed north, west, and south into Bmow Strait and Peel Sound, remaining in waters of 158-270 m depth for the final per id of transmitter operation.

Subsurface activity of the three study animals could be divided infa three recognisable categories: (1) near-surface, with sudacings at intervals of 1 min or less, i.e. dives of short

TIME (Mlna)

FIG. 1 . Typical profile of the type of deep dives described in this paper. This dive (No. 146 of beluga No. 5801) had an estimated bottom time of 4. I min, a mean descent rate of I . 88 d s , and an ascent rate decreasing fmm 1.95 to 1.15 rids. It was carried out at 19: 15 local time OW 21 July 1998 in water sf approximately 335 m depth at posi- tion 78E014'N, 90°59'W.

TlME BEFORE SURFACING gs) FIG. 2. Rate of ascent as the surface is approached after a deep dive (m - 39). Values represent the average rate over consecutive periods of 28 s. The right-most point is that of the final complete 20-s interval measured before surfacing, which ended randomly at any time between 0.1 md 19.9 s before the animal surfaced. The value on the horizontal axis for each point therefore represents the approximate midpoint of each consecutive interval.

duration, (2) '"pike" dives of 4 min or less, comprising a descent to 20-150 m depth with an immediate return to the surface, and (3) prolonged, flat-bottomed dives to depths sf 20-350 m. In this paper, we will consider in detail only these prolonged dives and, in particular, those to a depth of 100 m or more. These were of a very characteristic '%square" profile,

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with an almost linear rate of descent, a period of several min- utes spent at a constant depth, and an ascent which often slowed progressively as the surface was neared, particularly in the final 100 m (Fig. 1 and 2).

The whales normally surfaced very wear the point of sub- mergence after these deep dives. Estimated dive duration was in the range 560-820 s (9.3- 13 -7 min) including, where nec- essary, an extrapolation for any part of the descent which was not recorded within the 620-s '6window'7 of the transmitter. The duration of 23% of dives could not be estimated because the entire descent was missed. Some of these dives were prob- ably longer than 820 s, but the shape of the frequency distri- bution of known dive durations suggests that few exceed this limit. The mean duration of 39 dives to 150 rn or more was 686 s (1 1.4 min), but this is an underestimate based on the assumption that no part of the constant depth 'bottoam" seg- ment of the nine incomplete dive profiles (see Fig. 1) had been missed. The regression of maximum depth of the dive on dive duration was significant and positive (n = 36, $ = 0.53, p < 0.001).

The rate of descent, averaged over the entire descent, was in the range of 1.43-2.20 d s . Average ascent rates varied between 1.23 and 1.84 d s over the entire ascent, but the rate of vertical movement progressively diminished as the surface was approached. In dives to 150 rn depth or more, the rate of ascent (expressed as an average over each consecutive period of 20 s) fell from 1.7 1 d s about 100 s before reaching the surface (equivalent to a depth of about 140 m) to 1.18 d s in the last complete interval before surfacing (Fig. 2). Over periods of 20 s, the maximum recorded rates of descent and ascent were 2.55 and 2.35 d s , respectively. Regression of the average rate of ascent on the duration and maximum depth of the dive was significant and positive (sz = 36, r2 = 0.37, p < 0.081 and n = 40, r' = 0.43, p < 0.001, respec- tively). There was no significant relationship between the aver- age rate of descent and either dive duration or depth.

Measured "bottom" time, the period spent at constant depth after the descent, was in the range 180-420 s (3-7 min), typ- ically representing 45-60% of the total period of submergence. The upper time limit is almost certainly an underestimate because, as mentioned above, the end of the descent phase and beginning of the period at maximum depth was not recorded for nine of the longer dives.

The length of time spent at or near the surface between deep dives was rarely recorded because the recording 'window" was too short. Nevertheless, it was estimated on three occasions to be approximately 6 min between dives of about 13 min dura- tion, once to be 4 min between dives of at least 9 min, and once to be 1 min between dives sf 4 and 5 min. The proportion of time overall spent at or near the surface varied dramatically according to the behaviour of the whales and, in particular, whether they were engaged in bouts of prolonged dives. Whale No. 5803 spent five consecutive days in the shallow waters of Cunningham Inlet and rarely ventured to depths of more than a few metres throughout this time. In contrast, animal No. 5801, on 22 July 19% during a period of intensive deep diving, was estimated to spend 42% of the 24-h period at a depth of 8 m or more (Fig. 3).

All deep dives recorded for female No. 5801 were carried out, as accurately as it was possible to measure, to the seabed or very near to it in water depths of 170-350 m. The one dive recorded for female No. 5$02 was to a depth of 32 1 m. Although the location at which this dive was made, and there-

fore the water depth, was not known, there is so little water within range deeper than 380 rn and none deeper than 350 m, that this dive must also have been to or near the seabed. Whale No. 5803 canied out "square" profile dives both in midwater and to the seabed. In water depths of 200 & 30 rn, this animal made many dives of 6-10 min duration but only to 15-25 rn below the water surface.

Discussion

These results demonstrate, contrary to Klurnov and Dorofeev (1 936) and Kleinenberg et al. (1964), that wild belugas may routinely dive to considerable depths. Indeed, the small area in which female No. 5801 spent a period of almost 4 d diving to 300 m or more is the only one in Barrow Strait where such depths exist. The fact that two of three additional belugas which were equipped with location-only transmitters (providing no depth information) during July 1990 also spent several days in this same area suggests that it was a popular locality for this stock of whales, at least in that year.

We assume that the function of deep dives in this species is that of foraging. Animal No. 5801, and the two other belugas we know to have spent time in the area of the deepest water, moved very little distance horizontally, so there seems to be no link to travelling. This activity also occurred well ahead of the annual autumn migration of belugas to the east. We know from direct observation and our telemetry data that the periods of fastest horizontal movements are characterised by shallow dives.

If cpur assumption is correct and deep dives are carried out for foraging, then the periods of continuous diving (see Fig. 3) indicate intensive feeding in offshore areas, supporting pre- vious conclusions that belugas sampled in or near estuaries are not feeding in these shallow-water areas during summer months (Sergeant 1973; Fraker et al. 1979; Brodie 1989; Reeves and Mitchell 1987; Caron 1987; Doidge 1990). Significantly, an ability to dive to 350 m allows belugas access to most of the benthic production in the Arctic Ocean.

There is very little information about the identity, distribu- tion, or abundance of potential prey species at these depths in the Canadian high Arctic. Arctic cod (Boreogadus saida), a common prey of belugas and other Arctic marine mammals, is widely distributed and relatively abundant throughout nearby Lancaster Sound (Bradstreet et al . 1986). In hydroacoustic sur- veys of Lancaster Sound, Crawford and Jorgenson (1998) did find what they assumed to be Arctic cod at depths of >400 m, but not usually in significant quantities below 200 m. The high- est densities were located immediately below the ice layer which was present during their surveys. The assumption that most of their target echoes were of Arctic cod was based on the lack sf direct evidence of any other species in their study area, but we must consider the possibility that there are other prey species present. Greenland halibut (Reinhardtius hyperglsssoides) , which lives on or near the bottom in deep water (Bigelow and Welsh 1924; Scott 1982; Wudon 1990) and is patchy in its dis- tribution (Atkinson and Bowering 1987), is possibly taken by belugas in the deepwater areas of Barrow Strait sought by the whales in this study. From the examination of hunter-killed belugas, other prey might include shrimp, crabs, or other inver- tebrates, many of these available only on or near the seabed (Tomilin 1957; Brodie 1989).

Rates of descent and ascent are similar to those for seals (e.g. Kooyman 1989; Le Boeuf et al. 1989) and within the range

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BELUGA FEMALE 5801,22 JULY I990

TIME (Hours)

FIG. 3. Plot of depth against time for beluga No. 5801 on 22 July 1990.

reported for trained belugas (Widgway et al. 1984). The pro- gressive slowing of the ascent rate within about 148 m of the surface (Fig. H and 2) has not been previously reported, perhaps because the resolution allowed by the method of data collection was not sufficiently fine. Nevertheless, it is a characteristic of nearly all the deep dives we recorded for animal No. 5801 and may have a physiological function, perhaps to reduce the rate at which gas bubbles appear in blood or tissues as the ambient pressure diminishes. Since expansion of gas in the lungs and other air passages must progressively increase the buoyancy of the animal as it moves to the surface, the observed slowing of the rate of ascent must occur because of a reduction in swim- ming speed, a flattening of trajectory, or both.

The discovery that wild beluga whales may remain under- water for per ids of 13 min or more, and for up to 42% of any 24-h period, has important implications for the interpretation of survey results. At least in deep water, all three of our study animals spent a substantial proportion of time well below the surface and would probably have been missed by observers aboard a ship or aircraft. Although l age numbers of high Arctic Fxlugas are commonly seen in bays and estuaries, or within 500 m of the coast in summer. this study supports the findings of Smith et al. (1985) and Richard et al. (1990) that a signifi- cant proportion of the stock occurs offshore. Future surveys should employ a correction factor for those animals estimated to be out of sight when the observation platform passed. In coastal waters, visual observations from land may be sufficient to allow the calculation of an appropriate correction factor (BroBie 197 I), but similar information for animals offshore can only realistically be gathered using radio telemetry.

Acknowledgements

We are indebted to the Polar Continental Shelf Project (PCSP) for generous logistical support and W F - U K for financial help. We thank

Gilly Banks, Kathy Frost, Haakon Hop, George Horonowitsch, and Gary Sleno for invaluable field or logistical support and Oliver Cox, Charlie Chambers, Ailsa Hall, Colin Hunter, Bernie McCorenell, Kevin Nicholas, Jeremy Tomlinson, and Last Engineering for help with preparation of the transmitters or data analysis. Drs. Mike Fedak and Sam Ridgway provided welcomed guidance on the interpretation of the results. Mimi Breton kindly provided the French translation of the abstract.

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