J. Zool., Lond. (A)(1986) 208, 1-7 Pattern and depth of dives in Northern elephant seals, Miruunga angustirustris B. J. LE BOEUF,D. P. COSTA,A. C. HUNTLEY Department of Biology and Coastal Marine Studies, University of California, Santa Cruz, California 95064 USA G. L. KOOYMANAND R. W. DAVIS Physiological Research Laboratory A-004, Scripps Institution of Oceanography, University of Calqornia, Sun Diego, La Jolla, California 92093 USA (Accepted 12 February 198.5) (With 1 figure in the text) A time-depth recorder was attached to a female Northern elephant scal at the end of her lactation fast before she entered the sea to feed. The animal dived continuously during its first 11 days at sea, the period recorded for a total of 653 dives. Mean dive time was 21 min, with the longest submersion lasting 32 min. Mean surfke interval between dives was 3 min, resulting in a total surface time of I1 %. Mean dive depth was 333 m and the deepest dive was 630 m, the deepest ever recorded for a pinniped. A depth histogram recorder attached to another female yielded a similar frequency distribution of dive depths. Contents Page Introduction ...................................... 1 Mcthods ........................................ 2 Results .......................................... 2 Discussion ........................................ 4 References ........................................ 5 Introduction Although the onshore behaviour of the Northern elephant seal, Mirounga angustirostris, has been studied extensively (e.g., Bartholomew, 1952; Le Boeuf, 1974; Reiter, Panken & Le Boeuf, 1981), little is known about the animal’s behaviour at sea, where most of each individual’s life is spent and where feeding takes place. Adult females and males spend 84% and 67% of the year at sea, respectively. The adult female annual pattern is as follows. Five weeks on land for parturition, nursing and copulation are followed by 2.5 months at sea feeding; 2-4 weeks on land moulting are followed by approximately 7.5 months at sea feeding and then a return to the rookery to give birth and complete the annual cycle (Le Boeuf & Peterson, 1969; Le Boeuf, Whiting & Gantt, 1972; Le Boeuf, 1981). There is little information on dispersal of elephant seals at sea after they leave the breeding colonies, and animals of both sexes and all ages are seldom seen by mariners (Bonnell, Le Boeuf, Pierson, Dettman & Farrens, 1978; Condit & Le Boeuf, 1984).We obtained acomplete dive record of an adult female for her first 11 days at sea after she gave birth and weaned her pup, and a frequency distribution of dive depths made by another post-parturient female during her first seven days at sea. 1 0022 5460/86/001001+7 S03.00/0 @ 1986 The Zoological Society of London 2 B. J. LE BOEUF ET AL. The period between weaning her pup and moulting is an ideal time to record a female’s activity at sea. Because females fast throughout lactation and lose a third of their mass (Costa, Le Boeuf, Ortiz & Huntley, 1986), we reasoned that they begin feeding soon after departing the rookery. Methods On 16 February 1983, we immobilized 2 lactating females at Aiio Nuevo Point, California, using ketamine hydrochloride (Briggs, Henrickson & Le Boeuf, 1975). The drug was injected into the hind quarters with a COZ powered pistol (Cap-Chur Darts and Pistol, Palmer Chemical Co.). The females were measured, rolled into a nylon stretcher and weighed with a 1000 kg dynamometer (Chatillon WT-1000+2.5 kg) suspended from an aluminum tripod. A time-depth recorder (TDR) was attached to the ankle of one female and a depth histogram recorder (DHR) was attached to the ankle of the other (Kooyman, Gentry & Urquhart, 1976; Kooyman, Billups & Farwell, 1983). Both instruments were attached with padded straps, a technique successful with Weddell seals, Leptonychotes weddelli (Kooyman, 1966). The TDR weighs 500 g and is designed to record the absolute time and depth of all dives for about 14 days; the DHR weighs 95 g and records only the depth of dives in 4 pre-set intervals, which ranged from 6.8 to 667 m in this study. The instruments were calibrated prior to attaching them to the animals (Gentry & Kooyman, 1985). Both females gave birth on 28 January, nursed their pups daily, weaned them and returned to sea to feed by 19 February. They were expected to return to the same location to moult during the last week in April, approximately 9-10 weeks after departure, thus allowing retrieval of the recording instruments. This prediction was based on studies of marked females; during this period, mean duration at sea ( f 1 S.D.)for 18 females in 1981 was 64.9+ 14.4 days, and for 33 females in 1982 it was 664 f8.5 days. The day after each female returned to the rookery, she was immobilized, weighed and the depth recorder retrieved. The TDR film record was developed, enlarged and digitized; a computer program calculated and summarized all dives into frequency distributions for depth, duration and time of day (Gentry & Kooyman, 1985). Results Both females returned to the site later than expected. The female bearing the DHR spent 93 days at sea during which she gained 57 kg. The female bearing the TDR spent 127 days at sea, gained 19 kg and was already moulting when she appeared on land. In comparison, a marked, uninstrumented female that went to sea at the same time gained 43 kg in 92 days. In 1982, the weight gain of three marked, uninstrumented females was 132 kg in 63 days, 25 kg in 66 days and 54 kg in 68 days. The ankles of both experimental females, to which the instruments were attached, were sore from the chafing of the straps. We emphasize the TDR record because it yielded more information. The TDR recorded all diving activity for the female’s first 11 days at sea. Several aspects of the record are noteworthy. Most remarkable was the pattern and consistency of the record. From the time the seal went to sea, until the TDR ran out of recording film 11 days later, the seal dived repeatedly for a total of 653 dives. The female’s diving pattern is summarized by the data in Table I. On average, the seal dived 61 times per day, each dive lasted about 20 min followed by 3 11min (mean & 1 S.D.) on the surface, resulting in a daily surface time of 1 1 %. Only 13 surface intervals exceeded four minutes. The maximum dive time of 32 minutes was more than a 50 % increase in duration over the mean daily dive time. PATTERN AND DEPTH OF DIVES IN NORTHERN ELEPHANT SEALS 3 TABLEI Dive characteristics of the TDRfemale during her,first I I days at sea (see text).Means & one standarddeviationfrom the mean Surface Maximum Mean Maximum Mean No. of time depth depth dive time dive time Ddy dives (%I (4 (m) (min) (min) *I 38 10 588 257 f I40 30 22 + 3.7 2 60 14 468 294+ 78 30 21 3.8 3 57 17 521 283 i-106 32 21 i 5.3 4 55 10 498 340% 67 28 24i 1.8 5 60 7 603 324+ I09 28 21 6.0 6 63 10 563 314+127 28 20 i5.8 7 59 11 550 347 + 112 26 21 f 3.9 8 62 11 530 370f 88 25 21 i 1.6 9 64 9 584 356i 95 23 20+ 1.4 10 66 12 630 371 1 I4 24 19k3.2 **I 1 69 12 58 1 387+ 95 25 20% 1.6 Average 61 11 333 21 Total 653 ~ ~ * 15.5h ** 26h Neither day 1 nor 11 included in overall average 251 Dive depth (m) FIG.1. Frequency distribution of dive depths of the female to which a time-depth recorder was attached. Total dives = 653. The mean depth of dives was 333 f42.7 m. The maximum dive depth recorded was 630 m. With the exception of the first 1 1 dives, when the animal swam near the bottom as she was departing the rookery, there were few shallow dives. A frequency analysis of dive depths (Fig. 1) shows that only 6 % of all dives were less than 200 m. From the first dive of 55 m, the dives got progressively deeper in increments of about 10 m, until dives 12, 13 and 14 when the depths reached 108 m, 234 m and 338 m, respectively. It was at this time that the seal probably reached the edge of the continental 4 B. J. LE BOEUF ET AL. shelf. The time lapse from the first dive until this sharp increase in depth at the 14th dive was 5 h. If the seal swam at about 2 m/sec directly offshore, it would have travelled 36 km. A bottom contour map of the Aiio Nuevo area shows that the continental slope is 10 km to 48 km offshore, depending on the direction taken. The DHR record provides similar data on the depth of dives. The record ran for the female’s first seven days at sea during which the seal dived 510 times; 9.2 % of the dives were between 6.8 and 82 m, 31.2 % between 82 and 199 m, 45.3 % between 199 and 442 m, and 14.3 % between 442 and 667 m. Discussion Several conclusions and questions can be drawn about Northern elephant seal dive capacities and behaviour from these records. As predicted from various aspects of their physiology (Bartholomew, 1954; Harrison & Tomlinson, 1956; Bryden & Lim, 1969; Lythgoe & Dartnall, 1970; Lane, Morris & Sheedy, 1972), elephant seals dive deeply and spend much of their time at depth.