DIVING DEPTH OF NORTHERN FUR SEALSIN THE FEEDING TIMETADAYOSHI ICHIHARA* AND KAZUMOTO YOSHIDA*
Northern fur seals (Callorhinus ursinus) are sometimes taken by fishing gears in theopen waters. Scheffer (1946) reports seven cases of fur seals taken accidentallyby the floating drift net along the Oregon coast of the eastern Pacific and he estimates that fur seals can dive 24 feet (7 meters) to 240 feet (73 meters) below thesea surface. Kenyon ( 1952) states that fur seals off Sitka, Alaska, have no evidence
which they descend much deeper than 30 fathoms (55 meters). He estimates thediving depth of fur seals from the depth of fishing lines on which fur seals sometimes get hooked. In these two reports, there is no description on the time when
fur seals were taken by nets or lines.
Generally, fur seals at sea show the inactive behaviors in the day time, except
for the migrating period from the breeding islands to wintering areas or from thelatter to the former. In the wintering area off the Pacific coast of the northern
Ja pan, sleeping fur seals often are observed in the day time. Japanese research
vessels for fur seals have observed and recorded the behavior of seals before theseseals are taken at sea. Provided that swimming or moving to some directions
indicates the active behavior and sleeping or resting at a location does the inactive
behavior of seals, we can examine from the observation records when fur seals move
about at sea. The activity index (M) of seals at sea, therefore, will be expressedby the following formula.

10090£> 80:;:;u"'~cQ)

u70
60m0.. 50
403045678910 11 12 13 14 15 16 17 18 19 20
Hour in the day

Fig. I. Activity offur seals at sea in each hour of the day, in the Pacific coast of thenorthern Japan from March through June.

* Far Seas Fisheries Research Laboratory, Fisheries Agency.
Sci. Rep. Whales Res. Inst.,
No. 24, 1972, 145-148.

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ICHIHARA AND YOSHIDA

M=a / a+i X 100
Where, a is the number of active seals and i is the number of inactive seals
at the time of observation. Fig. 1 is given when the value of M is arranged byeach hour in a day on the basis of observation records from March through June,
when is the latter half of the wintering season of fur seals. Number of seals observed comprised 1483 males and 1677 females from 1958 to 1967. In both sexes,
while seals are the most inactive at 1000 in the morning, they are active before
0600 in the morning and after 1800 in the afternoon. Hence, 5 of 10 seals are in
the state of sleeping or resting at 1000. This pattern of activity is maintained inspite of the progress of month. Although females are relatively more active thanmales, it is clear that fur seals are the nocturnal animal. According to several
records, fur seals unexpectedly observed at night always were quick in action.
Such a nocturnal behavior relates directly to feeding activity of fur seals. Mostof seal stomaches collected in the day time has no food or food under digestion.
According to the report of the Japanese pelagic research in 1958 off the coast of
the northern Japan, about 90% of seals taken at 0500 has food in their stomachs.
With the progress of hour in the day time, the occurrence of empty stomach increases and reaches 80% of the total samples collected at 1800 in the afternoon.

---------~...,•• 1"11""••*"'4UW:sa •au 11·--· t·'M050Fig. 2. Images of diving fur seals recorded by the echo-sounder of 28 KHz, in theJapan Sea.

After examining the digestive stages of food in the stomach by each hour, thisreport suggests that fur seals generally take food before sunrise. When the feeding
activity of fur seals reaches the maximum is not known because the hunting of seals
at night is difficult. Probably the searching activity for food at night determines
the nocturnal behavior of fur seals.
In the pelagic research in the Ja pan Sea, we fortunately took the records ofSci. Rep. Whales Res. Inst.,
No. 24, 1972.

DIVING DEPTH OF NORTHERN FUR SEALS147

diving fur seals with eco-sounder of 28 KHz. At 1900 on May 3 in 1970, theresearch vessel Tokai University II, 702.61 ton, stopped to sail as it became dark
at the location of 39-47 N, 132-47 E in the western Japan Sea. It was foggy inthe evening and the visibility was about 150 meters. In the afternoon of that day,no seal was observed along the sailing course of the vessel. Induced by the lightfrom the deck, however, 5 seals began to swim about the vessel and sometimes passed
beneath the bottom of vessel. The sonar which operated continuously throughthe voyage to examine the vertical distribution of marine organisms, took the swimming images of fur seals on the recording paper. As the vessel was equipped withthe sonar at the bottom of the bridge, from the bridge we could confirm seals coming
to surface to respire. The other mammals than fur seals were not observed in thewaters near the research vessel. The click of dolphins recorded through 28 KHz
echo-sounder by Shibata and Nishimura (1969) was not observed in our recordingpaper. Fig. 2 shows the diving images of seals within the beam of 28 KHz sound.
Such clear images were recorded from 1937 through 2245 at night and afterwards
1 or 2 seals repeated to enter the beam of sonar.

0M.•·~, ....

50Fig. 3.

Experimental image of a killed fur seal recorded by the echo- sounder of28KHz.

In order to confirm the image of animal observed on the record of sonar, weSci. Rep. Whales Rep. Inst.,
No. 24, 1972.

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ICHIHARA AND YOSHIDA

tried a test by using a killed fur seal. On May 19, we took a female seal of age
5 in the Pacific coast of the northern Japan. The body weight and the body lengthof the animal was 34 Kg and 128 cm respectively. Two small boats were prepared
for the test in both sides of the research vessel. The weighted seal tied up witha long rope was pulled rapidly in the water along the center of beam of sound.
When the seal passed through the bottom of the research vessel, an image wasrecorded as shown in Fig. 3. We can find the similarlity in images between Fig.
2 and Fig. 3, although the recorded depths are different between them.
In Fig. 2, fur seals swim in the depth of 100 meters and over from 40 meters
under the drifting vessel. The record of sonar shows that deeply diving seals do
not move horizontally but move rapidly to the deeper waters. As fur seals pursued
after schools of squids which were induced by the light of vessel, it is reasonable
to suppose that these records indicate the diving depth in the feeding periodof fur seals. The species of squids was not determined. The water temperature
was 10.1°C in 0 meter, 9. 7°C in 10 meters, 8.6°C in 25 meters, 5.3°C in 50 metersand 2.8°C in 100 meters deep at the position of recording, and no remarkable
thermocline was observed within 100 meters in depth. These fur seals were notbull at the naked eye observation on their sizes in the sea surface. This record
indicates that fur seals descend the depth over 100 meters in the feeding and thatthey can dive deeper than in the previous records.
REFERENCESJAPANESE FISHERIES AGENCY, 1959. Report of Japanese fur seal research in 1958. Tokai Regional Fisheries
Research Laboratory, Special publication, 6: 32 p.
KENYON, K. W., 1952. Diving depths of the steller sea lion and Alaska fur seal. J. Mamm., 33 (2): 245-6.
SCHEFFER, V. B., 1946. How deep will a fur seal dive? Murrelet, 27 (2): 25.
SHIBATA, K. and M. NrsHIMURA, 1969. Analysis offish-finder records-VIII, Classification and interpretationof echo trace on the tuna fishing ground. Bull. of the Faculty of Fisheries, Nagasaki University, 28: 43-67.

Sci. Rep. Whales Res. Inst.,
No. 24, 1972.