Abstract.- Nine lactating north- 

 ern fur seals CaUorhinus ursinus 



from St. Paul Island, Alaska, were in- 

 strumented with time-depth record- 

 ers and head-mounted radio trans- 

 mitters in July and August 1985. 

 Seven females were subsequently 

 located at least once while at sea. 

 Diving patterns obtained from fe- 

 males' time-depth recorders were 

 then associated with their foraging 

 locations. Generally two diving pat- 

 terns were found; shallow-diving and 

 deep-diving. The deep-diving pattern 

 appears to be associated with feeding 

 throughout the day over the conti- 

 nental shelf in water less than 200 m 

 deep. The shallow-diving pattern is 

 generally restricted to nighttime 

 hours and probably occurs mostly 

 over deep water. An analysis of the 

 occurrence of food in stomachs of 

 lactating fur seals collected in the 

 eastern Bering Sea from 1958 to 

 1974 also suggests that seals col- 

 lected over the continental shelf 

 were more likely to be feeding dur- 

 ing the day. We examine differences 

 in the way shallow- and deep-diving 

 females forage and discuss possible 

 prey associated with the two diving 

 patterns. 



Diving Patterns and 

 Foraging Locations of 

 Female Northern Fur Seals 



Michael E. Goebel 



John L. Bengtson 



Robert L. DeLong 



Roger L. Gentry 



Thomas R. Loughlin 



National Marine Mammal Laboratory, Alaska Fisheries Science Center 

 National Marine Fisheries Service. NOAA. 7600 Sand Point Way NE 

 Seattle, Washington 981 15-0070 



Manuscript accepted 12 December 1990. 

 Fishery Bulletin, U.S. 89:171-179 (1991). 



The sexually dimorphic northern fur 

 seal CaUorhinus ursinus is a polyg- 

 ynous colonial breeder. Arrival of 

 adult females and pupping are highly 

 synchronous; most pupping occurs 

 between 21 June and 31 July. Fe- 

 males give birth to a single pup with- 

 in 1-2 days after arrival at the same 

 site each year. Copulation occurs ap- 

 proximately 5 days after parturition, 

 and females remain on shore 1-2 

 days more before going to sea to feed 

 (Gentry and Holt 1986). The first 

 feeding trip is the shortest, and sub- 

 sequent trips gradually become long- 

 er (Gentry and Holt 1986). Periods of 

 feeding at sea alternate with visits 

 ashore to suckle their pups. The peri- 

 od from birth to weaning is approx- 

 imately 125 days. 



Compared with the land phase of 

 the fur seal's life history, little is 

 known of their life and behavior at 

 sea. Kooyman et al. (1976) first re- 

 ported on depth and duration of dives 

 for a lactating northern fur seal. 

 General patterns of diving behavior 

 for breeding females were described 

 by Gentry et al. (1986c). They found 

 that individual females exhibit two 

 discrete types of diving patterns: 

 shallow and deep. Some females, ex- 

 hibiting both patterns, showed the 

 deep-diving pattern on the first and 

 last days of a trip and the shallow 

 pattern on other days. Fur seals ex- 



hibiting the deep-diving pattern typ- 

 ically dive to depths greater than 

 75 m without changing depth during 

 a diving bout and dive at all hours of 

 the day (Gentry et al. 1986c). 



Feeding locations of individuals ex- 

 hibiting different diving patterns, 

 however, were unknown. Loughlin et 

 al. (1987) initiated studies to deter- 

 mine what foraging areas in the Ber- 

 ing Sea were critical for lactating 

 females. In 1984 using a ship, they 

 located 11 females equipped with 

 radio transmitters and tracked 4 of 

 them to foraging locations. In 1985, 

 they located 20 females with radio 

 transmitters at sea from aircraft. 



This paper reports our efforts to 

 link fur seal diving behavior and 

 foraging locations by instrumenting 

 females with both radio transmitters 

 (to determine location) and dive re- 

 corders (measuring depth of dive). 

 We examine differences among fe- 

 males in diving behavior and their 

 patterns of foraging, and discuss how 

 diving behavior correlates with possi- 

 ble prey species. 



Study area and methods 



This study was conducted at Zapadni 

 Reef rookery on St. Paul Island, Pri- 

 bilof Islands, Alaska, from 19 July to 

 16 August 1985 (Fig. 1). Nine female 

 northern fur seals with pups were 



171 



