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Fishery Bulletin 89(2). 1991 



Figure 1 



Study area with plotted locations of seven female northern 

 fur seals equipped with time-depth recorders and their rela- 

 tion to the continental shelf break (the 200-m contour). 



captured with a noose pole, removed from the rookery, 

 and placed on a restraint board (Gentry and Holt 1982). 

 Each female was tagged on the fore-flippers (Allflex 

 sheep ear tag) and equipped with a radio transmitter 

 (164 Mhz, Advanced Telemetry Systems, Isanti, MN) 

 and a photomechanical time-depth recorder (TDR) 

 (Meer Instruments, Solana Beach, CA). Radio transmit- 

 ters were attached to the top of each seal's head with 

 quick-setting epoxy resin (Devcon EK-40) (Loughlin et 

 al. 1987) and TDRs were attached to seals by harness 

 (Gentry and Kooyman 1986). After the epoxy resin had 

 hardened (20-25 minutes), each female was returned 

 to her capture site and released. Females were recap- 

 tured with a hoop net on their next visit ashore and 

 the TDR was removed by cutting the harness. When 

 possible, the mass for each female was determined at 

 each capture. 



Females were located at sea using a twin-engine 

 airplane equipped with a two-element Yagi antenna 

 mounted on each side of the fuselage (Loughlin et al. 

 1987). A total of 60 hours was flown on predetermined 

 transects within 300km of St. Paul Island at speeds 

 of 100-120 knots and at 1200 m altitude. 



Film from recovered TDRs was developed in either 

 Agfa Rodinol or Kodak D-19. Each record was repro- 



duced on paper with a 7 x enlargement using copy flow 

 xerography. At least three points of each dive were 

 digitized on an electronic digitizing pad: the start, the 

 end, and the maximum depth. These points allowed 

 computation of dive duration, depth, and interdive sur- 

 face interval. Each record was analyzed for bouts of 

 diving using the same criteria used by Gentry et al. 

 (1986c) for Callorhinus: five or more dives with less 

 than a 40-minute surface interval between each dive. 



The duration of diving bouts, number of dives per 

 bout, percent time below the surface, and number of 

 dives per hour were calculated as indices of the pat- 

 terns of foraging. 



The percent time spent below the surface was calcu- 

 lated separately for all dive bouts that occurred at mean 

 depths of less than 75 m and for bouts greater than or 

 equal to 75m. An ANOVA on percent time below the 

 surface was made after an arcsine transformation of 

 the data. All comparisons for significant differences in 

 depth were made using ANOVA with significance ac- 

 cepted at a < 0.05. 



The number of dives per hour was calculated for 

 daytime (0700-2259) and nighttime (2300-0659) hours 

 and for the entire record for each female. Percent of 

 total dives for day and night hours was also calculated. 



To further test the relationship between foraging 

 locations and dive patterns, we used data from the Na- 

 tional Marine Mammal Laboratory's pelagic fur seal 

 database, which provided a large sample of lactating 

 females that had been collected from July to Septem- 

 ber, 1958-74, from the eastern Bering Sea. We com- 

 pared the proportion of stomachs with food to empty 

 stomachs of lactating females collected over the con- 

 tinental shelf (<200m depth) and for those collected 

 off the continental shelf (>200m depth; chi-square 

 analysis). For the comparison, we used specimens ob- 

 tained between the hours of 0600-1000 and 1400-1900 

 (we chose 0600 and 1900 because they were the start 

 and the termination of sampling, and we arbitrarily 

 selected 1000 and 1400). 



Results 



Four of the nine females (nos. 195, 566, 651, 695) in- 

 strumented with TDRs and radio transmitters were 

 deep divers, diving at all hours of the day; the other 

 five (nos. 214, 308, 713, 767, 854) were shallow divers 

 and dived primarily at night (Fig. 2). 



Seven of these females, three deep divers and four 

 shallow divers, were located at sea at least once. 

 Female 195 was located on the third day of a 12-day 

 trip at the continental shelf break. At this location the 

 bathymetry changes rapidly, and because aerial loca- 

 tions are only approximate this female could have been 



