Fishery Bulletin 89(2). 



1991 



et al 1986b). Fur seals foraging over the shelf were 

 likely to feed on walleye pollock Theragra chalco- 

 gramma, Pacific herring Clupea harengus pallasi, and 

 capelin Mallotus villosus (Kajimura 1984). Each of 

 these prey items is distributed throughout the water 

 column over the shelf, depending on the sex and age 

 of the individual and time of day; however, they are 

 principally found near the bottom (Bakkala and Waka- 

 bayashi 1985). Even when prey are near the bottom 

 over most of the shelf floor, they are shallower than 

 the maximum diving depths observed for most fur seals 

 and are accessible during all hours of the day. 



The results of foraging effort for shallow and deep 

 divers in this study are consistent with the results of 

 Gentry et al. (1986c) in their study of diving of females 

 from St. George Island. Costa and Gentry (1986) used 

 isotopic turnover methods to measure food intake and 

 metabolic rate in female fur seals instrumented with 

 time-depth recorders. In that study of two deep-diving 

 and two shallow-diving fur seals, the deep divers ate 

 less food and expended less energy but gained similar 

 body mass on a single trip to sea. Our results confirm 

 their conclusion. In this study, using four measures- 

 bout duration, dives per bout, dives per hour, and time 

 spent below the surface-deep divers expended less 

 effort on foraging than shallow divers. Deep divers 

 apparently obtain greater energy per dive. 



The difference in foraging effort of diving types 

 may be explained by differences in energy content 

 of prey, success rate for prey capture, and/or average 

 size of prey captured. If success rate per dive and 

 average size of prey were similar for both diving types, 

 then energy content for the prey of deep divers would 

 have to be greater. If prey were of similar size and 

 energy content, then the prey of deep divers would 

 have to be easier to capture. If success rate and energy 

 content were similar, then the prey captured by deep 

 divers would have to average a greater size per dive. 

 Unfortunately, no data exist on success of individual 



dives. 



Data on the energy content of prey items of northern 

 fur seals suggest that Gonatus sp. and walleye pollock 

 have similar energy content (M.A. Perez and T.R. 

 Loughlin, NMFS Natl. Mar. Mammal Lab., Seattle, 

 unpubl. data). No data exist for energy content of 

 bathylagids. Perez and Bigg (1986) report on the 

 size range of prey found in the stomachs of northern 

 fur seals collected during 1958-74: walleye pollock, 

 4-40cm (1721 prey from 71 stomachs); gonatid squid, 

 5-24 cm (>59 prey from 10 stomachs); deep-sea smelts, 

 8-12cm (986 prey from six stomachs). The size range 

 for Myctophiform fish prey and the mean size of prey 

 were not reported. 



Transit times 



The most frequently used measure of transit time, 

 derived from time-depth records, has been the time 

 from shore to the first diving bout and from the last 

 dive bout to shore (Gentry and Kooyman 1986). Data 

 from the current study show that female fur seals fre- 

 quently feed while traveling to very distant feeding 

 locations and that the time from shore to the first 

 diving bout is only a subset of the total time in transit. 

 There was no difference in the time from shore to 

 the first dive bout for the two types of diving patterns 

 observed in this study. Similarly, the times from the 

 last dive bout to arrival ashore were not different. Fur- 

 thermore, no correlation was found between these 

 times and' feeding trip duration. It has been suggested, 

 however, that a correlation exists between feeding trip 

 duration and distance to feeding area (Gentry et al. 

 1986a). Gentry et al. (1986a) found that regressing 

 transit time of individuals upon their trip duration 

 resulted in a poor fit (r 2 0.357). When they compared 

 species averages using two tropical and two sub-polar 

 otariid species, the correlation was much greater (r 2 

 0.761). They concluded that transit time may largely 

 determine trip duration for the species, but its effect 

 is partly obscured by the large variation in transit times 

 and trip duration of some individual animals. 



The results of Loughlin et al. (1987) and of this study 

 show that females feed while in transit to primary 

 foraging areas. A correlation may exist between trip 

 duration and total time spent in transit; however, 

 measuring the time from shore to the first diving bout 

 and from the last diving bout to shore is only a subset 

 of actual time spent in transit and therefore an inade- 

 quate measure of total transit time. Without knowledge 

 of either the swim velocity or the location of females 

 (either through radio-tracking or with some instrument 

 carried by the animal to record location) it is not possi- 

 ble to discern from a record of a time-depth recorder 

 the actual time spent in transit. 



Classification of diving patterns 



It is important to point out that any classification of 

 diving patterns gives the impression that they are more 

 discrete than they really are. It is more accurate to view 

 any particular diving record as fitting into a continuum 

 from strictly shallow diving at night to exclusively deep 

 diving at all hours. The terminology used to categorize 

 these diving patterns may overemphasize the impor- 

 tance of depth. It should be remembered that though 

 deep- and shallow-divers are classified as such, the 

 depth of dives may not be as important for identifying 

 the pattern as the time of day in which diving occurs. 



