578 



Fishery Bulletin 101(3) 



153°0'W 



152''0'W 



58°0'N- 



57°0'N' 



•58°0'N 



■57°0'N 



152°0'W 



isro'w 



Figure 8 



Figure showing the progressive increase in distance of locations from 

 shore for a B-nionth-oId sea lion over time. This animal (identification 

 number: PTT 21094) was equipped with a satellite transmitter near 

 Kodiak Island in March 2000. Early trips were <15 km from shore in 

 water <50 m in depth near the capture site. Trips became progi"essively 

 greater as the animal matured through May 2000 when it was venturing 

 over 50 km from shore in water >100 m in depth. 



Ontogeny of diving ability has been studied in two other 

 otariids. Baker and Donohue (2000) used data loggers 

 (which they termed "time wet recorders") to measure time 

 spent in the water and diving behavior of northern fur 

 seal {Callorhinus iiniiriiis) pup.s on St. Paul Island, Alaska. 

 These pups began spending substantial time in the water 

 at approximately 40-50 d of age that coincided with growth 

 of the under fur and increases in sea surface temperature. 

 Time spent in the water increased up to about 100 d of age; 

 diving to depth did not occur until they were much older and 

 about to migrate. Horning and Trillmich (1997) conducted 

 an extensive study on the ontogeny of diving behavior in 

 Galapagos fur seals (Arctoccphaliis f^alapagnensis), a spe- 

 cies that weans no sooner than 2 years of age. They found 



that in young the development of diving behavior was close- 

 ly linked to dependence on the mother and that substantial 

 diving activity did not occur until one year of age; but even 

 then the young fur seals were still nutritionally dependent 

 on their mothers and did not dive as deep, or for as long, as 

 mature females. The weaning date for Steller sea lions is 

 unknown but is assumed to be between 4 and 12 months, 

 and most pups are weaned just before the next breeding 

 season ( 11-12 months) (Porter, 1997). The change in diving 

 characteristics that we report is interesting in that it coin- 

 cides with this period. Prior to weaning these pups forage 

 in the company of their mother and learn to forage on their 

 own; the need to dive deep for long periods to acquire food 

 is compensated by nursing from the mother. Once weaning 



