566 



Abstract — Understanding the onto- 

 genetic relationship between juvenile 

 Steller sea lions iEiimetopiaa jubatus) 

 and their foraging habitat is key to 

 understanding their relationship to 

 available prey and ultimately their 

 sur\nval. We summarize dive and move- 

 ment data from 13 young-of-the-year 

 (YOY) and 12 yearling Steller sea lions 

 equipped with satellite dive recorders 

 in the Gulf of Alaska and Aleutian 

 Islands (n=18), and Washington (/i=7) 

 from 1994 to 2000. A total of 1413 d of 

 transmission (.v=56.5 d, range: 14.5- 

 104.1 d) were received. We recorded 

 222,073 dives, which had a mean depth 

 of 18.4 m (range of means: 5.8-67.9 m: 

 SD= 16.4 ). Alaska YOY dived for shorter 

 periods and at shallower depths (mean 

 depth=7.7 m, mean duration=0.8 min, 

 mean maximum depth=25.7 m, and 

 maximum depth=252 m) than Alaska 

 yearlings (.?=16.6 m, 0=1.1 min, .v = 

 63.4 m, 288 m), whereas Washing- 

 ton yearlings dived the longest and 

 deepest (mean depth=39.4 m, mean 

 duration=1.8 min, mean maximum 

 depth=144.5 m, and maximum depth= 

 328 m ). Mean distance for 564 measured 

 trips was 16.6 km; for sea lions slO 

 months of age, trip distance (7.0 km) 

 was significantly less than for those 

 >10 months of age (24.6 km). Mean trip 

 duration for 10 of the 25 sea lions was 

 12.1 h; for sea lions slO months of age, 

 trip duration was 7.5 h and 18.1 h for 

 those >10 months of age. 



We identified three movements types: 

 long-range trips (>15 km and >20 h), 

 short-range trips (<15 km and <20 h) 

 during which the animals left and 

 returned to the same site, and transits 

 to other haul-out sites. Long-range 

 trips started around 9 months of age 

 and occurred most frequently around 

 the assumed time of weaning, whereas 

 short-range trips happened almost 

 daily (0.9 trips/day, n=426 trips). Tran- 

 sits began as early as 7 months of age, 

 occurred more often after 9 months of 

 age, and ranged between 6.5 and 454 

 km. The change in dive characteristics 

 coincided with the assumed onset of 

 weaning. These yearling sea lion move- 

 ment patterns and dive characteristics 

 suggest that immature Steller sea lions 

 are as capable of making the same types 

 of movements as adults. 



Manuscript approved for publication 

 29 October 2002 by Scientific Editor 



Manuscript received 4 April 2003 at 

 NMFS Scientific Publications Office. 



Fish Bull 101:566-582 (2003). 



Diving behavior of immature Steller sea lions 

 iEumetopias jubatus) 



Thomas R. Loughlin 



Jeremy T. Sterling 



National Marine Mammal Laboratory 



Alaska Fishenes Science Center, NMFS 



7600 Sand Point Way, NE 



Seattle, Washington 98115 



E-mail address (for T R Loughlin) torn loughlin®noaa gov 



Richard L. Merrick, 



Northeast Fishenes Science Center, NMFS 



166 Water Street 



Woods Hole, Massachusetts 02543 



John L. Sease 



Anne E. York 



National Marine Mammal Laboratory 

 Alaska Fishenes Science Center, NMFS 

 7600 Sand Point Way, NE 

 Seattle, Washington 98115 



Steller sea lions range throughout the 

 North Pacific Ocean rim and are declin- 

 ing in numbers in most of Alaska and 

 Russia (Loughlin et al., 1992; Loughlin 

 and York 2000). Studies of mitochon- 

 drial DNA suggest that at least two 

 stocks exist: an eastern stock (Califor- 

 nia through southeast Alaska) and a 

 western stock (Prince William Sound 

 and areas west) (Bickham et al., 1996; 

 Loughlin, 1997). For the western U.S. 

 stock (west of 144°W), counts of adults 

 and juveniles have fallen from about 

 110,000 individuals in the late 1970s 

 to about 25,000 individuals in 2000— a 

 decline of almost 80%. Although the 

 numbers of sea lions that died were 

 greater from the late 1970s to the 

 early 1990s than at present, the rate of 

 decline has remained high. As a result of 

 this decline the U.S. government desig- 

 nated the western stock as "endangered" 

 in 1997 under the U.S. Endangered Spe- 

 cies Act; the eastern stock is designated 

 as "threatened." Reasons for the decline 

 in numbers are unknown but may be 

 hnkcd to reduced availability of prey 

 caused indirectly by environmental 

 changes or commercial fishing activi- 

 ties, or both (Loughlin and Merrick, 



1989; Merrick, 1995). Severe environ- 

 mental perturbations and commercial 

 fishing, both resulting in changes in the 

 abundance or availability of prey, have 

 been implicated in the alteration of pin- 

 niped foraging behavior and declines in 

 pinniped abundance (e.g. Trillmich and 

 Ono, 1991; Melin, 2002). One method for 

 studying the effect of reduced prey avail- 

 ability on pinnipeds is to measure diving 

 behavior and foraging ecology by using 

 either a time-depth-recorder ( Kooyman 

 et al., 1983; Gentry and Kooyman, 1986) 

 from which dive data are retrieved after 

 the animal returns from a feeding trip 

 (e.g. Goebel et al., 1991; Boyd et al., 

 1994; Werner and Campagna, 1995), or 

 by using a satellite-linked time-depth 

 recorder (SLTDR; the newer version is 

 called a "satellite dive recorder" SDR), 

 which transmits dive and transmitter- 

 status information to orbiting satel- 

 lites and thus eliminates the need to 

 recapture the animal (e.g. Merrick et 

 al., 1994). 



Few data are available concerning 

 the foraging ecology of Steller sea lions. 

 Merrick et al. (1994) and Merrick and 

 Loughlin (1997) presented information 

 on the dive characteristics and foraging 



