Loughlin et al ; Diving behavior of immature Eumetopias jubatus 



575 



~ -20 



E -40 



t -60 



-80 



Circles = Washington Stelter sea lions (n=7) 

 Squares = Alasl<a Stelter sea lions (n=18) 



i^ri^^ 



T 



N 



^ 



H 



— r- 

 10 



! 13 14 

 Age in months 



1 1 1 1 1 1 1 1 1 1 r 



15 16 17 18 19 20 21 22 23 24 25 26 



Figure 5 



Mean dive depth and duration as a function of age in months for Washington and Alaska 

 Steller sea hons. Error bars represent the standard error. Each connected symbol represents 

 one individual sea lion monitored over several months. 



of age, the mean trip duration was 7.5 hours (;i=307; 

 SD=7.5 h; range: 1-81.3 h; median=6 h) and for sea lions 

 >10 months of age, the mean duration of all trips was 18.1 

 hours («=237; SD=34.2 h; range: 1-344 h; median=10.3 h). 

 Averaged across individual animals, the mean duration of 

 trips at sea ranged between 6.2 to 21.4 hours; this range 

 was 6.2 to 17.2 hours for the younger animals and 10.3 and 

 21.4 hours for the older animals. The analysis of the re- 

 peated-measures ANOVA on the logarithm of trip duration 

 showed that the older sea lions had longer trip durations 

 (P<0.001). We could not test for gender and gender x age 

 effects because there were no measured trip durations for 

 females >10 months. Among the younger animals, there 

 was no gender difference in mean trip duration (P=0.11). 



Types of movement 



We identified three types of movements for the sea lions at 

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

 (<15 km and <20 h), and transits to other haul-out sites 

 (Fig. 9). Long-range trips most likely were foraging trips 

 and began around 9 months of age. These trips had a mean 

 of 48.7 km (SD=55.7 km; max=240.8 km) and may coincide 

 with the assumed onset of weaning; they represented 6% of 

 all trips to sea. The most numerous trips (S&'Yc ) were short- 

 range foraging trips (v=3.6 km; SD=0.4; max=21.0 km), 

 which happened almost daily (0.9 trips/d, n=A2& trips). 

 Transits were movements from one haul-out site to another 

 haul-out site; these trips were characterized as the straight 



line distance from one haul-out site to another and began 

 as early as 7 months of age but occurred more often after 

 9 months of age. Transit trips represented 6% of all trips 

 at sea and had a mean distance of 66.6 km (SD=83.7 km; 

 range: 6.5-341.9 km). 



Discussion 



The differences in diving behavior between young Steller 

 sea lions in Washington and those off Alaska are intriguing. 

 Possible reasons for these differences include variable habi- 

 tat type, prey resources, or morphological or genetic differ- 

 ences. However, there is no evidence, based on morphology 

 or genetics, to either support or refute differences in the 

 diving behavior that we observed. The evidence of genetic 

 differences between the western and eastern stock of Steller 

 sea lions is based on mtDNA haplotype differences for a seg- 

 ment of the mitochondrial D-loop which does not code for any 

 structural proteins (Bickham et al., 1996; Loughlin, 1997). 

 One morphological difference between the two stocks 

 is a progressive increase in mass of Steller sea lion pups 

 from east to west (Merrick et al., 1995), but whether this 

 difference in mass continues with increasing age is un- 

 known. Large animals typically dive deeper and longer 

 than smaller (and younger) animals (Schreer and Kovacs, 

 1997). Larger animals have less drag per unit of mass and 

 generally have more blood than smaller ones and thus are 

 able to store more oxygen. Larger animals also have lower 



