254 



Fishery Bulletin 103(2) 



(Pascual and Adkison, 1994; York, 1994; NMFS 2 !, one 

 might expect the animals in the declining population 

 to show signs of nutritional stress compared to those 

 in the stable population. The results for pup size and 

 growth give no indication of food stress during early 

 lactation. In fact, pups from the declining population 

 on Seguam, Yunaska, and Chirikof Islands grew faster 

 than pups from the stable population on Lowrie Island 

 during the first six weeks. Similar results were also 

 found in a study of pup BM (Merrick et al., 1995), in 

 which pups were weighed on rookeries from Oregon to 

 the Aleutian Islands in late June and early July from 

 1987 to 1994. Although the pups' ages were unknown, 

 weighing date was used as a covariate in the analysis. 

 Merrick et al. (1995) found a continuous increase in 

 pup BM from Oregon to southeast Alaska and to the 

 Aleutian Islands. These investigators also concluded 

 that pup BM was on average greater in the declining 

 population. 



In most other studies of declining populations or dif- 

 ferences among rookeries, such contradictory results 

 have not been seen. A study of California sea lion pups 

 during an ENSO (El Nino Southern Oscilliation) event 

 revealed lower pup growth during the period of food 

 stress (Boness et al., 1991). Trillmich and Limberger 

 (1985) have also seen clear effects of low food avail- 

 ability during an ENSO in Galapagos fur seals and 

 sea lions. Antarctic fur seals are affected in predictable 

 ways (increased pup mortality and increased female for- 

 aging time) during times of decreased food availability 

 (Costa et al., 1989). Hood and Ono (1997) found that in 

 the declining California population of SSLs, pups spent 

 less time suckling when adult females made longer for- 

 aging trips in 1992 than in 1973 when the population 

 was larger. The longer foraging trips suggested less 

 abundant food resources. 



Considering the results for SSL pup growth in light 

 of the population decline, we suggest three alternative 

 hypotheses: 1) food availability was never a factor in 

 the population decline; 2) food availability caused the 

 overall decline, but lactating females and their pups 

 were not affected during early lactation; or 3) our study 

 was conducted when pups and lactating females were no 

 longer experiencing decreased food availability. 



Faster rates of pup growth may be normal for the 

 Aleutian Islands and western Gulf of Alaska despite 

 the population decline. The declining and stable popula- 

 tions are genetically distinct (Bickham et al., 1996), and 

 perhaps the differences seen in our study are normal 

 differences between the two populations. It is impos- 

 sible to determine if growth and foraging behavior have 

 changed over time because historical data on maternal 

 investment are sparse. Juveniles rather than neonates 

 may be the affected age class in the declining popula- 

 tion (Merrick et al., 1988), whereas lactating females 

 are feeding on either different prey or age classes and 

 not experiencing decreased food availability. York (1994) 

 constructed a population model for SSLs in Alaska and 

 concluded that the current population decline could be 

 accounted for by increased juvenile mortality. 



Alternatively, because our study was performed late in 

 the decline, the higher growth rates could be the result 

 of lower population density and less competition for food 

 in the declining population. Trites and Bigg (1992) re- 

 ported larger body sizes in northern fur seal populations 

 during a period of decline. The northern fur seal popula- 

 tion in the Pribilof Islands in the Bering Sea increased 

 from the early 1900s to the 1950s. During this period, 

 adult body size decreased. From 1950 to the 1970s the 

 population declined and there was a concurrent increase 

 in individual body size (Trites and Bigg, 1992). Scheffer 

 (1955) hypothesized that increased body size was due 

 to decreased competition for food, which in turn would 

 be due to the lower population density. It is possible 

 that the same density-dependent effects are occurring 

 in the declining SSL population because our study was 

 performed late in the decline, after the original cause 

 may have abated. More information will be needed to 

 determine the cause of the SSL decline and whether it 

 is related to availability of food, especially for different 

 age classes, and to different times of the year. 



Acknowledgments 



We thank T. Adams, R. Andrews , D. Bradley, J. Burns, 

 M. Castellini, J. K. Chumbley, W. and S. Cunningham, 

 J. Davis, F. Gulland, D. Gummeson, B. Heath, D. John- 

 son, S. Kanatous, D. Lidgard, R. Lindeman, R. Merrick, 

 D. McAllister, L. Milette, K. Ono, L. Polasek, T. Porter, 

 D. Rosen, J. Sease, T. Spraker, U. Swain, W. Taylor, A. 

 Trites, D. van den Bosch, T Williams, and the captain 

 and crew of the RV Mecleia for assistance in the field. 

 We thank K. Andrews for the map and D. Brandon for 

 assistance in data collection and analysis. G. Worthy, A. 

 Trites, T. Lacher, D. Owens, and M. Reynolds reviewed 

 an early version of this manuscript. Funding and logis- 

 tical support in the field were provided by the Alaska 

 Department of Fish and Game, the National Marine 

 Fisheries Service/National Marine Mammal Labora- 

 tory, Texas A&M University, and the Texas Institute 

 of Oceanography. This research was conducted under 

 Marine Mammal permit no. 846 and 963. 



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