Proxies of food intake and energy expenditure for estimating the time-energy budgets of lactating northern fur seals Callorhinus ursinus

Northern fur seal Callorhinus ursinus numbers on the Pribilof Islands have been in decline over the past two decades while numbers have increased at other sites, such as in the Kuril Islands in Russia and at Bogoslof Island in the Aleutian Islands. Although these divergent abundance trends remain unexplained, the potential influence of food limitations could be studied by examining the time-energy budgets of individual lactating fur seals. Our goal was to find suitable proxies for food intake and energy expenditure of lactating seals so that we might provide time and energy budgets of an increasing population on Lovushki Island, Russia that could be compared with the declining Pribilof fur seal population. Between 2005 and 2008, we outfitted 13 lactating fur seals with instruments that collected data on depth, external temperature, stomach temperature, GPS location, and acceleration. With these data we investigated proxies for 1) food intake, including time spent at sea, time spent diving, vertical travel distance (VTD), wiggle behavior, and changes in stomach temperature; 2) involuntary costs, including indices of basal metabolism (BMCI), thermal exposure, and instrument hydrodynamic drag (TAG); and 3) locomotive costs, including overall dynamic body acceleration (ODBA), horizontal travel distance, and flipper strokes. We also considered an index of work (WORK) calculated using horizontal distance, ODBA, and seal mass. Multimodel inference was used to determine which proxies best described changes in seal mass at sea when considering two sets of candidate models that differed by excluding or including locomotive costs (n = 13 and n = 7, respectively). Seal mass changes were positively associated with VTD and TAG and negatively associated with BMCI (p < 0.001, adjusted R2 = 0.84) when locomotive proxies were excluded. With locomotive proxies considered, models were improved and mass changes were best described by being positively related to VTD and negatively associated with WORK (p < 0.001, adjusted R2 = 0.95). We suggest that relative energy budgets for individual lactating northern fur seals could be compared using proxies for energetics that are easily monitored with standard data logging instruments.