California Sea Grant College Program

I investigated diving behavior, movement, and spatially explicit foraging behavior of the sexually dimorphic male California sea lion and evaluated the extent to which blood and muscle oxygen storage capacity influenced behavioral patterns. Satellite-linked dive recorders were used to investigate the movement and diving patterns of 25 sub-adult and adult male California sea lions during 2003-04 and 2004-05. Male diving behavior was consistent with that of a typical shallow epipelagic to mesopelagic foraging strategy and a strong diurnal pattern. Variation in diving behavior was explained by the presence of three diving strategies among individuals including shallow, mixed depth, and deeper divers. Males dispersed, rather then a previously described migration, traveling north and south while foraging almost exclusively over the continental shelf during short trips in 2003-04. Ocean conditions were highly anomalous in 2004-05 and males altered their foraging effort during previously undocumented offshore excursions up to 450km offshore. Trips during 2004-05 were more than twice the distance and three times the duration of trips during 2003-04. Diving patterns also shifted with increased search time underwater with deeper and longer duration dives, and significant shifts in diet occurred that reflected changes in the diet during the 1997-98 El Niño.

I also examined the extent to which blood and muscle oxygen stores varied as a function of age, body size, and sex. Blood oxygen storage capacity was not fully developed by the end of the dependency period and muscle stores were not developed until animals were sub-adult size. Decreased myoglobin in adult males resulted in decreased mass-specific oxygen stores compared to females, which was unexpected given their greater body mass. Delayed development of body oxygen stores coupled with differences between the sexes resulted in different aerobic diving capacity among different size, age, and sex animals. Males dove well within their physiological diving capacity the majority of the time. This apparent excess in physiological dive capacity may provide male sea lions with the plasticity to develop alternative individual foraging strategies, and buffer them against fluctuations in prey distribution and abundance resulting from environmental perturbations such as those observed during 2005 and El Niño events.