California sea lions (Zalophus californianus) are the most numerous pinniped species found in California. Consequently, sea lions have the potential to consume large quantities of fish and cephalopods (i.e., squid and octopus), some of which are commercial and sport species. Sea lions also interact with commercial and sport fisheries by taking their catch and damaging fishing gear.
Mark Lowry collecting a scat sample at San Nicolas Island, California.
The objectives of the sea lion diet study being conducted by Coastal Marine Mammal Program are to:
- Identify prey species being consumed by sea lions in order to determine which are most important to the sea lion population. Prey species are identified by hard parts, such as fish otoliths and squid beaks found in scats. See the recently published
- Monitor seasonal, annual, and multi-year variability in consumption of prey species.
- Determine if dietary changes can be used to signal when the population reaches carrying capacity,
- Estimate consumption by the sea lion population.
- Use sea lion diet data as a fishery-independent index of abundance for commercial species of fish and squid.
California sea lions pup and breed at four of the nine Channel Islands in southern California. Since 1981, CMMP has been conducting a diet study of sea lions at San Clemente Island (a small rookery) and San Nicolas Island (a large rookery). Information on the diet of sea lions is obtained from analyzing scats (i.e., fecal samples) and spewings (i.e., vomitus) collected at those two rookeries in January (winter), April (spring), July (summer), and October (autumn). Otoliths (a crystalline structure within the ear organ) from fish and beaks (mandibles composed of chitin) from cephalopods are recovered from the samples by washing each sample through sieves of varying mesh size. Otoliths and beaks, which are shaped and sized differently for each species of fish and cephalopod, respectively, are used to identify and enumerate fish, and cephalopods consumed by sea lions. Also, otoliths and beaks are measured for estimating size of prey being consumed by sea lions. Northern anchovy (Engraulis mordax), Pacific sardine (Sardinops sagax), Pacific whiting (Merluccius productus), Pacific mackerel (Scomber japonicus), jack mackerel (Trachurus symmetricus), shortbelly rockfish (Sebastes jordani), and market squid (Loligo opalescens) are the main prey of California sea lions in southern California. They also consume a variety of other fish and squid, but at lesser amounts than those just mentioned. The study has also revealed that not only are there seasonal cycles to consumption of certain prey (such as market squid being consumed mainly in autumn and winter), but that multi-year cycles in consumption of prey also take place that appear to be governed by multi-year differences in abundance and availability of various prey species. The study has also determined that the diet of sea lions becomes more variable during El Nino oceanographic events due to decreased abundance and availability of their preferred prey. It is anticipated that the dietary response to El Nino will also occur when the population reaches the maximum size sustainable by the environment (i.e., it will reach carrying capacity).
The principal investigator is Mark Lowry of the California Current Marine Mammal Assessment Program (CMAP).
Throughout the years of the study, various personnel from CMMP and Marine Mammal and Turtle Division (PRD) have helped with the study. Information about this project can be obtained from the following publications:
Lowry, M. S., C. W. Oliver, C. Macky, and J. B. Wexler. 1990. Food habits of California sea lions, Zalophus californianus, at San Clemente Island, California, 1981-86. Fish. Bull., U. S. 88:509-521.
Lowry, M. S., B. S. Stewart, C. B. Heath, P. K. Yochem, and J. M. Francis. 1991. Seasonal and annual variability in the diet of California sea lions, Zalophus californianus, at San Nicolas Island, California, 1981-86. Fish. Bull., U. S. 89:331-336.
Lowry, M. S. and J. V. Carretta. 1999. Market squid (Loligo opalescens) in the diet of California sea lions (Zalophus californianus) in southern California (1981-1995). CalCOFI Rep. 40:196-207.