Quantifying errors associated with using fecal samples to determine the diet of the California sea lion (Zalophus californianus)

The purpose of this study was to quantify the errors associated with using fecal samples to determine the diet of the California sea lion (Zalophus ca lifornianus). Fishes and squids of known size and number were fed to five s ea lions held in enclosures with seawater-filled pools. Enclosures were was hed and pools were drained periodically so that sea lion feces could be col lected using a 0.5 mm mesh bag. Fish otoliths and squid beaks were collecte d from feces and used to estimate number and size of prey eaten. An average of 50.7% (SE = 6.4%) of 430 fishes and 73.5% (SE = 12.0%) of 49 cephalopod s fed to sea lions were represented by otoliths and beaks in feces, respect ively. Estimated lengths of fish from feces were less than lengths of fish fed to sea lions by an average of 30.1% (SE = 2.8%). Beaks were not digeste d significantly; estimated lengths of squid were underestimated by an avera ge of only 3.3% (SE = 1.5%) relative to actual lengths. Passage rates of ot oliths varied, but more than 70% were recovered within 48 h after the fish was consumed. Passage rates of beaks were generally less than those of otol iths; six beaks (11%) were collected in feces 4 days after the squid were e aten. Correction factors were created to more reliably estimate the number and size of fishes and cephalopods eaten by California sea lions.