A comparison of indexes for prey importance inferred from otoliths and cephalopod beaks recovered from pinniped scats

In this study a similar data set was used to compare results produced by using four indexes used for quantifying the California sea lion prey. Hard structures like Sagitta otoliths and cephalopod beaks have traditionally been used in pinniped diet analyses. They are important since their shape and inner features can be species or genus specific and aid in the identification of prey. Scats are currently the most used sample type in pinniped diet analyses because they can be easily collected, are abundant, noninvasive and provide enough otoliths and cephalopod beaks. Traditionally, food habit studies of the California sea lion Zalophus californianus based on scats have considered the Percent Number and Percent Occurrence as measures to determine prey importance. There are two other lesser-used indexes: Split-Sample Frequency of Occurrence and Index of Importance that have also been used for quantifying the California sea lion prey. A total of 251 hard structures (fish otoliths and cephalopod beaks) recovered from California sea lion scats collected at Isla San Pedro Mártir in the Gulf of California, México, were examined. Prey taxa were identified using voucher specimen material. The California sea lion diet was estimated using the four indices described above and the results were compared using χ² tests and correlation coefficients. Results obtained from four measurements were consistent in ranking the three main prey taxa but different in relative importance. A strong correlation was found among indexes, but the standardized residuals indicated that linear regression was not an adequate model to describe the relationship between PN and any other indexes. Representing the diet using a single measurement such as IIMP can facilitate the interpretation of results, allow comparisons and promote consistency in pinniped diet studies based on scats.