Abstract
Analysis of teleost sagittal otoliths contained in scats has been widely used to determine the diet of seals. This method is based on the assumption that relative frequencies of otoliths in scats faithfully reflect those offish in the diet. This assumption has rarely been tested experimentally. We compared the ratios of herring (Clupea harengus) to sprat (Sprattus sprattus) otoliths in faeces (output) of captive California sea lions (Zalophus californianus) and South American fur seals (Arctocephalus australis) with the ratios at feeding (input). Sea lions and fur seals showed no consistent differences in recovery rates and partial digestion of otoliths. Output ratios deviated only slightly from input ratios, the smaller sprat otoliths being underrepresented in the output by 8%. Only about 40% of the otoliths fed to the seals were found in the scats. For both species partial digestion of otoliths led to a 16% underestimation of fish length and a 35% underestimation of fish mass.
Highlights
Analysis of teleost sagittal otoliths contained in scats is a frequently employed method for assessing the composition of pinniped food
These studies have shown that otoliths can decrease in length during passage through the intestinal tract, that recovery of ingested otoliths can be very low (Hawes 1983; DaSilva and Neilson 1985), that the probability of an otolith being digested depends on the fish species but appears to be a random event for otoliths of a given species in a meal (Murie and Lavigne 1985), and that otoliths of different fish species may not appear in scats in the same ratios in which they were eaten (Hawes 1983)
We report feeding experiments on captive individuals of two species of otariid seals, the California sea Printed in Canada I Imprime au Canada lion (Zalophus californianus) and South American fur seal (Arctocephalus australis)
Summary
Analysis of teleost sagittal otoliths contained in scats is a frequently employed method for assessing the composition of pinniped food. Total or partial digestion of otoliths has been recognized as a serious problem by previous authors (Prime 1979; North et al 1983; DaSilva and Neilson 1985; Prime and Hammond 1987), few experimental studies have been conducted. These studies have shown that otoliths can decrease in length during passage through the intestinal tract, that recovery of ingested otoliths can be very low (Hawes 1983; DaSilva and Neilson 1985), that the probability of an otolith being digested depends on the fish species but appears to be a random event for otoliths of a given species in a meal (Murie and Lavigne 1985), and that otoliths of different fish species may not appear in scats in the same ratios in which they were eaten (Hawes 1983). While estimates of total biomass intake derived from otoliths in scats appear unreliable (unless complex corrections for total and partial digestion are made), it is not so clear whether the relative frequencies of otoliths in scats provide a reliable estimate of the relative number of fish ingested
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