Limitations of Using Otoliths Recovered in Scats to Estimate Prey Consumption in Seals

Abstract

Of three length classes of herring, Clupea harengus (30–35, 20–25, and [Formula: see text] total length), fed to a captive harbor seal, Phoca vitulina, otoliths were recovered only from herring 30–35 cm long; of these otoliths only 4% were recovered. Using surface area to volume ratios of otoliths, other fish species were predicted to be similarly underrepresented in scats. After digestion, otoliths showed fewer annuli and a reduction in length and width. Herring otoliths in harbor seal scats indicated the consumption of large herring ([Formula: see text] total length) only and could not be used to estimate numbers of prey consumed. The partial digestion of herring otoliths will result in underestimates of fish age, length, and weight if calculations are based on standard relationships between these parameters and measurements from undigested otoliths.