Digestion of capelin, Mallotus villosus (Müller), herring, Clupea harengus L., and polar cod, Boreogadus saida (Lepechin), otoliths in a simulated seal stomach

The ability of otoliths to withstand digestion in a seal stomach appears to differ between fish taxa with the otoliths of gadoids being particularly resistant to gastric erosion. Few studies have examined the effect of otolith size on the rate of otolith dissolution. We exposed 50 otoliths (total sagittal length: 1.4–7.0 mm) from capelin (Mallotus villosus, n = 20), herring (Clupea harengus, n = 10), and polar cod (Boreogadus saida, n = 20) as well as nine whole capelin (total body length = 100–120 mm) to digestive solutions assumed to mimic the gastric environment of Arctic seals in vitro (pH: ∼2, temperature: ∼37°C, pepsin concentrations: 0–750-μg ml−1 solution). Pepsin did not affect the dissolution rates of sagittae but resulted in a complete digestion of whole capelin within 10 h. Irrespective of species examined, sagittae fitted the same pattern of dissolution rate with the smaller sagittae being dissolved faster than did the larger ones. We suggest that scaling controls sagittal dissolution rates (DR, μm min−1), and that the sagittae from the three forage fish combined dissolve according to sagittal length (SL, mm) as shown by the equation: DRCommon = 27.348e−0.120 SL (n = 46, r2 = 0.688, p < 0.001). Given this relationship, the numerical importance of prey taxa with the smaller otoliths – i.e. also the small-sized gadoids (polar cod) – is likely to be underestimated in feeding studies with Arctic seals.