Stomach-content analysis can be used to identify the species and size
of prey fishes eaten by marine mammals, and has been used for both qua
litative and quantitative estimates of their food requirements. Method
s that use mass or volume to determine the amount of fish eaten depend
on the degree of digestion and the time required for the stomach to e
mpty. The transit time of digesta was estimated by recording the initi
al defecation time and the time required for the stomach to empty in f
our harbour seals (Phoca vitulina). The seals were given meals varying
in size from 0.5 to 2.1 kg and in caloric content from 4940 to 12 770
kJ/kg, and containing carmine or barium sulphate as a marker. The tim
e from eating until red faeces were observed was between 2 h 35 min an
d 6 h 15 min. Also, barium sulphate was injected into the food given,
and X-rays were used to estimate the time required for the stomach to
empty. The stomach started to empty less than 1 h after the meal. Some
prey remained in the stomach 5 h after eating.