This was inevitably destructive and is now widely considered unacceptable. However, stomach contents of seals can be obtained without killing the animal using lavaging or stomach pumping techniques. For seals, the second method is collection of faeces from haul-out sites. Some seals also regurgitate undigested hard parts of prey. Finally, some information can be obtained by direct observation of feeding.
Seals often bring large fish to the surface to eat.
Identifying and quantifying prey remains This usually comes down to identifying fish otoliths or bones and cephalopod beaks. There are various guides available but reference material is essential. Quantifying diet can be done on the basis of numbers or frequency of occurrence but usually the proportion by weight of each prey species is estimated. The original weight of prey can be estimated by measuring otoliths, bones or beaks. Thus, the sample should include males, females, young and old in proportion to the amount of food consumed by each component of the population.