There are three main sampling methods. The traditional approach used to be to examine stomach contents. 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.