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Finding what seals eat
Scientists have several ways of finding out what seals eat. One way is to look at the bits of the fish and squid inside a dead seals stomach. Although this is not a very nice way of finding out what they eat.
Another way is to look in their poo. By sifting through seal poo, scientists can find out what the seals have been eating. Hard bones of fish and mouth-parts of squid are not digested by the seals and are passed by the seal into their poo. Each type of fish has distinctive shaped bones. By sifting through seal poo, scientists can identify the bones found in it. The number of bones from each type of fish gives you an idea of how many of those fish are being eaten.
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The above image displays primary otoliths (ear bones) of a fish called Red bait. Each fish has two of these, a left one and a right one. Otoliths help fish keep their balance (or stay upright) in water.
In addition to finding prey hard parts in seal poo, you can extract prey DNA from the poo. Every animal has unique DNA. Not all DNA is digested by the seals so scientists can take poo samples, look at the DNA in it and work out what sorts of prey the animal has been eating. While extracting DNA is trickier than just taking out the hard-parts, an advantage is you can detect prey that leave no hard-parts.