In this book, Buck and the other sled dogs seem to subsist solely on fish. This was a kind of food that would have been easy to get up in that area during this time and it would have been relatively light. It would have been light because, as we are told in Chapter 2, what they were eating was sun-dried salmon.
In Chapter 2, at least, when Buck is working for Francois and Perrault, his ration for each day is a pound and a half of the sun-dried salmon. There will be times in the book where he gets more or less than that, but he always seems to be eating fish.