Nate the Great and the Fishy Prize by Marjorie Weinman Sharmat illustrated by Marc Simont

Rosamond makes a prize for the smartest pet contest but it disappears.  Nate the Great and loyal dog Sludge solve the case just in time!
2017 update-
My first grader just read this book to himself.  He says, "It was a great book!  The problem is that Rosamond lost the prize for the smartest pet contest.  The case is solved by Smudge, Nate the Great's dog, sniffs the bag.  Nate the Great comes over and sees the tuna fish can in the bag.  Then they can begin the contest!!"