Bring a large pot of lightly salted water to a boil. Cook macaroni for 8 to 10 minutes, or until al dente; drain and rinse under cold water until cool.
In a large bowl, mix together macaroni, peas and onions, salmon, and mayonnaise. Season with onion powder, salt, and pepper, and mix well. Cover, and refrigerate 2 hours, or until well-chilled.