If something is considered a caviar substitute, more than likely that something is salted non-sturgeon fish roe. Of the 30,000 species of fish on planet earth, only 27 of them can produce true caviar while all rest give us caviar substitutes. Since we are talking about roe from potentially thousands of different species of fish, there is no one easy way to classify how non-sturgeon roe compares to sturgeon roe.
Generally, it can be said that caviar substitutes have a less unique flavor than caviar, lacking the complex buttery notes of an Osetra, or the clean brine taste of roe from a Siberian Sturgeon. They are often described as more salty and fish flavored than roe from a sturgeon. However, this description is more accurate for certain caviar substitutes than others.
Many types of roe, such as red caviar and tobikko, have a much more mild taste and simple "fish-like" flavor compared to sturgeon caviar. Despite being quite different, these affordable roes each have a uniqueness that you do not get from a sturgeon. Often consumed more for the texture than the taste, caviar substitutes of this type can add a nice crunch or pop to certain dishes such as those served at a sushi bar.
When comparing salmon, trout, whitefish and tobikko roe to something like paddlefish roe, it can be said that some caviar substitutes are more of a “substitute” than others. Paddlefish is not a sturgeon but produces eggs that look and taste similar to true caviar, even though it is technically is not "caviar".
In the U.S. it is very common for popular caviar substitutes such as those mentioned above to be labeled as "caviar" even though they're just salted fish roe. Since there is so much caviar in today's marketplace, it is always important to know your species when making a purchase to determine if you are getting the real deal, or a substitute.
With that being said, if you want to buy caviar but sturgeon roe is looking a bit too expensive, try a substitute like paddlefish caviar. You'll get close to the taste of true caviar without the cost.