What should your training heart rate be in order to safely develop cardiovascular fitness? 50%?

Cardiovascular fitness is defined as  body’s ability to produce and deliver   oxygen to the muscles. The more blood that is pumped, the lower the resting heart rate and the less risk an individual is for cardiovascular disease or other chronic conditions like obesity or type 2 diabetes.  

If the normal heart rate is about the range of 60 and 100 beats per minute, then lower this number is, the better one's cardiovascular fitness and health.
The higher the heart rate is, the more work the heart is doing.

That is true! many fish species traveled to the u.s. because of it.