From MarineSpecies Introduced Traits Wiki
A group of squirrelfish (Holocentrus ascensionis
) that are shoaling, not schooling: though swimming as a group, their speed and direction are not synchronised. This is a typical example of a marine population. 
Definition of Population:
A population is a group of individuals of one species
that live in the same geographical area at the same time.
This is the common definition for Population, other definitions can be discussed in the article
- ↑ http://en.wikipedia.org/wiki/Fish
- ↑ Solomon, E. P., Berg, L. R., & Martin, D. W. (2002). Biology, sixth edition. (N. Rose, Ed.). Stamford, CT: Thomson Learning.